Radio Boulevard
Western Historic Radio Museum

Vintage Test Equipment  (from 1900 up to 1970)

Laboratory R-C-L Devices,  Diagometers & Test Kits,  RF Signal Generators,
Freq Measuring Instruments,  AF Oscillators,  Voltage Measuring Instruments,
C-L-R-Z-Q Measuring Instruments,  Vacuum Tube Testers,  Oscilloscopes,
Panadapters,  Xmtr Modulation Monitors,  Digital V & F Measuring Instruments,
 Test Bench Power Supplies,  Variable Voltage Xmfrs,  Miscellaneous Instruments


photo: Post-WWII National Co. engineer adjusting a General Radio 805-C RF Sig Gen while testing a Nazi-copy HRO receiver  

Test instruments that are built to a high-quality standard are generally referred to as "Laboratory Equipment." Some of the test equipment used in early laboratories may seem archaic and useless today but their purpose and operation is interesting and historically important. Some of the test and monitoring equipment for the military was the best that could be produced at the time and no expense was spared in its construction. At the budget-built end, there were the countless test-gadgets intended for radio repairmen or radio hobbyists that might seem to barely be able to perform the intended task but clever designs sometimes resulted in very useful test instruments. The following is a sample of some of the very best laboratory-quality vintage test instruments along with some of the most elaborate (expensive) military test gear and also some service test equipment from the other end of the price spectrum -  H. Rogers Nov, 2014

Laboratory R, C, L Devices

Otto Wolff, Berlin - Imported by James G. Biddle, Philadelphia

Resistance Decade Boxes

These two precision resistance boxes were built in Berlin by Otto Wolff and date from around 1900. James G. Biddle of Philadelphia imported these instruments and each box has his tag attached. Biddle was also a very early major builder and supplier of wireless equipment with Reginald Fessenden being one of his prominent customers over a long period of time. Otto Wolff, from 1900 to 1909, produced the Reichsanstalt-type of resistance standards used at the National Bureau of Standards (NBS.) At the turn of the last century, many precision measurements had to be calculated because the measurement couldn't be made directly since many of the instruments we take for granted today hadn't been developed. A known precision resistance allowed one part of the equation to be selected and the remainder was then calculated. These types of boxes were used in DC Resistance bridge circuits for precise measurements of an unknown resistance (Wheatstone Bridge circuits) or in other circuits for calculating current flow, low voltage levels or comparison evaluations. Both of these Otto Wolff resistance boxes were used a Mount Wilson Observatory in Southern California. Many experiments were carried out at Mt. Wilson and the facility there had (has) a lot of very early test gear. I was given these boxes by my old Virginia City friend, Maynard Clark who worked at Mount Wilson Observatory (Maynard designed much of the computer software for the various telescope drive systems at Mt. Wilson.)


James G. Biddle Company - "JAGABI"

Adjustable Slide Rheostat-Potentiometer

These large, adjustable wire-wound rheostat-potentiometers were generally used in college and university physics laboratories when performing experiments in electricity where fairly large currents were involved. Most of these types of devices used nickel-chromium wire wound on ceramic forms.  All of these rheostats will have a slide adjustment to vary the value of resistance (or voltage when connected as a potentiometer) as necessary for the experiment. Also, these types of rheostats were available in many different values of total resistance and maximum current flow ratings. This JAGABI has a screw-thread drive within the square tubing to allow minute adjustment of the slide contact position. The large knob is original and has JAGABI embossed on it - JAGABI was the trade name for James G. Biddle Company in the 1920s.

The JAGABI rheostat shown is 52 ohms total R, rated at 5.0 Amps. It's about two feet long. The small German rheostat shown is 48 ohms, only rated at 1.5 Amp - six inches long. 


Central Scientific Company

Shunt-pin Resistance Boxes

Central Scientific Company was located in Chicago, IL and generally built various types of devices for laboratory experimental use, mainly for schools, colleges and universities. Early resistance boxes require some calculation as to how to achieve the desired resistance from the placement of the various shunt pins into the numerous junction slots. Each junction slot can shunt the resistor selected by inserting the shunt-pin. To select a R value one first has to know the total resistance of the box. In the case of the Central Scientific box shown, the total R is 1100 ohms. Next one has to look at the specific resistance values shown next to the junction slots. These values show how much resistance is to be subtracted if the junction is shunted by a pin. With the box shown, each junction is a different value of R starting at 400 ohms and reducing in value down to .01 ohm. One has to calculate the value that needs to be subtracted from 1100 to result in the value of R desired and then insert the shunt pins into the necessary junctions to have those values of R total the subtracted value. The end result is the desired resistance at the large binding posts. Also, one could calibrate a device by using the box and moving the shunt pins around for the desired result and then total up the shunted R value and subtract that from the total R (1100 ohms for this box) and that would give the calibration resistance necessary to achieve the result desired. All very laborious and antiquated. However, it shows the process necessary before "dial in" decade boxes were available. The long shunt pin with knurled nut allows connection as a potentiometer if desired.


Leeds & Northrup Company

Precision Dial Resistance Decade Box

Morris Leeds and Edwin Northrup started Leeds & Northrup in 1903 in Philadelphia, PA to supply precision instruments for scientific experimentation within laboratories, colleges and universities and precision test and calibration instruments for industrial applications. Northrup was a professor at Princeton and only stayed with Leeds for about seven years but his name remained with the company. L&N was a major supplier of scientific instruments for colleges and universities, usually in the Physics Department. Various types of laboratories also purchased L & N equipment.  L&N was a major supplier of various types of devices that were used for industrial calibration of temperature sensors and associated temperature monitoring instruments. These would be thermocouples, RTDs and other temperature sensors or transducers. L&N built precision resistance devices including decade boxes, various types of elaborate potentiometers, fixed resistances along with sensitive galvanometers, individual component-type Wheatstone Bridges and many other types of instruments. Some of the Resistance Bridges were used to locate shorts in long runs of multi-conductor cable. L & N was purchased by General Signal Company in the mid-1990s. Today there is a Leeds & Northrup doing business out of the UK specializing in industrial furnace temperature monitoring devices.

The R decade box shown is probably from the 1940s. It has a range of 10 Ω up to 99.99K Ω. This L&N box differs from the "shunt-pin" type of R box in that the resistance value desired is just "dialed-in" with the switches.  The box is mahogany and the top is hard rubber. The hard rubber used in L&N equipment has a tendency to turn brown with long-term exposure to light. Originally these panels were black. As can be seen from the property asset decal on the front, this instrument was used a Mount Wilson Observatory.


The Eppley Laboratory, Inc.

Standard Cell


One of Edward Weston's products was "The Weston Standard Cell" which was a mercury-cadmium battery that provided a very specific DC voltage that didn't change over long periods of time. The Weston Std. Cell voltage was nominally 1.018638vdc for most of its life. A Standard Cell provided a reference voltage for calculation of various circuit values or a specific voltage supplied for precision measurement type comparisons. Anywhere that a precise DC potential of known value was needed, the Standard Cell provided that function. Most Standard Cells were used in college and university physics laboratories as the source of an exact DC potential that could be then calculated into the known circuit values for determining an unknown value. In the industry, Standard Cells provided the same function - a precise known DC potential for various purposes. Since any older style DC voltmeter connected across the Standard Cell's terminals would add a "resistive load" (usually 1000 to 5000 ohms per volt) to the circuit that would affect the actual circuit potential, one had to rely on the fact that the Standard Cell was going to provide exactly 1.018638vdc and use that as the DC potential in the equation.

Many companies built Standard Cells and Eppley Laboratory, Inc. was a popular source for school, college or university laboratories.


Leeds & Northrup Company

Type K Potentiometer

Precision potentiometers were used in conjunction with sensitive galvanometers and usually with a Weston Standard Cell (or similar Standard Cell) to provide a method to measure very low voltages. Used in various hook-ups a variety of measurements were possible although some had to be calculated from other data. Early designs used straight length resistance rods but these proved to be difficult to handle and store. The idea of using a resistance wire coil that was wound in a spiral configuration saved a lot of room and allowed for better portability. The maximum voltage that could be measured was usually around 1.8vdc but special external precision resistors could be connected to increase the voltage limits. The galvanometers used with Type K potentiometers was usually a sensitive device that used a mirror mounted to the moving coil and a light source. A shadow-graph scale was at one end of the galvanometer allowing very sensitive measurements to be made (see "Voltage Measuring Instruments" section below for L&N 2420B Mirror Galvanometer.) There are several binding posts on the rear of the Type K potentiometer to allow connecting various instruments, i.e., the Std. Cell, the galvanometer, external EMF inputs, etc. The controls on the right side of the potentiometer are for "rough" R adjustments and then the large drum is turned to fine adjust the potentiometer. The scale on the drum dial appears to change as the potentiometer arm and drum rides up the resistance wire spiral. The 0-100 scale of the drum is read through the window that has 10 levels shown giving resolution of 1000 (100 x 10) for the span of the potentiometer. Switches allow for selecting either an external Std. Cell for source voltage or for connecting an external source of EMF that could be used by the potentiometer. Also, there is a control to compensate for slight changes in the Std. Cell output. Scaling can be selected via the voltage switch.

Most Type K Potentiometers were either in colleges or universities (in Physics classrooms usually) but were also found in laboratories where a precise measurement of low level DC voltages were necessary. Within an industrial environment, the K Pot could be used for special test and calibration. Testing and measuring thermocouples was a common use in industry.

The photo left shows the construction underneath the K Pot. Note that the hard rubber underside is black since it was protected from exposure to light.

Photo right shows the K Pot with its protective cover installed. The wooden base and the cover are made out of mahogany.


Leeds & Northrup Company

Type K-3 Universal Potentiometer
Catalog No. 7553

This is the updated version of the Type K potentiometer described above. The K-3 performs the same functions with much easier to read dials, adjustments and connections. The K-3 was produced in the late-fifties and early-sixties. Various inputs are "guarded" which are protection circuits for the K-3. Circuitry inside appears quite different from the Type K with modern color coded wiring along with many plastic forms and parts. This unit doesn't have the metal "box" that the entire unit is mounted in although, even out of the box, the entire potentiometer is enclosed and protected.

These types of potentiometers were used to measure very small voltages accurately. Generally, the industrial use was in testing or measuring thermocouples (TC.) The TC uses two dissimilar metal wires in a junction that when heated will generate a small voltage. The voltage is incredibly small but changes in temperature to the TC junction cause a linear change in TC output voltage. TCs are used in the industry for measuring temperature inside furnaces or other high temperature areas. Before accurate low-level digital metering was available, potentiometers and galvanometers were used to test and measure low-level voltages.


W. M. Welch Scientific Co., Central Scientific Co., Herman H. Sticht Co., Rex Rheostat Co.

Adjustable Slide Rheostats

Many different companies built these slide rheostat-potentiometers. Some of the slide contacts use carbon brush contacts that were spring-loaded. Others used metal finger contacts against the windings. Some forms are ceramic while others are a molded slate-like material. Some of the slides are gear driven while other use a "pinch" type of release for adjustment. Some rheostat-potentiometers have a "100 - 0" logging scale with an arrow pointing to the right with the notation "INCR AMPS." Left to right in the photo are Herman H. Sticht Co., Central Scientific Company, W. M. Welch Scientific Co. and Rex Rheostat Co.

Most of these large rheostats were sold to colleges and universities for their physics laboratories but rheostats were also sent to various industrial sites for test and repair uses. As would be expected for their typical users, the various slide rheostats shown in the photo came from St. Martin's University located in Lacey, WA.


Leeds & Northrup Company

NBS Precision Oil-Filled Resistor Standards


L &N produced these precision "standard" resistors for the laboratory and for industry where the resistance value was traceable to the National Bureau of Standards (now NIST - National Institute of Standards and Technology) for accuracy. Each wire wound resistance used manganin wire* that was treated and aged for stability and then the unit was mounted into a cylinder that was filled with insulating oil. The oil provided a way to allow for greater dissipation of heat and the hole in the top of the resistor allowed for venting due to expansion or allowed the users to insert a thermometer to monitor the heat build-up. The four-terminal connections are part of the Reichsanstalt-type resistor standard description. The terminals are 0.25" in diameter for high current and low IR drop. 

These NBS resistor standards could be purchased in a set and were sometimes supplied in a wooden storage box. Each resistor usually had paperwork showing its traceability to the NBS.

These NBS resistor standards are values of 1 ohm, 10 ohms, 100 ohms and 1000 ohms. They were used at St. Martin's University in Lacey, Washington.

* Manganin wire is an alloy of copper, manganese and nickel. It was developed by Edward Weston in the late-nineteenth century as a stable zero-coefficient of resistance material.


Cornell-Dubilier Electronics Division
(Federal Pacific Electric Company)

Capacitance and Resistance Decade Boxes

Cornell-Dubilier built these small hand-held Decade Boxes that provided both resistance and capacitance in various wattages and working voltages. Generally used in prototype construction or "factory select" component calibrations where a value R or C needed to be connected into a circuit. Left to Right and Top to Bottom:

Model RDA = 10W - 0 to 110 Ω
Model RDB = 0 to 11K Ω (wattage not marked, probably 1/2 watt)
Model RDC = 2W - 0 to 1.1K Ω

Model CDB = 0 - 1.1µf   600wvdc
Model CDC = 0 - 10µf   600wvdc
Model CDE = 10 to 150µf  450wvdc Electrolytic Capacitors

Today, most of these types of decade boxes are not reliable or accurate. The capacitors are probably leaky and the carbon resistors may have drifted in value. Test before using.

These CDE boxes were a set from Mt. Wilson Observatory.



Model 250  Power Resistance Decade

Many of the "decade" boxes that are found are very low current devices that allow the user to substitute or select switchable resistances into a circuit under test or construction. These "low current" boxes are not suitable for power supplies or for high current loads. Many times the internal resistors are only good for 0.5 watt. Enter the Clarostat Model 250 Power Resistance Decade. This rack mount "decade box" is entirely made up of high current WW resistances - usually 50 watt but some are 100 watt resistances. The user can "dial in" the desired resistance from 1 Ω up to 999,999 Ω. You only have to be careful if your desired resistance is <10 ohms or so and you are using the box as a current load. Maximum current is shown below each step switch. A series connection is provided (the red binding posts) for inserting a current measuring instrument and the resistance connection is via the black binding posts. Note that the schematic for the binding post connections is on the front panel. This "decade box" is very useful and it's always on my test bench (even if I don't use it for every project.)


General Radio Company

Type 526C  Potentiometer


General Radio Company was founded by Melville Eastham in 1915 to provide high quality radio parts, radio testing devices and high-quality laboratory test equipment. G-R was originally located in Cambridge, Massachusetts. Eastham was also associated with the Clapp-Eastham Company, a producer of wireless equipment and later radio sets. Eastham sold out his part of Clapp-Eastham in 1917. G-R supplied the "top-of-the-line" laboratory test equipment throughout the thirties up into the sixties. By the late-1970s, G-R changed their name to "GenRad" and concentrated on the automatic computer-driven PC board testing equipment (ATE, automatic test equipment.) In 2000, Gen-Rad was purchased by Teradyne for their ATE business. IET Co. now supplies some new GenRad instruments and online manuals for the older GR test equipment.

Many times, a smaller, three terminal variable resistance is needed for calibration purposes. Depending on how it is connected, the variable resistance can be a potentiometer that can act as a low current voltage divider for various calibrations or adjustments. Utilizing just two terminals (arm and one end) can provide variable resistance values for low current calibration requirements. This GR pot is 0 to 100K ohms and is rated at 11mA maximum. The panel of the 526C is phenolic and the potentiometer assembly is mounted in a black wrinkle cylindrical housing with nickel plated binding posts. Even this small instrument shows the quality of construction provided by all General Radio Company devices. The GR 526C is from the 1940s.


General Radio Company

Type 107-K  Variable Inductor


General Radio supplied various types of precision components for laboratory and test facility use. Some types were fixed values of capacitance or inductance while other types were variable capacitors and variable inductors. Also, capacitance, inductance and resistance decade boxes were available from GR. Shown to the right is General Radio's precision variable inductance. There were five ranges available and the one shown is a Type 107-K which has the L range of 90 to 500 uh.

The inductor inside the unit is basically a variometer but the user has the option of connecting the two coils - the rotor or the stator - either in series or in parallel. Depending on the position of the moveable link, either self-inductance (total L) can be utilized or mutual inductance (mutual L) can be used. The scale is accurate to 1% for a series connection for total L at 1kc. For a parallel connection, the scale is divided by four. Mutual L is accurate to 2.5% at the maximum value. Mutual L range is 0 to110uh (or .25% of FS.)

The rotor and stator coils are impregnated and baked to assure stability. Litz wire is used in most of the versions that were available. There were five versions J thru N providing the range of 9uh up to 500mh if one had all five versions of the Type 107. The case is walnut and the panel is phenolic. The Type 107-K shown is from the late-1950s and sold for $85 in the 1959 catalog. Earlier versions of the Type 107 had a silver dial rather than the black dial on the Type 107-K shown in the photo.


General Radio Company

Type No. 722-CB  Precision Variable Capacitor

As with the Type No 107-K precision Variable Inductor shown above, the Type No. 722-CB is a Precision Variable Capacitor. The 722-CB is an aluminum air-variable capacitor that has ball-bearing support and is worm-gear driven with a precision readout that is scaled in pico-farads (pf or uuf are the same) and with a vernier scale for precision reset capabilities. A scale correction chart is supplied and mounted on the front panel. 1100pf shown is the maximum value of capacitance with the variable a full-mesh. The minimum value is 50pf. The output of the 722-CB is "three terminal" with two GR-874 connectors and one grounded binding post.

The maximum voltage rating of the 722-CB is 1000 volts peak.

Selling price was $205 in 1959. The calibration stickers are from Honeywell and date from the early-1970s. The lower left metal tab had a calibration "seal" that would show if the instrument was tampered with between calibrations.


General Radio Company

Type No. 1454-A  Decade Voltage Divider

This device allowed the user to connect a voltage to the input terminals and then, by adjustment of the four switches provided, create a voltage-divider circuit that resulted in the desired reduced voltage at the output terminals. The decade switches are set to a desired ratio of voltage reduction. For example, if 10.0vdc were the input voltage and the first decade switch (marked 0.1) was set to 5, this would be a ratio of 1 : 0.5 (or .5 which is one half) and the resultant output voltage would be 5vdc. The four decades allow for precision adjustment for the desired, divided voltage at the output. Price was $145 in the late-fifties.


Diagometers & Radio Test Kits

When a radio repairman had to make a house-call he couldn't bring along a lot of test gear. The diagometers were portable signal generator, tube tester, voltmeter, current meter in one case. Normally, the repairman could remove a tube from the radio under test and place it in the diagometer and then plug in the proper cable to the radio's tube socket. The repairman could then measure "in circuit" what was going on with that particular radio stage. Some of the more elaborate diagometers will have built-in oscillators to act as signal generators for test or alignment purposes. These oscillators are usually crude and simple devices that don't have any calibration of output level or frequency calibration. However, in the mid-to-late 1920s, few superheterodyne receivers were in use by the general public and the normally encountered TRF AM BC receiver was pretty simple to get tracking with the oscillator provided.

Later "test kits" might include a VOM and a Capacitance Tester in one box. Other types of instruments were also available. Triplett offered a "test kit" that had a VOM, an Oscillator and a Tube Tester.

Diagometers were popular instruments through the late twenties and very early thirties but as radio sets evolved and used higher gain stages (that required shielding) most of the diagometers wouldn't work with the interconnecting cable strung out and the radio set's tube installed in the diagometer where it was completely unshielded. Most of the later-1930s radio stages would oscillate or feedback when interconnected with a diagometer. 

Shown in the photo to the left is the Supreme Radio Diagometer Model 400 which has an assortment of adapters to fit to the radio tube socket. The radio tube is then plugged into the panel socket and then the service man can read various voltages and current to diagnose the problem. This test kit probably dates from the late-twenties to the early-thirties.


Shown in the photo to the right is a Weston Diagometer that works in a similar manner to the Supreme instrument however it is much smaller and more portable. The case is bakelite. This is an earlier unit than the Supreme since it only has four pin and five pin test sockets. Probably dates from the late-1920s.



Precision Apparatus Company 

Test Panels

Precision Apparatus Company was started by S.M. Weigast in Brooklyn, New York in 1932. Precision aimed their products at the radio serviceman and supplied a variety of instruments from tube testers to voltmeters. Signal Generators were also produced along with diagometer-type panels. By 1957, the name PACO was being used and, in 1966, Dynascan acquired the Precision stock and name, adding it to their name B&K (there after called B&K Precision.)

Most radio repairmen liked to have a work bench that was easy to use and already had the test equipment interconnected and ready to use. The Precision Test Panels accomplish that by incorporating many functions into one unit. Included are voltmeters, current meters, ohm meters, battery voltages, etc.

Shown in the photo above right is a Precision Test Panel Pattern 829. This panel is designed to mount in a cabinet or wall just above the test bench. That allowed the radio repairman to have easy access to all switches and be able to read the meters. Internal batteries provided +45vdc and -4.5vdc for various circuits under test. To the left side of the panel are mounted the adapter sockets that would plug into the end of the test cable. The test cable would then be plugged into a radio set's tube socket and the tube then plugged into the panel. In this manner of operation the repairman could make measurements while the set was operational (or more like non-operational.) John Ridgway W3ON had a radio repair business in Maryland in the thirties. His shop was equipped with two of these 829 panels (I do have both panels.)

RF Signal Generators

RF Signal Generators for the Service Industry

The Clough-Brengle Company

 Model OMA - Wobulator


The Clough-Brengle Company was founded by Kendal Clough and Ralph Brengle, both formerly associated with the Silver-Marshall Company. The location of the factory was in Chicago and the year was 1932. C-B produced laboratory test equipment and also radio service-type equipment. C-B was most active throughout the thirties and early forties. After WWII, the company changed hands and, though listed in some Chicago directories during the fifties and early sixties, didn't advertise much and produced other types of test equipment.

Shown in the photo to the right is the C-B Model OMA "Wobulator." The "Wobulator" is a mechanical FM signal generator. Using a variable speed electric motor that turns a small air-variable capacitor that is connected to the oscillator tuning capacitor, a varying frequency is created. The sweep frequency is controlled by the motor's speed and the deviation is based on the ratio of the variable's maximum capacitance to the tuning capacitor setting (frequency) that the generator is tuned to. Wobulators were mainly used for sweep aligning IF sections in superheterodyne receivers.

The C-B Model OMA is an early FM signal generator but it is for alignment of IF sections of superheterodyne receivers. Under the vented area on the right side of the panel is a rotating indicator to show motor speed. The motor speed is controlled by the "Phase" control. This was adjusted to give a "steady" image on the oscilloscope. The band switch controls the frequency range while the frequency base line is adjusted with the center dial. Since early oscilloscopes didn't have elaborate sweep circuits the wobulator had to be adjusted for a steady image of the IF passband.

Model OCA RF Signal Generator

Shown in the photo to the left is the C-B Model OCA RF Signal Generator. Despite its small size, the Clough-Brengle OCA RF Signal Generator offers quite a few functions. The frequency tunes from 100kc to 30mc. The modulation is either 400~ or external. The modulation input is via the phone jack on the front panel. The RF output is the typical C-B connector that looks like a pilot lamp socket (but isn't.) The charts on the front panel give the user an idea of where in the RF spectrum the OCA is set although accuracy (by today's standards) would require the use of an external frequency meter.

Both of these C-B instruments came from K6QY.

  Further down this page, listed in Oscilloscopes, is the Clough-Brengle "Graphoscope" Model 126.


Precision Apparatus Company

Precision Signal Generator Series E-200


Shown in the photo left is the Precision Signal Generator Series E-200. This signal generator was a good seller for Precision probably because it was easy to use and was accurate enough for the time. Frequency coverage is from 90kc up to 44mc. A 400~ modulator was provided. This E-200 dates from around 1940 although they were produced for probably 20 years longer. This E-200 belonged to John Ridgway W3ON and it was likely purchased just before WWII.

Precision started to use PACO in the late-1950s and when Dynascan purchased Precision in 1966, "Precision" was added to their B & K brand.


Simpson Electric Company

Model 340  Signal Generator

Ray Simpson got his big break with his association with Charles Lindbergh's 1927 flight across the Atlantic. Simpson designed, built and installed an indicator device to work with the Spirit of St. Louis' earth indicator compass. This fame allowed Simpson to develop his company that built meters and testing devices. Simpson grew through the thirties and forties, mainly in providing high-quality panel meters. Their bakelite-cased Volt-Ohm-Meter, or VOM, was probably their most popular product. It's interesting that in 1985, Simpson Electric Company was purchased by the Lac du Flambeau Band of Lake Superior Chippewa Indians. Simpson is still in business.  

Simpson is best known for their VOMs but they also produced other types of test equipment. The Model 340 Signal Generator is perhaps the thinnest generator ever built. It is only three inches deep. It can be used upright (as shown) or it can be operated lying flat. The AC power cord exits at the top of the unit. The large dial has a multitude of different scales. Frequency coverage is from 75kc up to 120mc. A 400~ modulator is included. Two output levels are provided along with an adjustable gain control. Dates from the late-1940s


Simpson Electric Company

Model 415 Signal Generator


The Simpson 415 is a standard Radio Serviceman's Signal Generator. RF output is from the connector on the lower right while AF output is from the connector on the lower left. The connectors are not standard (or they were not used for very long) and require a specific Simpson plug. The cable on the lower right of the panel is an original. The 415 has seven frequency ranges and covers 75kc up to 108mc. Audio modulation has an adjustable level control and is 400 hz. Using the selector switch the Audio Output connector can be used as an audio input for external modulation purposes. The 415 dates from just after WWII, around 1946 or so.


The Hickok Electrical Instrument Company

Model No. 188X - Signal Generator

Robert Hickok started in the test equipment business in 1910 in Atlanta, Georgia. Soon, he moved to Cleveland, Ohio where there was more of an industry that could use his products. By the early twenties, Hickok was developing methods to test vacuum tubes. By the mid-thirties Hickok was offering many types of radio test equipment including voltmeters, tube testers, signal generators and even an early oscilloscope. Hickok went on to produce many types of tube testers that were used by the military and by civilian radio and television repairmen. In 1956, Hickok bought out Supreme Instrument Company. Hickok Electrical Instrument Company went into the automotive diagnostic instrument business in 1984 and today produces not only those products but also aircraft and locomotive indicators and gauges. Manufacturing was moved to the old Supreme Instruments location of Greenwood, Mississippi in 2000.

Although now mainly known for tube testers, Hickok produced test equipment of all types for the service industry, radio repairmen initially and then later television repairmen. The Model No.188X Signal Generator first appeared in a mail order catalog in 1942, so the actual production of these signal generators probably began in late-1941. Many were produced and used during WWII and generally were given a military designation as TS-465A/U although some units were merely civilian models with military acceptance stampings. The 188X had a frequency range of 100kc up to 110mc covered in seven bands along with crystal controlled fixed frequency 100kc and 1000kc reference outputs. The output db meter had three ranges. FM radios were being sold by Scott, Philco and Zenith by 1940, so the 188X provided the radio service man with FM sweep signals that were selectable for aligning FM receivers as well as sweep aligning AM radio IF amplifier sections. A sweep synchronizing voltage was available to allow a stable oscilloscope pattern when aligning by this method. AM modulated signals could be selected as fixed frequency modulation at 400hz or variable frequency using the audio oscillator frequency control that had a range of 1hz up to 10Khz. Pin jacks on the lower front panel provided external input to the db meter, external modulation and sweep synchronized voltage. Main output was via a coax cable that exited out the front panel. There was also a Model 177X that eliminated the db meter. The 188X was probably available after WWII for a short while. Hickok introduced the 188X's replacement by 1950 and it's shown below.


The Hickok Electrical Instrument Company

Model 288AX Universal & Crystal Controlled Signal Generator



The 288X was a post-WWII upgraded and modernized version of the 188X (profiled above.) The first version 288X signal generator was available around 1950. There were other versions of the basic 288X design that were minus the output meter or with an external tuning dial. All of the 288X versions would do FM alignments and featured a sweep circuit. Frequency range is from 35kc up to 110mc. Various AM modulation frequencies were available both fixed frequency and variable. The variable audio oscillator could also be used externally. This 288AX is a very late version that shows that the "A" model was thoroughly modernized - at least for appearance. The 1967 price was an incredible $315.


RF Signal Generators for the Military

U.S. Navy  -  LP-5  RF Standard Signal Generator
CFD-60006-A - Signal Generator Unit
CFD-20080-A - Rectifier Unit

Federal Manufacturing and Engineering Corp.

The LP-5 RF Signal Generator (CFD-60006-A) is a "military contact" reconfiguration of the famous pre-war General Radio Company Type 605-B Standard Signal Generator. The LP-5 was built during WWII by contactor Federal Manufacturing and Engineering Corporation, a company that was mainly known for photographic equipment such as cameras and enlargers. As with many "contractor-built" units for WWII use, the LP-5 uses many primary source OEM parts and components in its construction. In this case, parts and components from General Radio Company. The same build method was used for the WWII Wells-Gardner-built RAO receivers that used many National Company parts in their construction.

The LP-5 was repackaged as a semi-portable RF generator built into a heavy-duty aluminum case. It can be operated from either its separate 115vac operated rectifier power unit or from a battery set-up that provides +200vdc for the "B" supply and +6vdc at 1.7A for the "A" supply.   >>>

>>>   Although the pre-war GR version mounted its PI-605 power unit in the same cabinet as the oscillator unit, the LP-5 set-up utilizes a completely separate rectifier unit, the CFD-20080-A, that is connected to the oscillator unit via a power cable. The CFD-60006-A's metal case had a screw-mounted cover that had the dummy antenna, a 10:1 attenuator and cables clip-mounted on the inside (these covers are almost always missing from units found today.)

The frequency coverage of the LP-5 was from 9.5kc up to 30.0mc in seven tuning ranges. An additional tuning range allowed the frequency coverage to be extended from 30mc up to 50.0mc although with reduced accuracy in frequency readout and reduced output levels. The internal modulator provides up to about 50% modulation (fixed 1000 cycle sine wave) with very little distortion but higher mod levels, although available, will increase the distortion significantly. External modulation is also an option. The LP-5 has a built-in VTVM that measures the RF output level, although not directly. The user adjusts the output level to a reference line on the meter and then the output attenuator scale is accurate when referenced to the multiplier setting. Modulation level is read directly on the meter scale. A constant "one volt" RF output is provided at the upper coaxial fitting to allow for various types of monitoring or measurement. The lower coaxial fitting is the attenuator output that is normally used for calibration purposes. The coaxial fitting use the standard Navy "snap in" coaxial plug.

The LP-5 uses five tubes, 76 RF Osc, 89 Separator, 76 Modulation Osc, 84 Modulation VTVM rectifier and a 955 RF Carrier VTVM. The rectifier unit uses an 84 tube which brings the total tube count to six.


U. S. Navy - SG-85/URM-25D RF Signal Generator

Various Contractors

The URM-25D was an upgrade from the previous model URM-25 that was introduced in the early 1950s. The URM-25D used nine tubes and one 6X4 rectifier tube in the separate power supply that was bolted to the inside of the case. The power supply was connected to the signal generator by a power cable that had a six pin Jones' plug on the end and by a small round two pin Jones' plug for the AC input routing that allowed the unit to be tuned on from the front panel. The RF unit is enclosed in a cast aluminum housing that virtually seals the circuitry and components. Within the enclosure are six tubes, the coil turret, the tuning condenser and crystal calibrator. Located on the side of the cast housing is the audio section of the signal generator that uses three tubes and a fiber component board with all of the components and wiring mounted to soldering turrets. The VTVM circuit operates the meter which can read RF output or modulation level. The attenuator allows changing output level ranges from X.1 up to X10K which can be then correlated to the VTVM for accurate output measurements (provided the impedance match is correct.) Various terminations and attenuator modules are contained in the lid and allow proper impedance termination for accurate output measurement. The frequency range is from 10kc up to 50mc. The adjustable level audio modulation is at 400hz or 1000hz and there is also an External Modulation input. The URM-25D was in production from various contractors from the late-fifties up into the 1970s. There were later suffix designations up to "L" with various upgrades to the units produced. The URM-25 RF Generators were in active use up into the late-1980s. The URM-25D shown in the photo to the right was built by Trad Electronics Corporation. The power cord is not original and the "butch plate" used for mounting the power cord strain relief isn't either. >>>

>>> Most URM-25Ds will have paper capacitors used on the Audio board. These are usually Micamold brand - probably some of the worst quality paper capacitors ever built. The style used a brown bakelite rectangular package. This style of Micamold capacitor is notorious for developing leakage current that will cause excessive current flow through the associated circuit resistors. Also, the heat build-up within the plastic case causes the body of the capacitor to bulge in the center. If your URM-25D's audio oscillator won't work, most likely the paper capacitors on the Audio board are bad. When replacing the paper capacitors be sure to check the value of the resistors on the Audio board to make sure excessive current hasn't caused them to drift in value. After capacitor replacement adjust R-157, the degeneration potentiometer, for the lowest distortion sine wave and verify that the Audio Oscillator will restart when switched on and off, that is, when switching from CW to MOD.

RF Signal Generators for Commercial or Laboratory Use

Measurements Corporation

 Model 82 Standard Signal Generator*

Measurements Corporation of Boonton, New Jersey was founded in the late-thirties by former employees of the Ferris Instrument Company. Harry Hauck and Jerry Minter are names usually associated with Measurements' formation. High quality was always obvious in Measurements equipment and they supplied the market up to 1954 when they were purchased by McGraw-Edison Co., who kept production going for a short while before closing down Measurements Corporation. 

The Model 82 is a laboratory quality RF Signal Generator with dual meters and adjustable attenuator. The frequency range is from 50kc up to 50mc and the Separate AF oscillator that can be used for modulation or as audio output. The AF output is adjustable from .02kc up to 20.0kc. AF output level can be monitored on the right side meter while the RMS output level can be monitored on the left meter. The RMS level is sampled before the adjustable attenuator but the attenuator has a calibrated scale that can be used to set the actual RF output level based on the measured RMS into the attenuator (of course you must use the correct termination load for everything to be accurate.) The Model 82 dates from the late-forties.

* "Standard Signal Generator" is a term that specifically describes a type of RF signal generator that provides the following functions. First, the carrier frequency must be adjustable and range from kilocycles up to megacycles. Usually, 15kc for the low and 50mc for the high although the ranges can vary depending on the manufacturer. The amplitude of the output must be adjustable and the user must have a means of measuring the output accurately. This is either a scaled attenuator or a built-in VTVM type (measuring RMS) meter or both. Modulation of the carrier is provided and is adjustable. A means of measuring the percentage of modulation is provided. Frequency of modulation is selectable at either 400hz or 1000hz and an external modulation input is provided. Output impedance is very low allowing the user to match the device-under-test impedance using series loading or various terminations.

   I found this old Measurements Model 82 in the "inactive" stockroom at Bently Nevada Corp. in Minden, Nevada around 1978 or so. BNC had purchased many pieces of surplus test equipment in the past and when the piece of equipment was no longer used it was sent over to the "inactive" stockroom. It was possible for employees to purchase various pieces to test gear out of the "inactive" stockroom (although this became more difficult by the 1990s.)

General Radio Company

Model 601-A  Standard Signal Generator


This is a battery operated signal generator that provided very accurate voltage measurements of the signal output and was used where it was necessary to have a portable signal source for testing or aligning various types of equipment. The 601A uses three type 30 vacuum tubes, one RF Oscillator, one AF oscillator and one VTVM tube. The AF oscillator was used to modulate the RF oscillator to provide 400~ modulated signals for alignment purposes. Frequency was adjustable for 500kc up to above 2000kc but for accurate measurement of the output frequency a heterodyne frequency meter was necessary. The built-in meter allowed measuring the output level and also allowed measuring the filament voltage and the plate voltage, both of which were provided by batteries that are mounted internally. The entire generator is housed in an oak case that can have the lid folded down and latched. Side-mounted carrying handles are provided for easy moving. The 601-A is from the early 1930s. I was given this GR instrument by KO6NM.


General Radio Company

Type No. 805-C  Standard Signal Generator

The enormous General Radio 805-C is probably the largest RF Signal Generator that was ever produced. It's 30" long by 16" high and 11" deep. Weight is over 100 lbs. The tuning dial is 8" in diameter. 12 tubes total which includes the Amperite 3-4 ballast tube. The RF Oscillator and the RF Output tubes are usually metal 6L6 tubes but this particular 805-C was equipped with 1614 tubes instead. The 1614 tubes are 20W plate dissipation, heavy-duty industrial versions of the 6L6 metal tube. These weren't end-user substitutions either - this 805-C has the General Radio "1614" tube identification tags installed. The RF Output is modulated by a 6L6. The power supply is electronically regulated using a pair of 2A3 tubes along with a 0D3 regulator tube. Two rotating turrets have the individual band coils mounted to them with the Oscillator turret and the Output turret rotating simultaneously with the band switching action. The entire RF box is fully shielded. Alignment can be performed with all shielding in place by way of the alignment holes in the front panel (they have metal hole plugs installed normally.) Frequency coverage is from 16.0 KC up to 50.0 MC. Modulation is selectable 400~ or 1000~ or External. The Output Attenuator allows signal outputs to be reduced to < 1.0µV while full output is measured in volts (2 vrms FS on the meter.) The Attenuator is also entirely shielded in its own metal box and has a 6AL5 tube inside that is part of the Output VTVM circuit. Metering allows measuring percentage of modulation and RMS output level. These massive, behemoth signal generators were the industry standard from just after WWII up to around the early-1960s. The 1951 selling price from GR was $1450 and by 1961 it had escalated to an incredible $1975.
GR-805-C SN: 1285 - Here's how I found this behemoth of a signal generator. Around 1997, I'd been running a "wanted" ad in the local flea market paper for radio receivers and transmitters. I had a fellow call on the ad and tell me, "I have a transmitter for sale." Questions to find out any further details were answered with, "I don't remember" or "I'd have to go look at it in the garage."  I got his address which was up in the north part of Reno in a community called Sun Valley. When I got there the fellow opened up his garage door and showed me this GR-805-C setting on the garage floor. He said, "There it is." I told him, "That's a Signal Generator." He replied, "Well, isn't that what a transmitter is?"  Hmmmm,...he had me there, I bought the GR-805-C.

The photo to the right shows the 805-C in the condition it was when I purchased it. Note that the PERCENTAGE MODULATION meter is damaged. There is a red pin jack that is identified with paper labels covered with tape. This was for testing the regulated B+ (that's adjustable) without taking the unit out of the cabinet. Note that the Attenuator knob is not original. Also, the plastic pointer on the carrier percentage vernier dial is missing. The output connector is not original and is an SO-239 "wedged" into the "drilled-out" remains of the original connector. The aluminum boarder around the panel is showing because the trim edge pieces are missing.

Below are some of the internal problems,...

Photo left shows the power supply section below the Modulator. Note that the 5U4G tube is broken and only the base is installed in the socket. Apparently, where this 805-C was used employed coffee drinkers that were a bit careless. The damage to the modulation meter was caused by spilled coffee. Coffee was spilled into the vents on top of the 805-C also. Lots of coffee residue in the power supply section too. Fortunately, dried coffee is water-soluble. The power supply filter capacitors are oil-filled paper dielectric types while other filtering capacitors are either can-type electrolytics or axial mount tubular electrolytics. The two large tubes in front of the power transformer are a pair of 2A3 tubes used as part of the electronic voltage regulation circuit. The power transformer is the typical General Radio red painted open-frame type.


There are two shields over the top of the RF Osc and RF Output sections that are missing. Also, the shield that covers the Modulator and Power Supply section is missing.

Photo above shows the RF box from the top. The two rotating turrets that carry the RF coils can be seen in box the RF Oscillator section and the RF Output section.  To the right of the RF box is the Modulator section. The sheet metal box to the left of the RF box contains the Attenuator.

Restoration of this 805-C has been "put off" many times over the years. Too heavy, too big, what would I use it for, etc.,...all the usual excuses. Finally, the restoration of this "over the top" piece of test equipment has begun - January 2015

GR 805-C  SN:1285 - Restoration

Rebuilding the Modulation Level Meter - This meter had dried coffee all over the front glass. The coffee had seeped into the meter and had made a mess of the scale, the pointer and the suspension. The meter suspension was "locked" in place due to "dried coffee." I discovered that coffee was pretty easy to remove. Water worked pretty well but Glass Plus really worked quite well. The meter housing, glass, scale and the suspension cleaned up fine and all traces of the coffee were removed. This freed the suspension but the needle was broken off just before the pivot points. I decided to try and "graft" a new needle to the remaining part of the pointer shaft. I found a needle that matched and was successful in using epoxy to attach the new needle. Of course, the new needle didn't have the same mass as the original and also the glue was adding to the total mass of the pointer. I had to add a little bit of weight to the counterbalance to get the pointer to "sort of" balance. This changes the amount of current necessary to drive the meter to FS thus affecting the calibration. The meter does work smoothly however the current required for FS deflection is twice that of original therefore the meter reads half what it should. The series resistance in the mod meter circuit is quite high (>100K) so minor adjustment of the external series resistor allowed compensation for the higher current requirement of the repaired meter. (Current FS was originally 200uA FS and the change to 400uA FS didn't require much adjustment of the series R.)

photo above: The damaged Percentage Modulation meter before restoration. It is unbelievable but the brown "gunk" is coffee residue (and a lot of it!) It's gotten inside the meter and has caused significant damage. The "gunk" has gotten into the suspension and has dried, "locking" the movement.

photo above: The Percentage Modulation meter restored. Note that there is a slight stain at 30% to 38% but a vast improvement in the appearance and functionality of the meter.

Attenuator Rebuild - Shown in the photo to the left is the disassembled Attenuator housing showing how the various 1% wire wound network resistors are mounted. Note the lower round opening (at ~ 8 o'clock) which is the location of the 61.5 ohm 1% series output resistor. This resistor and two resistors in the attenuator network were open. Since the wire wound elements are nickel-chromium wire (ni-chrome, or NiCr) the wire can't be soldered to with ordinary SnPb solder with a rosin core. NiCr wire has to be soldered with silver solder with an acid flux. Low temperature, silver-bearing solder can be used (melting temp around 450º F.) Shown in the photo to the right are the repaired resistors. On the left is the 61.5 ohm output series resistor which had to be wound with new 36 gauge NiCr wire. The middle resistor is a tapped resistor with 135 ohms and 16.7 ohms. The 16.7 ohm side needed new NiCr wire to repair. The black residue seemed to be a coating to protect the junction.
The resistor to the right is also tapped 135 ohms and 16.7 ohms. This resistor had one end of the 135 ohm wire loose. It was resoldered with the low-temp silver solder to repair. Since the low-temp silver solder uses an acid core these joints had to be cleaned with water afterwards.
More Attenuator Details - The attenuator shaft was a .375" diameter fiber rod that apparently had broken and was repaired using a .25" metal shaft that was glued with epoxy to the remains of the original shaft. I made a replacement shaft out of .375" diameter hard rubber. This allowed using the correct GR knob. Reassembly required installing the repaired NiCr resistors back into the housing and then testing that the resistance network was correct. Next the contact arm and shaft were adjusted so that the contact arm would be in the center of the segment when the detent was centered. Again, testing the resistance was done to assure that the attenuator was working correctly. The original Navy-type coax output connector was "long-gone" and had been replaced with an SO-239 connector. I rebuilt the remains of the original Navy connector to accept a GR 874 connector. This way, any GR 874 adapter can be used. The 874 connectors date from the late-forties so, although not original to the GR 805-C, they are from the correct time period. With the attenuator ready to install, next I had to get the GR 805-C running.
Powering up the GR 805-C -  The amount of coffee that was spilled onto and into the GR 805-C was amazing. Certainly this was a frequent "accident" that happened on more than one occasion. Once everything had been cleaned of "dried coffee" (which had also gotten into the modulator circuit and the power supply) I needed to go over all of the wiring to check for broken connections (two were found.) After that, I needed to check all of the filter capacitors. The power supply pi-filter uses oil-filled paper dielectric capacitors which were tested for shorts and for value with both caps meeting spec. The remaining filters were all electrolytic types. These had to be tested for shorts and then reformed. I used a variable power supply with current meter to check that the capacitors would charge and wouldn't draw excessive current at voltage. Surprisingly, all the electrolytics passed and were reformed. Next, I applied 115vac with the 5U5G removed. This tests all of the low voltage circuitry. I also checked the HVAC at the 5U4 socket and found it present. The 5U4 was then installed and the GR 805-C slowly brought up using a variac. B+ was checked and found to be within specifications. >>> >>>  With the B+ functional, the GR 805-C was beginning to "come to life." Although the carrier level meter didn't indicate any output, the oscilloscope on the output section showed a nice sine wave. There was no signal appearing at the input to the attenuator however. The problem was caused by a broken cable connection from the feed-thru capacitor that couples the output section to the attenuator input. I still had no carrier level meter function (connected to the attenuator input) but this was easily solved when I noticed that I had forgotten to install the 6AL5 tube for the VTVM function. A 6AL5 got the carrier level meter working. Now, even though there was signal going into the attenuator, there was no output. This was because I had not soldered the rotating link inside the attenuator that connected the network to the output series load. As long as I was going into the attenuator again, I figured I should probably clean the contact segments and readjust the rotating contact position again (just to be sure.). Re-installation of the attenuator had the output now completely adjustable with the combination of the output level control and the attenuator divider position.

A check of the calibration found that the frequency was fairly close but certainly requiring some adjustment to be within specifications.

Cabinet Front Panel Trim-Piece - When purchased this GR 805-C was missing the front panel trim-piece. On early versions, which SN:1285 is, a black wrinkle-finish metal angle was installed around the perimeter of the cabinet. This piece extended down about 3/4" over the edge of the front panel to give the panel the appearance of having a frame. I suspect that the original was one-piece in the form of a "ring" that was placed on the panel and mounted as the panel screws were installed. I didn't have a method to either bend or tig-weld the pieces to form a one-piece trim ring, so each side is a separate piece held in-place with the front panel screws. The pieces are made from 3/4" x 3/4" angle aluminum cut at 45 degree angles at each end to fit together and appear continuous when mounted. Each piece was painted gloss black on the inside wall and black wrinkle finish on the outer wall. The trim pieces and the 805-C panel are mounted to the cabinet using black finish machine screws. Completed GR-805-C - October 30, 2016 - Although I had the 805-C electronically complete and operational, it took several months to make the trim pieces to finish off the panel and cabinet. Actually, making the pieces only took a few hours and another hour to paint them. I guess I was delaying making the trim pieces because I thought I might be able to build a "one-piece" trim ring instead. After awhile I decided to just finish this project.

The GR-805-C is a huge piece of equipment. It's performance is at the top of what would be expected for the late-1940s. I certainly could use the generator for alignments but I'm already set up for using the HP-606B (also an excellent generator.) My thought is to set up an all-General Radio test bench with all GR equipment from the late-forties. Maybe that will be the next project.


General Radio Company

Type No. 1001-A  Standard Signal Generator

The GR 1001-A Standard Signal Generator is a laboratory quality RF signal generator that provided the user with frequency coverage from 5kc up to 50mc in eight direct-reading tuning ranges. The tuning scales are either silver for 5.0 to 15.0 or black for 1.5 to 5.0 or the multiples necessary for 5kc up to 50mc readout (although the 50mc range has its own scale.) The vernier knob also has a scale that reads to 0.1% per division for a frequency change from a known tuned frequency. Accuracy of the tuning dial readout is rated at +/-1%. The carrier output voltage is measured with the VTVM (the panel meter) and can be set to a measured level and then the calibrated attenuator used to set the actual output voltage. Accuracy of the measured output levels vary with frequency but are generally around 5%. A separate output stage is grid modulated by the internal 400~ oscillator or an external modulator can be used. Modulation levels up to 80% can be achieved without distortion. Modulation levels can also be read on the VTVM panel meter.

Tubes used are (1) 6C4, (1) 6L6, (1) 6AL5, (1) 6SN7, (1) 5Y3GT and (1) OC3/VR105. The output terminals are GR 874 coaxial type that can accept many types of 874 adapters (an 874 to BNC is installed.) The GR 1001-A usually came with cables, power cord, 50 Z ohm terminator, 874 adapters and a TO-44 adjustment tool. The case is welded aluminum and there is a storage compartment located on the top of the cabinet for cables and accessories. This relatively small Signal Generator weighed in at 54 pounds and had a selling price in 1951 of $675. By 1959 the selling price had risen to $860. The 1001-A was produced from about 1950 up to the mid-1960s.


Hewlett-Packard Company

 Model 606A and 606B RF Signal Generator


The HP 606 generators were laboratory-quality RF Signal Generators that were the industry standard from the late-1950s up to around 1970. Frequency coverage is from 50kc up to 65mc. Early models are basically all-vacuum tube design with voltage regulation provided by strings of 12B4 and 6AW8 tubes. The early models tend to run very hot due to the vacuum tube regulator circuits. Later versions (606B) redesigned the power supplies to be all solid-state which reduced the heat considerably. 19 tubes are used in the 606A and 11 tubes are used in the 606B. Although the 606 generators have a built-in crystal calibrator, the 606B added a fixed-RF output level specifically to drive digital frequency counters and also added a Delta Frequency adjustment that allowed the user to add or subtract slightly from the set frequency. The RF box is fully shielded and requires a special 11" long Allen-head wrench to remove four Allen head screws that are difficult to access - the wrench is supposed to be mounted in the fahnstock clips on top of the RF box shield (but it's often missing.) Full metering of audio modulation levels and RF output is provided along with a fantastic attenuator circuit. For maximum accuracy of the metered output measurement the 606 must be operated into a 50 ohm load. For higher Z requirements (>300 Z ohms) a 50 ohm output termination should be used if you want the output meter to read accurately.

It's interesting to note that the design of the HP-606 is very similar to the General Radio 805-C. Both instruments use rotating turrets that carry the coils for each tuning range. Both instruments use separate RF Oscillator and RF Output sections and screen modulate the RF Output with a separate audio oscillator. Both have regulated power supplies and a highly accurate attenuator on the output with full measuring capabilities. However, the HP-606 was probably one third the cost of the 805-C. It certainly weights less-than half as much and is half the size.

Shown in the photo above right is the HP-606A. This 606A was destined for destruction as it was in the process of being "parted out" when I found it. A quick cash offer allowed me to pick up the pieces and take it home. After reassembly, the 606A had 12 of the 19 tubes either missing or bad, a "hamster" mod to the voltage-doubler circuit that caused the +300vdc supply to run at +420vdc, one shorted B+ bypass capacitor and broken wires to the 20mc to 60mc RF Osc coil. After these problems were repaired, a calibration was necessary. Now, this HP-606A works quite well and is set up as the Sig Gen for the shop test bench.

Shown in the photo left is the HP-606B. The control on the 606B that is located where the fuses were on the 606A is the Delta Frequency control. The additional BNC connections were for an external Frequency Control and for an external Digital Frequency Counter connection. This 606B had three bad tubes and a bad Delta Freq board requiring a new 24 volt Zener diode and two new 1% resistors to get it operational. Re-calibration was then necessary because the 606B had been aligned when the Delta Freq Bd. was non-operational. I use this HP 606B as the Sig Gen for the house-upstairs test bench. This 606B came from NU6AM.


Frequency Measuring Instruments

Heterodyne Frequency Meters
US Navy - LM Series      US Army Signal Corps - BC-221 Series,

Heterodyne Frequency Meters provided a method of accurately measuring either a transmitted frequency or a received frequency of operating radio equipment. All receiver dials, prior to WWII, were vague in accuracy and didn't provide a precise readout of where exactly in the RF spectrum the receiver was tuned. The heterodyne frequency meter used a tunable oscillator to produce a frequency-accurate signal that could be "tuned" to the receiver's tuned frequency thus providing a heterodyne that provided the operator an accurate measurement of the receiver tuned frequency. All USN LM freq-meters provide an option of either a CW signal or a modulated (400Hz) output (for "MCW" receivers.)  U.S. Army Signal Corps models only provide CW output.

To measure a transmitter's output frequency required the user to put on the headset of the Freq-Meter (the headset must be plugged in to power up the BC-221 series.) The transmitter frequency is then tuned-in with the Freq-Meter acting as a receiver and, as the transmitter frequency is tuned-in, a heterodyne is heard in the headset. Zero-beat will be the transmitter frequency (or a harmonic there of.) All Freq-Meters will have a calibration book that is for the particular unit as all tuning dials are a micrometer type device in order to provide the necessary accuracy. Specific calibration frequencies are shown in the book that allow tuning to the built-in 1000kc crystal calibrator which then, using the "Corrector" control, allows the user to set-up for maximum accuracy.

Modern digital frequency counters have replaced the old Freq-Meter (as has synthesized tuning on transmitters and receivers) providing extremely accurate read-outs. However, it's fun to go through the methodology of using a Freq-Meter and get a feel for what was the "standard" for accurate frequency measurement - pre-digital frequency counters. You might be surprised at just how accurate the old BC-221 or Navy LMs are (with careful set-up, better than 1.0kc accuracy is normal.)

photo right: USN LM-18 and power supply. The power supply uses a type-84 rectifier tube and oil-filled paper capacitors as filters. The switches COMP 1 and COMP 2 allow the user to set the AC operating voltage with both switches up if AC is 110vac or less, switch 1 down and 2 up if AC is 120vac or higher and 1 up and 2 down is AC is between 110vac and 120vac. The interconnecting cable uses five-pin connectors although only four pins are used. The stock cable was nine feet long. The AC power connector is the same style connector but only three pins are provided. The LM-18 uses three tubes, a 77 heterodyne oscillator, a 6A7 crystal oscillator-detector and a 76 modulator-audio amplifier. The calibration book has metal covers and slides into a holder below the LM. Both the LM and the PS have shock-mount bases. Behind the metal protection dome on the power supply is a 120vac 6W pilot lamp (actually, an indicator lamp.) When original and complete, the serial numbers on the LM, the calibration book and the PS will all match. This LM-18, calibration book and PS is SN 222.

The upper left-most photo shows the USN LM-21 with its companion AC power supply. This unit was rebuilt at the Mare Island Naval Shipyard in the 1960s. It is complete with its original cables (not shown in photo.) Note the vernier "arm" on the CORRECTOR control. This modification is actually listed in the original Mare Island rework papers that came with this LM. Came from KG7DVA

The photo upper middle shows the US Army Signal Corps BC-221-J built by Zenith Radio Corp. during WWII. Like many BC-221s this unit has an added "homebrew" AC power supply in the battery storage area. The red pilot lamp is not original. Note on the BC versions - no MODULATION option. The BC-221-J came from KØDWC.

Shown in the photo upper right is the US Army Signal Corps BC-221-AK built by Philco. This unit is installed in the olive drab painted wooden box with canvas covers. The Antenna and Ground connections were placed on the front panel on these versions. Also, the controls are relocated on the panel with the Crystal and Freq Band controls slightly changed in their functions.  From KØDWC

The photo left shows the "official" AC power supply available (with regulated B+) for the BC-221 designated the RA-133-A. The power supply will fit into the battery compartment although some of the battery retainers might have to be removed. A short cable connects to the BC-221 A+, B+ and A-/B- terminals in the battery compartment  The AC power cable has an in-line switch and pilot lamp. All BC-221s were originally battery operated because they were used in the field. Since the RA-133-A is fairly hard to find, many BC-221s have had "homebrew" power supplies installed into the battery pack area of the unit. Most of these types of supplies don't have regulated B+.


U.S. Navy  -  Model LR-1

Combined Heterodyne Frequency Meter and
Crystal Controlled Calibrator Equipment

General Radio Company

The General Radio LR-1 is the "Rolls-Royce" of  Frequency Meters. With 21 tubes and weighing in at around 120 lbs, just in shear size, it dominates any radio landscape it inhabits. The LR-1 has just about everything GR could think of to put into a single box, albeit a very large box measuring in at 23" H x 18" W x 17.5" D. The circuit allowed for extremely accurate frequency measurement, whether measuring an incoming RF signal (transmitter) or determining a correct frequency for radio reception.

GR provided a very rapid and extremely easy method to measure frequency that allowed the user to just "dial in" the Heterodyne Frequency Meter (HFM) and directly read the frequency on the tuning dial scale. GR also provided a more thorough and extremely accurate method of measurement that could be used when needed. This accurate method used a crystal-controlled 100kc calibration oscillator that provided either a 10kc or 20kc signals by way of multivibrator circuits to heterodyne with the HFM's output that would be tuned to the nearest calibration point that was lower than the frequency to be measured. Then, using the Interpolator, these heterodyne beat notes could be measured with the large "arced" meter at the center-top of the panel (the meter is calibrated in kilocycles.) With internal filtering, each 10kc beat note would only actually respond up to around 5kc before the next heterodyne beat note would begin to "tune in." Each 5kc frequency change would produce either an increasing frequency or decreasing frequency as each heterodyne beat note was tuned through. The Interpolator was calibrated to measure the frequency of the beat notes and then drive the meter to the correct frequency indication (since a lower calibration "set" frequency was used, that provided an increasing frequency beat note that then caused an increasing meter movement.) The frequency indicated on the meter in kilocycles would then have to be added to the frequency that the HFM dial was set to. For example, if the HFM dial was set to a calibration point of 10,510kc (heterodyne heard in the 'phones) then the HFM dial tuned until the heterodyne is heard in the receiver's output, then, if the Interpolator Meter indicated 3.6kc the measured frequency would be 10,513.60kc. This method allowed for extremely accurate measurements since each calibration point and associated beat note was <5kc from the frequency that the HFM was set to. Above 15mc, 20kc is used for the calibration frequency.

Other circuits provided a Detector-Audio Amplifier to drive headsets with either local or remote outputs available. The Detector-Audio Amplifier also provided sufficient drive for the Interpolator. Tubes used are nine type 76 tubes, one type 75, one type 6SK7, two type 6C6, two type 884, four VR-105, one type 83 and one type 84 are used in the LR-1. The LR-1 operates on 115vac.

The serial number on the LR-1 shown in the photo above-right and all chassis photos is 1081. The contract date shown on the data plate is 7 April 1941 which is actually before the US became involved in WWII. However, the USN "acceptance tag" date is 1-6-44 which indicates that the LR-1s were certainly built during WWII. These "over-the-top" HFMs were typically set-up onboard ship along with the USN Type RBA longwave receivers and the RBB and RBC medium and shortwave receivers. They were also used at shore stations where accurate frequency measurements were necessary for both transmitters and receivers.

photo left: This is the right side of the LR-1 showing the detector and audio output circuits. The tubes (l-r) are 75, 76, 6C6 and 76. 

photo right: This is the left side of the LR-1 showing on the upper chassis the crystal oscillator and calibrator section. The tubes (l-r) are 6C6, 76, 76, 76 and 76. The lower chassis contains some of the Interpolator circuitry. The tubes (l-r) are 6Z4/84, 884, 884 and 76. The 6Z4/84 is shielded. The fuse board to the front of the chassis has provision for fusing the B+ plate (top) at .25A, provision for two spare fuses in the vertical position are below the B+ plate fuse. The next fuse is for the tube heaters at 5A. The lower fuse is 2A and allows selecting the operating AC voltage with the top position being 120vac followed by 115vac in the center and 110vac for the lower position. Note the Jones plug at the bottom. This is how AC is routed to the LR-1. The cabinet has extensive filtering of the AC input.

photo left: This photo is of the rear of the LR-1 and shows the massive wire wound series load resistors for the B+ to the various circuits in the unit. To the right of the resistors is the heated wooden box that contains the crystal for the oscillator. Below the top chassis is the huge tuning condenser of which the rear casting can be seen. The lower chassis contains the power supply showing the power transformer and one of the filter chokes. Note the several oil-filled filter condensers behind the choke.

photo right:
This photo is of the top of the LR-1 and shows the shielded tuning inductances to the upper left. The center chassis contains the type 83 power supply rectifier tube located under the tube shield.

photo above - (cont.) The four tubes just in front of the wooden box are VR-105 regulator tubes that are part of the Interpolator. In the lower chassis is a 6SK7 tube which is for the HFM. To the left of the 83 tube shield are two type 76 tubes that are also part of the Interpolator. Note the thermometer that is mounted to the top of the wooden box heater for the Calibration Oscillator. This provided a way to check if the crystal was at the proper operating temperature.
Using the LR-1 - Since all of the filter capacitors in the LR-1 power supply are oil-filled paper dielectric units, reliability is quite good. Add to that quality components and construction, I felt that it was likely that with a little careful pre-testing and a few other checks, this LR-1 might function on all original parts. My initial test had the HFM working great and the output from the RF Output to a short cable provided a very strong, clean signal to the R-390 receiver. Accuracy of the HFM by itself was impressive but resolution of the tuning dial didn't allow "to the kilocycle" accuracy. Next, was switching on the Calibration Oscillator and plugging in a set of 600Z ohm phones. Now, I could calibrate the HFM and then use the Interpolator to accurately measure where the R-390 was tuned. I used the onboard Crystal Calibrator on the R-390 to set it to 1808.0kc with an accuracy that is as close as the R-390's calibrator allowed. I next set the LR-1 to 10kc calibration and switched to Upper on the Interpolator. I cal'd the LR-1 to its calibration point at 1800kc. This is heard as a heterodyne in the phones. I next tuned the HFM until I heard the signal in the R-390 and then tuned the HFM to zero beat in the R-390. The Interpolator meter increased its reading until at zero beat in the R-390 the meter read 7.4kc (on the red scale since the range for 1.8mc is highlighted in red.) This was then added to the 1800kc calibration point and the total was 1807.4kc. About 600 hz discrepancy with only a 10 minute warm up on both pieces of gear. Pretty close, in fact, so close as to ask which is more accurate? The R-390 calibrator or the LR-1 or my setting up of either piece of equipment. Impressive performance from a 70+ year old piece of test gear.

Since the initial testing, I've had to replace one of the oil-filled caps. I rebuilt it so it has the same original bath tub housing but it has new caps inside. Even 70+ year old, high-quality, military components sometimes "give up." Performance is even better now with increased stability and a cleaner, quieter audio output in the phones. Before there was some "crackle" in the phones - nothing serious, but noticeable - now that is entirely gone with just a clean heterodyne present. Probably the bad cap was "breaking down" before it entirely shorted out. The LR-1 is going to be set up with the RBB and RBC receivers and hopefully will look something like these vintage photos below.

Vintage Photos of the LR-1 in Use

Shown in the photo to the lower left is a USN Radioman setting up the USN station W1RRF. This photo was taken in 1951 when Navy Reservists taught radio and radio operating. The station W1RRF is being set up to transmit code practice, note the tape sender unit that the radioman is adjusting. The LR-1 was used to set the transmitter to the exact frequency. (Photo from: Radio & Television News - May 1951.)

Shown in the photo to the top right is an LR-1 set up with the USN RBA receiver to the right of the LR-1 followed by the RBB and RBC receivers. This photo is of the radio room onboard the USS Mugford taken in 1946. Note that the cable connection to the LR-1 is to the RF Output and therefore it is being used to set exact tuned frequencies on the receiver. (photo from N6MKC)

The LR-1 was also used in USN land-based intercept and monitoring stations where it was necessary to know the exact frequency that the receiver was tuned to. The photo lower right is a receiving post equipped with National RAO receivers. Though this photo isn't dated, it probably is from late-WWII.

The RAO's tuning dial was vague at best and certainly couldn't resolve the tuned frequency to 1.0kc. At first glance the LR-1 doesn't appear to be connected up one can see there is an adapter installed on the RF Output connector and a wire strung across the top of the LR-1. This provided a strong enough calibration signal for any of the RAO receivers.  (photo from: BAMA Edebris - National RBH/NC-156)

Although today a complete LR-1 is a rarity, one can see from the photos they were used extensively by the USN. Undoubtedly the size and weigh of the LR-1 made it difficult to justify keeping them around when the LM HFM was almost as accurate and only weighed about ten pounds. Many LR-1s were "parted out" and today the arced Interpolator meter shows up fairly often since it has "General Radio" on it.


General Radio Company

Type 535-A  Frequency Meter- Monitor


This GR Frequency Meter is very early and is battery operated. Frequency readout is an arbitrary scale the must be correlated to the graph on top of the case. Note the magnifying glass for accurate readout of the dial (even though the scale is 0 to 100.) Operation is typical for heterodyne frequency meter use (see Heterodyne Frequency Meters in a section above.) The 535-A could also monitor signals if necessary although being a battery operated instrument this couldn't be of a long duration. The 535-A dates from the early thirties. In fact, the paper frequency calibration chart on top of the box is dated 12-12-1933. This GR instrument came from W6MIT who found it at a ham swap meet in Sacramento, California.


Lampkin Laboratories, Inc.

 Type 105-B Deviation Frequency Meter

During the fifties and sixties, the Lampkin Type 105-B Deviation Frequency Meter was the industry standard for accurate frequency measurement of transmitters and receivers. Each instrument had its own specific calibration chart that was correlated to ambient temperature and each instrument had its own built-in thermometer. If you weren't into math calculations then you'd never make it with a Lampkin. Since each instrument only tuned a small range of frequency, all measurements were of related harmonics. You had to use the specific unit's tuning chart, add other correlations and factor in corrections for temperature to actually get to the correct measured frequency. When everything was correct (including your math,) accuracy was unbeatable. Of course, today's digital frequency counters are much more accurate and read frequency out directly (little, if any, math required.)

The Lampkin 105-B shown belonged to Al Chin who used to maintain the police radio equipment for the city of Reno. I do have all of the original matching charts and original manual for this particular 105-B and I have used it. Remember, back when the 105-B was commonly used by radio techs, there were no digital calculators. All math was either by slide rule or long hand.



Western Electric

72A Audio Frequency Meter


Although this is a frequency meter and it does have an internal oscillator for determining an unknown frequency that's were the similarity to the typical Heterodyne Frequency Meter ends. The measuring range of the 72A is from 100hz up to 4000hz and with interpretation of various lissajous patterns or wave comparisons the range can be extended from 10hz to 40khz. The knobs at the bottom of the panel select the oscillator frequency by switches and the vernier control (tenths of a cycle.) Right to left each switch is units, tens, hundreds and thousands. With the unknown signal input to the dual jacks in the upper right area of the panel SEARCH is selected. The 'scope will now show the input signal on the vertical plates and the tunable oscillator on the horizontal plates. The FREQUENCY knobs are adjusted until a diagonal line is showing on the scope. This would be equal frequencies and in phase. A circle pattern would be equal frequencies and 180 degrees out of phase. The internal oscillator can be set to MEAS and then it's output is fixed frequency, crystal controlled at 4000 hz. This would be used for comparison of the two waveforms by looking at various ratios of the vertical versus horizontal. Many ratio patterns are shown in the manual. The internal oscillator can also be used to drive an external device or measuring instrument.

The 72A was used for calibration of the TD-2 radio system and for various other purposes in the Bell System. This 72A was used at Winnemucca Mountain near Elko, Nevada at the Bell System of Nevada (aka Nevada Bell.)


Absorption Wavemeters

James Millen Manufacturing Co., Inc.

James Millen was a Stevens Institute graduate that made his fame as Chief Engineer and General Manager of National Company, Inc. in Malden, Massachusetts. Millen left National in 1939 and formed his own company, James Millen Mfg. Co., Inc. Millen produced several "ham" products including transmitters, some test equipment and lots of high-quality small parts. However, he didn't make much money off of his "ham gear." Millen made most of his profits as a "contractor" building oscilloscopes for RCA and two-way radios for GE.

One of Millen's early products were these easy-to-use, calibrated wavemeters in a set of four. They were especially designed for hams, who at the time, were active "homebrewers." The absorption wavemeters allowed hams to directly measure an oscillating circuit by watching the tuned stage's grid current. As the wavemeter was tuned (and held near the oscillating circuit) the grid current of the circuit under test would drop slightly at resonance due to "absorption" of the oscillator's em field by the resonant circuit of the wave meter. This frequency was then read on the scale of the wave meter. Millen's Wavemeters were extremely popular and are found everywhere. At a time when the radio amateur "homebrewed" most of his equipment, a wavemeter set would be useful in determining if the designed circuit was actually going to resonate at the intended frequency.

In the photo above note that the wavemeters in the smaller case have a different construction and brown knobs. These are the older pre-WWII versions that have a frequency range of 3.5mc up to 150mc. These units also have smaller air variables and no plastic covers. The post-war units (with the plastic coil cover) have a tuning range from 1.5mc up to 30mc.


General Radio Company

Type 758-A Wavemeter


The General Radio Company Type 758-A Absorption Wavemeter is a UHF device with a frequency coverage of 55mc up to 400mc. The 758-A has a built-in small wattage lamp that will illuminate as sufficient energy is coupled into the device at resonance. If the 758-A can be coupled to the LC under test then it will operate as a typical absorption wavemeter and the operator can watch the grid current meter of the circuit under test for a "dip" at resonance. But, if the operator is testing an LC circuit where a meter is not present then the lamp will serve to indicate resonant frequency provided there is enough power in the field to illuminate the lamp. Some 758-A units were equipped with a neon lamp that would illuminate with less energy than an incandescent lamp. The lamp socket is the standard screw base (pilot lamp size.)

At first glance an inductance seems to be missing from the 758-A however close inspection will reveal that the inductance is recessed in the ceramic ring behind the tuning air variable capacitor. Actually, the lamp assembly contacts the inductor and "tunes" the inductance as the air variable is rotated thus providing a changing L and C as the wavemeter is tuned. This increased the tuning range of the wavemeter allowing coverage from 55mc up to 400mc with just one device. The LC and the lamp are protected by a plastic cover.

The 758-A was supposedly used during WWII for radar testing but GR still had them for sale in the early fifties for $40.


Grid Dip Oscillators

Measurements Corp. - "Megacycle Meter" Model 59


Where absorption wave meters required the oscillating circuit under test to be operational, a Grid Dip Oscillator can be used to test a non-active resonant circuit and show the user the resonant frequency. Using the same principal (but in reverse) the Grid Dip Oscillator (GDO) is a tunable oscillator and if the oscillating tank coil is placed close to a "resonant circuit" then, as the GDO is tuned, when it passes through the resonant frequency of the circuit being tested, the meter of the GDO will "dip" (show a low reading) due to the absorption of the tank coil field by the LC circuit under test. The LC tuned circuit under test can be just a coil and capacitor or it can be an antenna - any resonant LC circuit can be tested and it's resonant frequency measured. Additionally, with a little math an unknown capacitance or inductance can be calculated by knowing one of the values and the resonant frequency of the pair. This was a great help for hams that were homebrewing just about anything from transmitters to antennas. The GDO was very popular and available as very reasonably priced units provided by Heathkit and EICO up to the ultimate GDO - the Measurements Corporation "Megacycle Meter" Model 59. James Millen Mfg. also built an excellent, easy-to-use GDO that was very popular and is still considered "one of the best."

Shown in the photos above right and below is the ultimate set-up for the "Megacycle Meter" Model 59. This kit contains all three tuning heads, the standard head (the round one) and the LF head (hex head upper right) and the VHF head (hex head upper left.) This set up allowed using the Megacycle Meter from 100kc up to 300mc. The standard coil set is behind the GDO itself in the small pocket area. The LF coils are to the lower right. The VHF head is only one band so the coil is built-in. Also, manuals for each head and the Megacycle Meter. Even green, Measurements "Memo" books were included with the deluxe padded, suitcase-sized carrying case. I got this Megacycle Meter kit from KB6SCO.

Other Grid Dip Meters - Shown in the photo below are some other Grid Dip Meters. On the left is the Heathkit Model GD-1B from the late-1950s. This particular example has been modified to have a tuning knob on top in addition to the side thumb wheel tuner. The middle GDO is the EICO Model 710 dating from the late-1960s. Again, the EICO has been modified with a potentiometer and knob to replace the thumb wheel "Sensitivity" control. Note that the EICO also has the more modern, entirely polystyrene meter case that became popular in the sixties. The GDO on the right is Barker & Williamson (B&W) Model "600" Dip Meter. The B&W GDO has the meter mounted on the side of the unit which must have made tuning and meter reading somewhat difficult. The color coded dial scales matches the color coded plug-in coils. The B&W GDO is missing the power cord.

Since most of these lower priced GDOs were purchased by hams "to be used," it's common to find these types of instruments modified or repaired in a crude manner. Although these types of GDOs will function and perform adequately it's obvious to keep the selling price low enough to appeal to hams the quality is just not present when compared to the Measurements Megacycle Meter or compared with the Millen Grid Dip Meter (described in the next section below.)


James Millen Manufacturing Co., Inc.

Grid Dip Meter  Type No. 90651, 90661 & 90662

James Millen was always looking at ham radio designs that would show up in magazines like QST or CQ. The famous Millen "Swing Arm" VFO actually first appeared in a 1941 QST article. Millen obtained the rights to build what was essentially a copy of the VFO that appeared in QST. The same was true of one of Millen's most popular products, the Millen Grid Dip Meter. The basic design appeared in a CQ magazine article and Millen obtained the rights to build the design in 1949. The basic Millen GDO is identified as Type No. 90651. With carrying case and all the HF coils it was Type No. 90661 and the Industrial GDO with metal carrying case, all eleven coil sets (HF and LF coils) and a three-wire power cord was Type No. 90662.

Good mechanical design and quality parts set the Millen GDO apart from many of the less-expensive makes that were available for hams. It's easy to use and when in good condition functions really quite well. The power supply is built-in and uses a selenium rectifier for the B+ which runs around +100vdc. The Oscillator tube is a 9002 triode with a 6.3vac filament supplied by the power transformer. Toggle switches control Filament voltage for warm-up periods and a Plate switch for operation in most modes. Plug-in coils were available for 1.7mc to 300mc and if the low frequency coils were also purchased the coverage was from 220kc up to 300mc. The standard Type No. 90651 and 90661 used a two conductor "zip" power cord but the so-called "Industrial" model 90662 used a three conductor power cord with a ground connection provided at the power plug.

The Millen GDO could be used as a GDO (as described above for the Measurements 59) but it can also function as an "easy to use" signal generator. By utilizing the phone jack and inserting a set of phones, the GDO can operate as a heterodyne frequency meter. It can also be used as an absorption frequency meter by leaving the Plate voltage off. There are many uses for a GDO which was why they were so popular.

Shown in the photos top and lower left is the Millen Type No. 90662 "Industrial Grid Dip Meter." This version included all eleven coil sets available providing frequency coverage from 220kc up to 300mc. Shown with the GDO in the upper photo is one of the HF plug in coils (small one with two pins) and one of the LF plug in coils (larger with three pins.) The GDO itself has a heavy three conductor power cord with ground. The metal carrying case has fitted storage places for all of the coils, the GDO, the power cord and the manual. The 90662 shown is an early version with the black "leatherette" finish paint. Later 90662 versions were painted smooth gray finish.

Sometime in the past, this Millen GDO was probably dropped. The meter wouldn't mechanically zero. It was going to be necessary to disassemble the meter itself to correct the problem. Unfortunately, Millen didn't make the GDO disassembly for meter removal easy. It's necessary to remove the top metal plate that has the nomenclature on it in order to access the screws that mount the meter. This then requires that the toggle switches and phone jack be removed. So far, fairly easy. The difficult part is removing the power cable. Most of the Millen GDO power cords are so stiff after years of drying out, they can't even be flexed without the insulation breaking off in pieces. The power cable strain relief also dries up to be a non-flexible rubber mass that can't be removed without cutting. This means that most functioning Millen GDOs will have replacement power cables. So, now with the power cable removed, complete disassembly and accessing the meter was easy. The meter problem was the top suspension spring had slipped (probably from being dropped) and needed to be readjusted. Afterwards, the meter mechanically zeroed correctly.

The meter was also checked for accuracy and was found to be within 3% tolerance, which is normal. As to the rest of the GDO, I reformed the electrolytic capacitors, tested the 9002 tube and then tested the GDO operation by applying power using clip-leads. The GDO functioned fine, so it was reassembled and retested. I found that the frequency drum dial is fairly accurate. I tuned a receiver to 3.800mc and then tuned the GDO to zero beat. The drum dial read 3.8mc, so accuracy seemed pretty good by the 1950s analog dial standards. However, the dial scales are vague and generally only have index marks at 100kc divisions. This was all accurate enough back in the 1950s and 1960s when many receiver dials weren't any more accurate. If more accuracy is necessary in actual use, the Millen GDO puts out a strong signal that is easy to monitor with a digital frequency counter.

The Millen GDO with the seven HF coils was identified as Type No. 90651 and when this GDO was purchased a carrying case was not included. Shown in the photo to the right is the Type No.90651 in its original cardboard box. A metal stand was included for coil storage.

Many Millen GDOs were sold with the HF coil set and a carrying case. These were identified as the Type No. 90661. Early versions supplied wooden boxes as carrying cases. Later, molded plastic was used for the carrying case. The Millen 90661 GDO sold for around $55 in 1950.

All Millen GDOs are identified as Type No. 90651 on the front panel regardless of the accessory package that may have been sold as Type No. 90651, 90661 or 90662.


The James Knight Company

Frequency Standard  FS-344

Frequency Standards were crystal controlled oscillators that could be coupled to a receiver input and provide an accurate indicator of specific frequencies or harmonics. Usually the Frequency Standard had two crystals, a 1000kc crystal and a 100kc crystal to provide accurate 1.00mc signals (and harmonics) and 100kc signals for more accurate indicators of where the receiver was actually tuned. Many Frequency Standards also had a multi-vibrator circuit that would divide the 100kc oscillator by ten and provide a 10kc "marker" although many of these 10kc frequencies are not as accurate as the crystal controlled oscillators. Sometimes, lower priced Frequency Standards used only one 1000kc crystal and then used multi-vibrators to obtain the 100kc and 10kc markers but these were not accurate except on the 1000kc position.

Popular manufacturers were Hallicrafters (HT-7,) Meissner (a nice unit) and sometimes the Measurements Company version shows up. The James Knight Co. was best known for manufacturing crystals so it seems logical that they should offer a Frequency Standard. The FS-344 is a good unit that uses two crystals and a multi-vibrator circuit to achieve its marker frequencies.

The FS-344 came from my old Virginia City friend, Jeremy Babb.


Audio Frequency Oscillators

Hewlett-Packard Company

 200 Series Audio Oscillator

William Hewlett and David Packard were both graduates of  Stanford University in 1934 and had started their own business by 1938. Hewlett-Packard's first product was an audio oscillator that used a variable RC to tune the frequency that resulted in a stable, low distortion signal. In addition, a small incandescent lamp was used in the oscillator feedback loop. This is the well-known part of many HP audio oscillator circuits - an incandescent lamp in conjunction with a Wien Bridge Oscillator. That circuit was part of William Hewlett's Master's thesis. The incandescent lamp provides a positive temperature coefficient resistance in the feedback loop (along with unity gain) resulting in an AVC action that provides constant gain and low distortion. The Wien Bridge Oscillator depends on resistance and variable capacitance (RC) to create oscillation rather than the then typical inductance-capacitance (LC) oscillator. The Model 200A was the initial product (the first few 200As were built in a garage behind Dave Packard's house) with a selling price of $54.40. The frequency range of the 200A was 35hz to 35khz.

Around 1939, Hewlett-Packard was approached by an engineer from Walt Disney who was working on Disney's "Fantasia." The production was going to use eight-channel stereo for the audio reproduction that featured the Philadelphia Symphony Orchestra conducted by Leopold Stokowski. Only a select group of theaters were going to be set-up to reproduce the multi-channel stereo recording and Disney was going to need audio oscillators for testing and set-up. The HP 200A wasn't exactly what was needed though. Disney requested the 200A frequency range be modified to 20hz to 20khz. This audio oscillator became the HP 200B. Eight 200Bs were ordered by Disney at a price of $74.40 each.

Shown in the photo above is the HP 200B Audio Oscillator. The 200B was produced from 1939 up thru WWII and was available for a considerable time after WWII. By the early 1950s, the price had increased to $120. As mentioned, the frequency range covered 20hz to 20khz and the output was 1 watt into a 500 ohm resistive load (about 25 volts rms open-circuit.) To keep output distortion low, HP recommended that the 200B be operated with the VOL (AMPL on some models) control above 80 and that an attenuator be used to reduce the output level. This also reduced the amount of hum in relation to output level - at low amplitude levels the hum could add a small amount of distortion to the output but with the 200B near full output the hum was virtually non-existent. The 200B uses five tubes, (1) 6J7, (1) 6F6, (1) 6F5, (1) 6V6 and (1) 5W4 or 5Y3GT. Like many 200Bs, this one has been repainted a color very different than original, which was HP Gray. Also, note the company asset number engraved onto the plastic ID tag. The serial number is not on the plastic tag but is located on a paper label located on the transformer metal shield. 

200C Audio Oscillator

During WWII, the 200C was introduced. It increased the frequency range up to 200khz, or 20hz to 200khz. Hewlett-Packard supplied the military with 200C Audio Oscillators for various uses that required the increased frequency range. Some were used in analyzing intercepted signals while others were used for various test purposes.

During the later part of WWII, the USAAF was developing a remote control system for piloting bombers. The system used 10 different transmitted tones that were filtered at the airplane receiver to discriminate them into functions that would actuate various controls of the airplane. The manual specifies that the HP-200C was to be used in the test and calibration of this system. When the HP 200C was part of the test equipment required, it was tagged as the TS-382/U. The later version of the TS-382 with suffixes from A through F is a different model HP Audio Oscillator but the initial version (no suffix) is the HP-200C.

Shown in the photo to the left is the TS-382/U. Shown in the photo below-left is the chassis of the TS-382/U showing that it is indeed a Hewlett-Packard 200C. The differences for the TS-382/U are a power cord that exits out the front panel, a "bull's eye" dial pointer indicator (rather than the original plastic pointer) and carrying handles on the sides of the case.  Frequency range is from 20 hz up to 200 khz.

photo left:  Chassis of the TS-382/U. Note on the chassis the small incandescent lamp that is to the right of the first section of the tuning condenser.

The photo below is a close-up of the data tag showing that the serial number is 133 and that the contract date is 1945. This is shown by the suffix DE in which "D" = 4 and "E" = 5. Inside the unit the various panels are ink-stamped 1038-DAY-45.


200D Audio Oscillator

At the end of WWII, HP introduced the 200D Audio Oscillator. The main changes were to the frequencies generated. The 200D frequency range was from 7hz up to 70Khz. Internally, the single four-gang variable condenser was changed to dual, four-gang condensers. The oscillator circuit remained the same using the Wein Bridge with the incandescent lamp in the feedback. The frequency dial was changed to a gray color and the scale became 7 to 70. The Multiplier switch provided X1, X10, X100 and X1000 frequency ranges. The cabinet was larger than the 200B or C versions. The 200D was available from 1945 up into the 1950s.

Shown to the right is the HP 200D. This version is from 1951. The inspection tag on the frequency dial indicates "Limited Use" and refers to "NAV Service." There isn't any information as to who the end user was. The latest dates on the tag are from 1972.


Hewlett-Packard Company

202D Audio Oscillator

The 202D was introduced during WWII and produced up into the early 1950s. The 202D featured a new extended lower frequency range of 2hz with the upper range of 20Khz, which was similar to the earlier 200s, and added a range similar to the 200D but starting at 7.0Khz and extending up to 70Khz. The 2.0 to 20 scale is used with multiplier ranges of X1, X10, X100 and X1000 for 2hz to 20Khz while the scale marked "A" is used for 7.0kHz up to 70kHz. The complete frequency range of the 202D was from 2hz up to 70Khz.

Internally, the 202D uses basically the same circuitry as the 200D. Dual, four-gang, variable condensers are used in the Wien Bridge circuit with the incandescent lamp in the feedback loop. The components and the layout look very much like the 200D.

In 1952, the 200 series and the 202 series were given a "make-over" and repackaged to not look so much like WWII-vintage equipment. The HP oscillator below shows what the next-generation equipment was going to look like.

Hewlett-Packard Company

 202C Low Frequency Oscillator

The HP 200D and the HP 202D were given a packaging "make-over" in 1952. The result was the "CD" versions, the 200CD and the 202CD. The new look featured a narrower, vertical cabinet that was dominated by a large frequency dial. After the earlier versions were no longer available, HP dropped the "CD" designation and the 202CD became just the 202C. The 202C is from around 1958. This is a powerful Low Frequency Oscillator capable of driving many types of devices directly. The circuit uses the "H-P familiar" Wien Bridge Oscillator with lamps in the feedback loop. Frequency range is from a low of one hertz up to 100,000 hertz, or 1hz up to 100Khz. The output has an impedance of 600 Z ohms. This HP 202C was purchased from Bently Nevada Corporation in Minden, Nevada where it had been set up to drive a "shaker table" used to test and calibrate velocity transducers.

As will be noted in some of these descriptions, I indicate that some items were purchased from Bently Nevada Corporation. I worked for BNC in Minden, Nevada from 1973 up to 1997. I was in Manufacturing Engineering and Custom Products for a time. Also, in PC Board Test and Final Test for a while. Most of my time however was in the Factory Field Service & Repair Department. In the early days of BNC, unused, obsolete test equipment that was in the "inactive stockroom" could be purchased by employees. It required a bit of paperwork but the prices were usually cheap.


General Radio Company

Beat Frequency Audio Oscillator - Model 1304-B

This piece of laboratory test gear allowed the user to select any frequency from 20hz up to 20Khz with just the turn of one dial. No range switches or multiple scales to read - all frequencies covered in one sweep of the dial. The circuit that made this wide range possible used a variable frequency oscillator that heterodyned against a fixed frequency oscillator. Due to the "mixing" of the two frequencies a "beat frequency" was created that allowed the wide range of frequency coverage using only a single range and a single scale dial. Additional circuitry provided accurate output measurement and level control. A "Cycles Increment" control allowed adding or subtracting slightly from the set frequency. A "Frequency Range" switch allowed 20kc to be added to the output frequency which increased the tuning range up to 40kc. The output Z is 600 ohms with either balanced or unbalanced available. A precision piece of equipment that was used in the lab but also was found in radio broadcasting where a precision audio oscillator was sometimes necessary for testing and set up of the transmitter. Selling price was $625 in 1959. This 1304-B came from my old Virginia City friend, KB7VT.


General Radio Company

Type 723-C   Vacuum-Tube Fork

So, what's a "Vacuum-Tube Fork?" It's an electro-mechanical, fixed-frequency oscillator that has its frequency generation based on a precision tuning fork that is driven by a vacuum-tube circuit that places grid and plate driving-pick-up coils that surround said tuning fork. Since the frequency is determined by the physical dimensions of the tuning fork, the accuracy of the frequency produced is +/- .05%. Output Z is selectable at 50Ω, 500Ω or 5000Ω. The output can deliver about 50mw into a matched load. The VT Fork has a built-in AC power supply that uses a selenium bridge rectifier and a voltage regulator. The power supply is removable as a unit and batteries can be installed if portable field work is to be performed. The wooden front panel is removable to allow access to the power supply. Two tubes are used - one 1A5GT and one 0C3. Two types of VT Forks were available, the C version providing 1000hz output and the D version providing 400hz output. The VT Fork was used where a very accurate and stable fixed-frequency was required. This might have been as an external modulation source for an RF signal generator, or as a generator for precision measurements using various types of Z bridges or as a Tone generator for communications systems or for beacon transmitters. Original 1951 selling price was $165.

Operation: The VT-Fork is an electro-mechanical oscillator with the 1A5GT as the amplifier and the tuning fork as the feedback path. As the fork begins to vibrate it induces more voltage in the grid coils. This voltage is amplified in the tube and that increases the current thru the plate coils that are 180º out of phase with the grid coils. The push-pull affect of the coils increases the fork vibration amplitude until maximum output is attained. This takes about 30 seconds. Looking at a properly loaded output with an oscilloscope a sine wave of about 12 volts P-P is produced. With no load the output is around 50 volts P-P. The vibrating tuning fork is quite audible while the VT-Fork is in operation.


Voltage Measuring Instruments

Weston Electrical Instrument Corporation

Portable DC Volt-Ammeters

Edward Weston immigrated to the USA in 1870 and soon was involved in several ventures that included the electroplating business and later electrical lighting. He formed Weston Electrical Instrument Corporation in 1888 making it one of the early builders of meters and other electrical measuring instruments. Weston developed the first accurate portable current and voltage measuring meters and followed with watt meters. Eventually, Weston developed the first portable light meter. Weston also perfected the "standard cell" which was a 1.018638vdc mercury-cadmium battery used for calibration purposes. Daystrom bought out Weston in 1954.

Most of what we take for granted in the suspension and accuracy of the modern analog meter was developed by Edward Weston. Electrical current flow indicating devices date to the early-nineteenth century. The "galvanometer" is a current indicating "meter" that relies on current flow through a coil suspended within a magnetic field to create mechanical movement of said coil. Movement was due to the opposing magnetic fields of the moving coil field and the magnet's field. The early galvanometers worked something like a compass and had to be oriented to the earth's magnetic field in order to have the indicator return to a zero-current position. By the mid-nineteenth century, magnets had replaced the earth's magnetic field as the return force and mirrors with light sources projected a moving light-beam onto a frosted glass scale. By 1882, Jacques d'Arsonval and Marcel Deprez (working independently) had improved the galvanometer to use a permanent magnet with a rotating coil within its field. Pivot points were two small wires top and bottom that suspended the coil within the magnet's field. The d'Arsonval meter movement was the basis for the modern analog meter using a moving coil. Weston substantially improved the d'Arsonval meter by adding coiled suspension springs to mechanically return the meter needle to zero and using pivots and bearings to reduce friction. Weston also improved the linearity of the meter by reducing clearance between the moving coil and the permanent magnet. With Weston's meter design having an improved magnet, better pivot bearings and accurate linearity, the meters could now be taken into the field and didn't require delicate handling, thus they were called "portable meters."  

Early-twentieth century DC Voltmeter coils tended to have rather high current requirements and the resulting load on the circuit being measured could compromise the accuracy of the measurement. The sensitivity of the meter was specified in "ohms per volt" and the lower the "ohms" the greater the circuit loading was. Generally, 1000 ohms per volt sensitivity was typical before WWII. In such a meter with 1000 ohms per volt, using a 300vdc FS could measure 300vdc and the meter would have an input load of 300K ohms and 1 mA would flow through the meter coil. Improvements in coils and more sensitive meter movements with higher resistances used in the voltage divider circuits reduced the loading and post-WWII meters with 20,000 ohms per volt became common.

All "moving coil" analog meters use the current flowing through the coil, which is suspended in a magnetic field, to cause the coil and associated meter needle to rotate. With careful design and construction, this mechanical movement can be very linear and accurate. The point is that these meters respond to current. To measure voltage requires resistance to be added to the circuit and to then recalibrate the scale in volts. The resistance divider circuit allows a certain amount of current to flow through the meter coil (and the coil resistance is also part of the circuit) to achieve a full scale reading. The movement will be linear and therefore the scale can be produced to show voltage rather than the actual current that is causing the meter needle to move. Resistance can also be indicated by adding a small voltage from a battery and scaling resistors to have the meter's full scale indication represent the desired value in ohms. This was the basis for the Volt-Ohm Meter or VOM.

Shown in the photo above right is the Weston Model 540 DC Volt-Ammeter. This instrument was designed to set on the test bench or desk and provide the user with DC voltage measurements using three different scales - 3vdc FS, 15vdc FS and 150vdc FS. DC current could also be measured in three scales - .15 Adc FS, 1.5 Adc FS and 15 Adc FS. Meter sensitivity is 1000 ohms per volt. Terminals are binding posts. The meter can be closed up by folding the drop-down door up. Dates from the early-1930s.

Shown in the photo left is the Weston Model 430 DC Volt Meter. This instrument measures DC voltage in three scales however individual binding posts are provided rather than a switch. The three scales are 3vdc FS, 15vdc FS and 150vdc FS. Meter sensitivity is 1000 ohms per volt. This meter dates from the mid-twenties.   


Leeds & Northrup Company

Catalog No. 2420-B   Mirror Galvanometer

The basic design of the mirror galvanometer originated in 1826 by Johann Poggendorf but the improvements that made it a useable instrument came from William Thompson (Lord Kelvin) in 1858. The design has a coil with a small mirror mounted to it suspended within a magnetic field. Current flow through the coil will cause the coil and the mirror to rotate slightly. Several inches away a light source has a lens mounted in front of it and that lens focuses the light into a beam that is aimed at the mirror which reflects the beam to a semi-transparent scale some distance away. As the mirror rotates due to the current flow, the light beam is deflected and moves across the scale. Since the light beam had no mass (unlike a long indicator needle would) it could be used with large scales with tremendous resolution. These meter scales in such a system allowed incredibly small current flows to be seen since there was an apparent magnification due to projection of the light beam onto the scale. The further the distance from mirror to scale, the greater the apparent magnification and resulting sensitivity to minute current flows. Generally, the suspension of the coil-mirror was by a thin ribbon above and a coiled wire below. An early use of the mirror galvanometer was for testing underwater cables for wire telegraphy systems. It was also found that the mirror galvanometer (with minor modifications) could actually be used as the indicating device and "read" by the wire operators. Other uses were in various types of bridge resistance circuits where precision "null" indications were necessary for extreme accuracy. More modern versions of the mirror galvanometer will have the mirror-coil assembly in a small air chamber with a glass port for the light beam. This sealed housing prevents mechanical-physical interferences from deflecting the mirror during precise measurements. Most mirror galvanometers will have an external resistance across the input terminals to prevent damage to the suspension from mechanical shock or excessive inputs. The external R acts as a load on the coil and dampens its movement. Later applications use mirror galvanometers to direct laser beams for various purposes such as IC manufacturing, light shows and some types of medical instrument devices. 

This working Leeds & Northrup 2420-B shown to the right is from around 1940. The lamp operates on 6 volts. The resistor dampens the coil-mirror suspension when the galvanometer is not in use. The scale is "0" center and is +/- 20 millimeters. Generally, these instruments read in uA per mm or in uV per mm. Often used in Wheatstone bridge circuits where the galvanometer was a sensitive "null" indicator. Some mirror galvanometers have intense illumination that allows for projection of the light beam onto a large wall-mounted scale which is observed in a darkened room. The 2420-B however is intended to be used with the glass scale mounted at the front of the box.

The L&N 2420-B Mirror Galvanometer is read by observing the etched glass scale noting the 20-0-20 scale. When illuminated, a bright "disk of light" is projected onto the scale with a shadow "arrow" and "index line" within. The reading is taken at the point where the "arrow" and "index line" is on the scale.

Photo left is the interior of the 2420-B showing the horseshoe magnet, lamp and lens.
Photo middle shows the mirror-coil assembly of the galvanometer.
Photo right shows the lens and lamps assembly. 


Volt-Ohm Multimeters (aka VOM)

 Triplett Electrical Instrument Company, Simpson Electric Company


Ray Triplett started in business in 1904 in Ohio. He was formerly trained in the watch business but built a small handheld electrical meter and called his company "Pocket Meter Co." In 1907 he coined the name "Readrite." The name was used extensively as Triplett developed and in 1932, the name was changed to Triplett Electrical Instrument Company. Triplett supplied panel meters along with other types of test equipment including tube testers. Triplett was purchased by LFG/api in 1997 and in 2007 Jewell Instrument LLC bought Triplett products and is still in business in Bluffton, Ohio.

Prior to the hand-held digital multimeter, the standard for portable AC-DC voltage, current and DC resistance measurement was the VOM. These units were generally built into a fairly large bakelite case with a huge analog meter with a multitude of scales along with a multitude of selectable measurement ranges. In order to provide a method of measuring resistance a DC voltage was necessary and usually this voltage was provided by a small battery. Some models used two batteries due to the potential necessary to measure high resistances. Usually a 1.5vdc D cell was used for low resistances and a 30vdc "instrument" battery was used for the high resistances. Early DC voltmeters had a fairly high load on the circuit measured with 1000 ohms-per-volt being common. Later VOMs are usually 20,000 ohms-per-volt (on DCV) providing less circuit loading and more accurate measurements of circuits under test. Low impedance sources with high voltages such as power supply voltage levels, plate or screen voltages, etc. were generally not affected by the loading but trying to measure a high impedance low voltage source like grid voltages or very low current voltage-dividers could be a problem. The standard instruments were usually built by Triplett (630 Series) or by Simpson (260 Series.) Although today most technicians use a hand-held digital multimeter, sometimes these older analog VOMs can provide measurements of very high AC or DC voltages or high DC current that the modern instruments are incapable of.


Shown to the above-right is the famous Triplett 630A from the late fifties to early sixties. Sensitivity is 20K ohms per volt on DC and 5K ohms per volt on AC. Note the "Caution On High Volts" warning - the 630A isn't protected from over-voltage. The high voltage range is 6000 volts AC or DC. This completely functional 630A belonged to Dave Walker of Walker Electronics in Reno, Nevada. Walker started in business in the early-1950s and just recently retired (2012.)

Shown to the left is the later version Triplett 630-PLK VOM that sold for $85 in 1967. It is "over-voltage protected" and is capable of measuring AC or DC voltages up to 5000 volts or measuring DC current up to 10 amps. The 30vdc instrument battery required for the Triplett 630 is an Eveready 413 or NEDA 210. They are available by "after-market" suppliers. The low voltage battery is a standard D cell. 20K on DC and 5K on AC. This 630 is in excellent, operational condition and was given to me by my old Virginia City friend, Roger LaVake (SK.)

Shown to the below-left is the equally famous Simpson 260 VOM. It's difficult to say whether the Triplett 630 or the Simpson 260 was the most popular model of VOM. Certainly both models will be encountered often. This Simpson 260 belonged to Bently Nevada Corporation and it still has its BNC asset tag on the front of the meter. 20K on DC and 5K on AC. The Simpson 260 sold for $60.

Shown below-right is the Simpson 270 which is an upgraded 260 model with improved accuracy and a special low-current meter movement called "TAUT BAND." This type of movement uses thin metal ribbon-bands to suspend the coil instead of springs. The metal bands are kept taut by spring pressure. The movement is somewhat shielded by its construction and is especially useful when measuring very low currents that might be encountered in transistor circuitry. The 270 sold for $70 in the mid-sixties. This Simpson 270 belonged to my old ham-friend, W7TC(SK.)

With VOMs encountered today, condition is always an issue because of the fragile nature of the bakelite case. Many of these models of VOMs will have broken (and sometimes repaired) cases - normally from being dropped or knocked-off of the bench. Excellent condition cases show that either the meter was well-taken-care-of,...or, it wasn't used very often.

Meter movements can also be damaged. On VOMs without meter protection it's common to find bent pointers or open meter coils. Also, without the over-voltage protection, you may find some burned resistors in the VOM circuit.

Leaving batteries in a VOM was common and nowadays those batteries probably will have started to leak their electrolytic. Leaking batteries can cause corrosion damage to the battery contacts and sometimes will leak onto other parts of the circuit causing corrosive damage.

If the VOM found is in good physical condition with no case damage and a good meter movement then usually only minor clean-up and a new set of batteries will be needed. 


Vacuum Tube Volt Meters

Triplett, Measurements, Simpson

Many times it's necessary to measure operating circuits that are sensitive to how much of a load the measuring device adds to the circuit. The VOM, especially the older 1000 ohms-per-volt type, tended to load down a circuit under test and provide inaccurate measurements. Sometimes the load would stop the circuit from operating as in the case of some types of oscillators. The vacuum tube grid input impedance is very high and if it is used as an input stage to a voltage measuring circuit, the load added during measurement is very low - over 10 meg-ohms per volt. Thus circuits can be accurately measured without causing the operation to change. Most VTVMs would have an RF probe that would have a vacuum tube diode in the head of the probe. Necessary voltages were supplied via the cable and the signal would also travel up the cable. The RF probe allowed measuring very high frequency (RF) RMS voltages. The very low capacitance and high impedance of the vacuum tube input inside the probe head allowed for very high frequencies to be measured without being "loaded down" by long (high capacitance to ground) cable connections.

Usually the VTVM needed to operate on the AC line voltage in order to have a power supply built-in to provide filament and DC plate voltages for the vacuum tubes. However, some manufacturers did make portable VTVM that were built into the same types of bakelite cases that the popular VOM used. It was necessary to use a tube that could operate on a rather small filament battery, generally one D-cell could operate a 1.5v filament tube. Also, usually two of the 30vdc instrument batteries were used in series to provide 60vdc plate voltage.

The Triplett 631 (right) is a battery-operated VTVM and requires three internally mounted batteries for operation (sold for $75 in 1967.) The 631 is identical in size to the the 630 VOM. Internally, it's "crammed-full" of circuitry that includes the vacuum tube.

The Measurements Model 62 VTVM is shown in the photo left (note the RF probe with 6H6 tube mounted to the rear of the case.) This VTVM is built into a slanting type of metal case that provides the user with an easily viewable meter. Push-buttons are used for range switching. FS ranges are 1, 3, 10, 30 and 100 volts. AC or DC voltages can be measured but the Measurements 62 will not measure resistance. Dates from the 1940s. 


The Simpson Model 303 VTVM is shown in the photo right. Although in a bakelite case, the 303 does operate on the AC line and features a stand that allows the meter to set at an angle that allows easy reading of the meter scale. The Model 303 is a fairly modern VTVM with the capability of measuring AC and DC voltages and resistance in ohms. Sold for $85 in 1967.


These old analog VTVMs are still quite usable and are actually the best way to perform receiver alignments since it's so easy to observe the "peak" voltage while performing the adjustment.


More VTVMs below,...


General Radio Company

Vacuum-Tube Voltmeter -  Type No. 1800-A, 1800-B


1800-A - General Radio introduced the 1800-A Vacuum-Tube Voltmeter just after WWII (1946) for the hefty price of $345. The circuit uses 10 tubes which includes two neon glow lamps for regulators and a 3-4 Amperite ballast tube. Also, two type 9005 "acorn tubes" are used - one in the RF probe and one inside the instrument. The 1800-A would measure AC voltage with a frequency as high as 500mc. The RF probe is at the end of an eight foot cable and can be used directly connected to the source to be measured in which case there is slightly less capacitance allowing higher frequencies to be measured. One can also leave the RF probe "plugged in" to the internal connections under the top lid and use the front panel jacks for connections. Due to the higher capacitance of this type of connection, the measured frequency limits will be somewhat lower. DC voltages can be measured with a standard 10 meg ohm load to ground or with an "open grid" connection, which is "no load" on the grid of the input amplifier tube. This has to be done carefully to maintain accuracy due to variable loads affecting the grid to ground resistance. Most DC measurements are done with the standard 10 meg ohm load connected. Zero adjustment on the front panel is a "fine zero adjust" with the "coarse" adjustment located inside the unit.

The mirrored scale allows for eliminating viewing parallax to increase accuracy. Additionally, the meter is illuminated with two lamps mounted in the lower part of the meter housing. Full scale voltage ranges are .05, 1.5, 5, 15, 50 and 150. The top two scales are linear and are for voltages above 5 volts AC or for DC measurement. The three lower scales are non-linear and are for voltages 5 volts AC and below. The 1800-A does not measure resistance, only AC and DC voltages.

The top lid will open to reveal a storage area that contains a set of adapters for various kinds of GR connectors in addition to the RF probe. The carrying handle will fold back and allows the 1800-A to be on its back and slightly elevated to allow easy viewing of the meter. Serial number of the 1800-A shown is 1928 and it originally was used by the US Navy. "Property U.S.N." is stamped in red ink on the top of the lid.

This 1800-A functions quite well and is an accurate instrument.

1800-B - General Radio made a few changes to the 1800-A VTVM and these new models were designated as the Type No. 1800-B. Introduced in 1956, the changes were minor. The "on-off" function was moved from the AC-DC selector switch to the Voltage Range switch. This allowed the AC-DC switch to then become the AC, +DC and -DC selector. With the 1800-A, the user had to change polarity by physically moving the DC connections at the +/- terminals. With the 1800-B this could be accomplished by merely changing the switch position. Also, the nomenclature was slightly altered to have "+DC" in red and "-DC" in silver. All specifications remained the same as the 1800-A. The same accessories were included and the basic appearance remained the same (although the lid-mounted licensing/use tag was eliminated for the 1800-B.) Selling price was $435 in 1959. The 1800-B was available from 1956 up to 1962.

This 1800-B is also a functional unit. Its serial number is 4483. The photo to the left shows how the 1800-B (or A) can be angled using the carrying handle as a prop. Beautifully built, both the 1800-A and the 1800-B are accurate measuring tools and very reliable.

Like most test equipment, many examples will have been subjected to endless applications of calibration stickers that nowadays seem to defy removal. Additionally, vibro-etching company asset-numbers into the front panels or meter cases seems to have been another popular method of defacement.

The walnut cases can get pretty scratched up with years of use. Various types of oils or waxes will usually have the case looking acceptable again. Be careful with using oil-type furniture polishes as this seems to darken the walnut considerably. The darkening lasts until the wood dries out. I use a lightly-dampened cloth to clean the walnut finish as this removes the dust and dirt but doesn't darken the wood finish. The 1800-B photo to the left shows the normal color of the walnut finish that General Radio used.


McMurdo Silver Company

Vacuum Tube Voltmeter - "SILVER" VOMAX  Model 900

During the 1930s, McMurdo Silver was trying to run three companies simultaneously while also trying to entertain his various girl friends while driving around in his Packard convertible. Needless to say, McMurdo's only success was with McMurdo Silver Manufacturing Company located in Chicago. They produced high-end radios that were truly "custom-built" to order. Chrome chassis, large speakers, great audio and spectacular cabinets were McMurdo's trademark. That, and his fierce competition with Scott Radio Laboratory, the only other "custom built" high-end radio builder at the time. McMurdo eventually went out of business in 1938 (sold to Scott Radio Laboratories) but he formed another called McMurdo Silver Company located in Hartford, Connecticut and concentrated on small test equipment gadgets and other types of inexpensive devices. In 1947, McMurdo Silver was killed while cleaning an antique pistol. This is ironic since McMurdo had worked his way through college as an antique firearms dealer.

The VOMAX is one of the better instruments that McMurdo produced after WWII. The RF probe plugs into a cubby hole in the front panel. The scales provided are AC or DC voltage 3, 12, 30, 120, 300 and 1200. DCR scales are 2K, 20K, 200K, 2M, 20M and an incredible 2000M (that's 2 billion ohms!) The db scale is 0 to 50db into 600Z ohms and the current measurements are 1.2mA up to 12 Amps.

The RF probe can be installed as shown for low frequency RMS measurements but for high frequency RMS measurements the RF probe can be unplugged from the cubby and used directly.  

This VOMAX was donated by W6JRY


Hewlett Packard Company

Vacuum Tube Voltmeter  Model 410B

The 410B is probably the most popular of Hewlett Packard's early VTVM instruments. They are easy to find, usually are in good condition and normally not too expensive. Versatile, accurate and easy to maintain, the 410B is a good choice for a vintage "bench-type" VTVM.

Test leads are permanently connected to a plug-in cable assembly that fits under and just behind the front panel. The RF probe allows measuring RMS voltages up to 700mc or so. The 410B has six ranges of voltages for both AC and DC measurement, 1, 3, 10, 30, 100 and 300 volts FS. Resistance can be measured from .5 ohms up to 500M ohms. Six tubes are used in the 410B, (2) 12AU7, (1) 6X4, (1) 0B2, (1) 6-4 Amperite ballast tube and the RF diode inside the RF probe is a 2-01C. A three-wire power cord is used and the case is metal that is painted HP tan and the front panel is gray. A rear door can be opened to access a storage area.

Accuracy is 3% on voltage measurement but varies with frequency on RMS measurements and with other factors for resistance measurements.

Like a lot of popular equipment that was used in the industry, it's sometimes difficult to find a 410B that hasn't had dozens of calibration stickers applied to the front panel and many also will have asset tags or numbers engraved or worse. Luckily, this 410B belonged to Al Chin who worked on the Reno Police Department radio equipment so it only has "Test" in red paint on the top of the cabinet. I also have a second HP 410B that belonged to the USAF that is a more typical example with lots of stickers and decals. Both are functional and useable VTVMs.


Hewlett-Packard Company

   Vacuum Tube RMS Volt Meter  Model 400D


There are three ways to express AC voltage - Peak-to-Peak, Average or RMS (Root Mean Square.) By far the most often used is RMS and this Hewlett-Packard Vacuum Tube AC Voltmeter is scaled in RMS or in DB (decibels.) The scales provided are a match for the attenuator scales used on the HP 606 signal generators and provides the user with the capability of measuring RMS voltages down to a scale of .001 vrms FS.

This 400D was used at Bently Nevada Corp. in Minden, Nevada for years. It still has the asset tag on the front panel. Note that this meter was routinely maintained by the BNC Metrology Lab and during one such routing the Output red binding post was replaced. BNC would send unused equipment to the "inactive stockroom" where it could be purchased by employees.





Audio Frequency Output Voltmeters

           Weston Electrical Instrument Corp.        Simpson Electric Company


The Audio Frequency Output Voltmeters shown are handy, useful little instruments, especially for receiver alignment. The meter is connected across the speaker voice coil connections where it measures the audio output level of the receiver. For alignment purposes the signal generator must provide a modulated waveform and usually 400 hz is the standard (1000 hz is also a standard.) Best results will be with the receiver's AVC off and the audio output level controlled with either the signal generator level or the audio gain control of the receiver  If the output is measured directly at the plate of the output tube then the connection must be to COND. which places a 0.1uf capacitor in series with the meter. +/- is common and 2, 10 and 50 are the full scale voltage levels. 2 is the most useful for voice coil measurements. Both the Weston 687 and the Simpson 427 are nearly identical in design, use and performance.


Frequency Selective Voltmeters (aka Wave Analyzers)

Hewlett Packard Company - 310 Wave Analyzer

Wave Analyzers are frequency tuned voltmeters that were generally used for measuring RF noise or RF interference on telephone lines, cable runs, data lines, etc. - anywhere that signals of some sort were routed down a cable and might be susceptible to interference from leakage or other problems. Most Wave Analyzers will tune from a very low frequency up to a few megahertz. Many will have features like a radio receiver, i.e., selectable sideband detection, BFO and an audio output that is accessible - usually a phone jack for a monitoring headset. What they won't have is AVC since the whole point is to measure the interfering signal's level without any control circuitry. Many Wave Analyzers used analog tuners but some were mechanical digital and more modern versions are all digital. Since the Wave Analyzer or Selective Volt Meter is essentially a radio receiver, it's possible to connect an antenna to the input and use the instrument as a LF receiver using the audio output monitoring to drive a set of headphones or loudspeaker (Z matching may be required.) Some versions work quite well in this application. Others don't.

Shown in the photo right is the Hewlett-Packard 310 Wave Analyzer - the HP-310 is an excellent performer as a VLF/LF/MF receiver because it tunes from 1 khz to 1500 khz, has a BFO, selectable modes (AM, USB, LSB) and has a mechanical digital frequency readout. It is easy to over-drive the output because there is no AVC but usually, if the noise is low, it can be operated on the highest sensitivity without any problems. In higher noise conditions, the second highest sensitivity must be used. Sensitivity is controlled by the setting of the FS meter level - lowest FS level = greatest sensitivity.

Sierra Electronics Corporation

Frequency Selective Voltmeter Model 125B


The Sierra Model 125B was used for the same purposes that the HP Wave Analyzers were. Sierra was a common brand used by many different telephone companies. The 125B is a vacuum tube unit and is fairly well built. However, its usefulness as a LF receiver is limited. Although it does pick up signals when an antenna is connected to the input, those signals are difficult to hear because the 125B lacks a BFO. Without a BFO, it is very difficult to hear the carriers of MCW signals, such as Non-Directional Beacons. Virtually all signals in the MF, LF and VLF ranges are data type signals (no voice) and without a BFO, most of these signals will go unnoticed when tuning the 125B. The tuning range on the 125B is from 1hz up to 620khz.


DC R, Capacitance, Inductance, Z, D and Q Measuring Instruments


Leeds & Northrup Company

Gray Instrument Company

Wheatstone Bridge


The invention of a resistance bridge circuit to measure an unknown resistance is credited to Samuel H. Christie in 1833 but the instrument's name, the Wheatstone Bridge, is credited to Sir Charles Wheatstone who popularized the circuit in 1843. The Wheatstone Bridge could measure an unknown resistance by substituting the unknown R into a "bridge circuit" consisting of two series-parallel R branches made up of four resistances (including the unknown and one fixed-value R in one series branch and the adjustable R and another fixed-value R in the other series branch with both fixed-value resistors of equal value.) A source of DC potential was connected to each end of the series-parallel R branches (+ to one end and - to the other end.) Then a sensitive "zero-center" galvanometer was connected between the two series-parallel branches at the series junction of each of the resistors pairs. The galvanometer would indicate a "null"  (zero-center) when the "unknown R" was equal to the adjustable R since equal current flowed in both series branches. Before long, it was discovered that not only DC resistance could be measured but capacitive or inductive reactance could also be measured if an AC signal was substituted for the DC voltage in the Wheatstone Bridge. By using an oscillator to provide the AC signal (generally at one specific frequency) values of unknown capacitance and inductance reactance could be measured and from that Xc or Xl measurement the actual component value could be calculated. Prior to vacuum tube oscillators, mechanical buzzers and "hummers" were used as an AC source.

Shown to the top right is a Leeds & Northrup Wheatstone Bridge. There are binding posts for connection of a DC potential labled "BAT" with + and - posts. Also posts for the Galvanometer connection and for the "unknown" R value. Push switches are provided to connect the battery or the galvanometer. The decade switches allow the user to select the value of R that is in series with the fixed R. Internally the two fixed R values are comprised of a center tapped R that provides the two equal fixed R values. When the user sets the switches he watches the galvanometer and when it shows zero, the unknown value is equal to the value shown on the selected switches. The level of DC applied is not important but as the value of the unknown R becomes higher more potential might be necessary to see significant movement of the galvanometer and thus see a good null indication. With a DC potential connected, only values of R can be measured. If an AC voltage is applied then the user must know its exact frequency so the proper calculation can be made to convert the reactance value to the component value. However the galvanometer will not function on AC so usually a headset is connected and an auditory null is used. Note that the Gray Instrument bridge (bottom right) shows "TEL GA" indicating either a galvanometer or telephones connection to these binding posts.

Today, most of the uses of the Wheatstone bridge are in physics classrooms or laboratories in colleges or universities where they are used to demonstrate the principles of the bridge circuit. It's a good exercise to go through the procedure and see how measurements were made when there were no digital readouts, hand-held calculators or even direct ways to measure values of R, C and L.

Both of these Wheatstone Bridges came from St. Martin's University in Lacey, Washington. Note that the Gray Inst. Co. bridge has "St. Martin's Colle" hand engraved on the panel. Also the hand-written notes regarding the accuracy of the bridge resistors were written on white paint that was then covered with Scotch Tape. Note that with both bridges that the layout of controls and binding posts are exactly the same - that is - standard for Wheatstone Bridges.


Leeds & Northrup Company

Cable Testing Set using DC R Bridge Circuit


This test box is an example of the type of "industrial" test equipment that Leeds & Northrup built. It's also an example of how the test equipment that was used in those types of environments is usually found nowadays in "well-worn" condition. Note that the numbered skirts on the R knobs have most of the numerals worn off right down to the celluloid base material. Also, the instruction card in the lid is virtually unreadable due to wear from having the test leads kept wrapped-up on the panel and then forcing the lid closed.

This Cable Testing Set is essentially a DC Resistance Bridge that incorporates a built-in galvanometer and enough switches to create different types of loop circuits to find faults within a run of cable. Both opens and shorts can be located with the test set. With a short-circuit within a multi-conductor cable it would have been possible to measure the resistance from the test set to the short-circuit (and back.) This could then be correlated to the actual distance to the short-circuit by comparing the resistance per foot for the size and material of the conductors within the cable to the measured resistance. An external battery was required for testing. This Cable Testing Set has a serial number of 84070 which dates it to the early 1920s.


General Radio Company

Type 650-A Impedance Bridge    Type 650-PI Oscillator-Amplifier

The Wheatstone Bridge could not only measure an unknown R value, if an AC voltage was applied to the bridge circuit, capacitive or inductive reactance could be measured. From the Xc or Xl, the actual unknown value of the component could be calculated. Since reactance is what is actually being measured, this application of the bridge was usually called an "Impedance Bridge." Early Impedance Bridges used mechanical oscillators that were something like buzzers and would work with a DC voltage, creating an oscillating voltage. Early Bridges used separate devices to connect a voltage source to separate resistive components and to the galvanometer to complete the bridge circuit. By the early twentieth century, Bridges were available as a complete test instrument although most required separate potential source and galvanometer. Later Impedance Bridges would have all components built-in with an AC line operated power supply for the potential source for DCR measurements and a built-in electron tube oscillator for Z measurements.

Shown to the right is the General Radio Company Type 650-A Impedance Bridge. When battery operated, a mechanical oscillator called a "hummer" was used. This device used the battery supplied DC voltage to cause a small pendulum to mechanically vibrate and that was picked-up by a carbon microphone inside the unit. The output was sufficient to drive a hi-Z headset. Early versions were battery only operation but in the late-forties, the 650-PI was introduced. The Type 650-PI eliminated the need for the hummer and an electron tube oscillator (1kc) was used in its place. The amplifier gave higher output levels for better detection of the null by listening to the headset. Most of the voltages provided by the 650-PI are for its oscillator and amplifier. The DC voltage provided for the DCR measurements is a substantially higher (~180vdc) than that provided with battery operation. This results in better accuracy at higher resistances. The 650-A shown to the right has the 650-PI installed. The solid walnut case is lined with copper sheet for full shielding. Selling price in the early fifties was $410 with the 650-PI installed and $260 if the battery operated version was desired. The 650-PI by itself was available for $150.

photo right: The typical production GR 650-A Impedance Bridge SN: 9052 from the early-1950s

photo above: This is a late production 650-A showing that these impedance bridges will sport black dials in place of the earlier silver dials. The serial number on this particular 650-A is SN: 12030 and it probably dates from the mid-to-late-1950s just before the introduction of General Radio's new 1650-A impedance bridge (1959.)
Using the General Radio Type 650-A - To measure DCR only requires that a small DC voltage be applied to the bridge - use the 650-PI with DC selected and the output connected to EXT IN on the bridge. An unknown resistance can then be connected to the "R" binding posts on the bridge. The galvanometer is switched to "Shunted Galv" before the unit is turned on. This protects the galvanometer from being "pegged" to full scale. R is selected with DQ scale switch and the bridge power supply switched on. Select a position with the range switch that moves the galvanometer needle to near the center with the C-L-R dial set to 1.0. Now, tune the C-L-R dial for a zero indication on the galvanometer. Next, switch the Shunted Galv to Galv which removes the shunt for better meter sensitivity. Now retune the C-L-R for zero on the meter. The DC resistance is read on the C-L-R dial times whatever multiplier is selected by the range switch. The galvanometer is only used for DCR measurements. Accuracy is about 1% to 2%. AC resistance can also be measured using the 1kc oscillator rather than the DC voltage.

Measuring C and L is accomplished with the 1kc sine wave input from the 650-PI unit connected to the EXT IN on the bridge. An unknown C or L is connected to the "CL" binding posts. For C measurements set the DQ dial to 0 and for L measurements set the DQ dial to 10. Select C or L measurements with the DQ scale switch. Connect a hi-Z headset to the "phones" binding posts on the 650-PI unit. Turn on the 650-PI. A 1kc oscillation will be heard in the phones. Adjust Oscillator Gain and Audio Output Gain for a relatively strong 1kc signal as heard in the phones. Then adjust the C or L range switch for the quietest oscillation note as heard in the phones. Now adjust the C-L-R dial for the best null (quietest signal in the phones.) Adjust the DQ dial for a further null and re-tune C-L-R for an even better null. If "Q" is selected (with the DQ scale switch,) then the Q dial can also be adjusted for L and adjust for a null. Select "D" for capacitance dissipation. Use the appropriate multipliers when Q or D is selected. Value of C or L is read on the C-L-R dial with the appropriate multipliers used. If the actual impedance value (Xc or Xl reactance) was desired for some reason it must be calculated from the component value measured with f = 1kc. Overall accuracy is about 2% for value and about 10% for Q or D. Better accuracy is attainable with short, low R test leads. Same is true with DCR measurements. Be sure to check the calibration of your GR 650-A for best accuracy. The manual specifies that another R bridge must be used for adequate calibration accuracy however a modern digital multimeter in ohms will be just as accurate for calibration.

As can be seen from the above, using the 650-A is a hassle compared to modern digital C-L-R-Z-Q measuring instruments. Still, it's fun to go through the process and see what it was like to use these instruments when they were "the best and most accurate" way to measure component and impedance values. Since all of the readouts and measurements require use of multipliers or conversions and reactance (impedance C or L) and other implied values must be calculated, competent math ability was a definite requirement. And,... remember, this was before digital handheld calculators. Everything was either longhand calculations or slide rules,  And,...speaking of slide rules,...the photos below show some vintage slide rules (in case you've forgotten what they looked like.)

photo above: Upper slide rule is a Pickett Model N902-ES (Simplex Trig) from the mid-1960s. This slide rule is aluminum that is painted yellow which was supposed to be "easy on the eyes" after hours of calculations. Below is the Keuffel & Esser Co. Model N4053-3 "Polyphase" from the mid-1930s. This slide rule is mahogany with white plastic scales. Also, two ruler scales on top and bottom edges.

photo above: This "Circular" slide rule, made by Concise for Motorola, was a very small "space saver" that easily fit in your shirt pocket. Concise was a Japanese company located in Tokyo.  The circular slide rule was made famous by Peter Sellers in the movie "Dr. Strangelove."

General Radio Company

Type 1650-A  Impedance Bridge

By the mid-1950s, the old GR 650-A Impedance Bridge was beginning to show its age and really looked like a relic from the 1930s. The 650-A was a large, heavy instrument that was sort of difficult to move around and, if left set-up, took up a lot of bench space. Around 1959, General Radio announced their new replacement for the venerable 650-A, the very modern Type 1650-A Impedance Bridge. Price in 1959 was $440.

The new 1650-A was in a metal cabinet that was half the size of the old 650-A. Additionally, the very cleverly designed lid remained attached and could be swung-around the cabinet and would provide a base for the cabinet that would allow any viewing angle that was convenient for the user. When transporting the bridge around, the lid could be swung back and locked into place providing panel protection and a carrying handle to allow easy moving of the unit. By the introduction of the 1650-A, General Radio was beginning to change the old black crackle finish panels and was now using dark gray crackle finish along with gray knobs.

The 1650-A eliminated the three separate D, Q and DQ dials used on the old 650-A and combined the function into one large dial with four scales. Also, to eliminate the difficulty of finding a "null" with low-Q inductances, "Orthonull" was provided. "Orthonull" allowed combining the tuning of the CRL dial and the DQ dial in one action. It also allowed the user the ability to separately adjust the DQ dial if necessary and not affect the adjustment of the CRL dial. For the most part, "Orthonull" was only used if there was some inter-action between the CRL and DQ dials that changed the "null" with each adjustment. Most of the time, component values just "dial in" when in the "Normal" position (CRL and DQ dials operated separately.)

The internal 1.0 kc oscillator was "fully transistorized" as was the detector. This allowed the 1650-A to run entirely on 6vdc which is provided by four D-cells. If necessary, an external power supply could also be used. The batteries load into a cylindrical opening on the top of the cabinet with the retainer cap being the negative connection. At the time (1950s-70s,) batteries were prone to leakage if left installed a unit that was going to be stored for lengthy time periods. Be sure to check the condition of the battery compartment on any 1650-A you intend to purchase. Some 1650-As will be found with corrosive damage where the batteries were left installed over a long period of time. Modern batteries are much less prone to rupturing but still they should be removed when the unit is not in use for long periods of time.

The 1650-A is a very easy Z bridge to use, especially when compared to its older brother, the 650-A. All component values seem to just "dial right in" and the DQ dial action provides "easy to see" nulls. Since the NULL meter has an amplifier circuit and also has adjustable sensitivity, excellent "null action" results and headphones are not necessary for AC measurements.

General Radio Company

Type No. 821-A   Twin-T Impedance Measuring Circuit


Twin-T describes the particular type of "bridge" circuit that the 821-A uses. The advantage of the Twin-T is that it allows a common ground to be used between the generator and the detector providing interconnections to the "unknown value" via coaxial cable. This allows high frequency measurements of capacitance, inductance, C dissipation factors, Q of coils, Z of coaxial cable runs, antenna-transmission line Z, resonant Z of parallel tuned circuits and Z matching networks to be made accurately. An external fixed value capacitor is required for some types of measurements. Additionally, usually some mathematical calculations are required to determine many of the final measurements. An external "generator" source is required and this can be a laboratory RF signal generator that provides frequencies from 460kc up to 40mc with an adjustable output level. An external "detector" (null detector) is also required and this can be a communication receiver that tunes within the frequency range required. The receiver should have a manual RF gain control and also have the ability to remove any AVC control of the signal. The receiver should also have a BFO (required for some set-ups.) The 821-A was supplied with two coaxial cables for interconnecting of the RF generator and the receiver to the Twin-T circuit. GR-874 connectors were used for the generator and detector connections to the 821-A. All circuitry within the 821-A is passive and all signal source and null detection are external to the 821-A. The selling price in 1951 was $520.

General Radio Company  -  Catalogs


The photo to the left is from General Radio Catalog M (1951.) It shows a laboratory engineer measuring an inductance using a Type 821-A Twin-T Impedance Bridge like the one shown above. The signal generator shown is the GR Type 1330-A Bridge Oscillator. The communication receiver shown is a National Company NC-173.

General Radio Company catalogs were first class books that not only showed the company's product but also included detailed write-ups on theory of measurement and use of the various types of bridges or oscillators and an abundance of other information including charts, tables and graphs to assist any user of GR equipment. The catalogs always had great photos and artwork and printing was always on high-class "slick" paper. Catalogs were assigned letters that were published chronologically but each catalog was considered current for about two years. Catalog M is dated October 1951. I also have Catalog P which is dated April 1959.


Boonton Radio Corporation

Type 260-A  Q-Meter

Boonton Radio Corporation was formed in 1934 by William Loughlin and C. L. Franks, both from Radio Frequency Laboratories. Franks had developed an instrument to measure inductance along other coil parameters and this became the first Q-Meter which was Booton's first product. Later, although Boonton made other types of test instruments, they were primarily known for their Q-Meter. Boonton's first Q-Meter was the 1935 Type 100-A. By 1941, the Type 160-A was in production. In 1953, the Type 260-A was introduced. The 260-A was in production from 1953 to 1970. Hewlett-Packard bought out Boonton in 1959 but continued the production of the 260-A. Note that the Type 260-A shown to the right has original H-P type knobs installed.

The Type-260 has a tunable oscillator and a calibrated variable capacitor allowing the user to measure resonant frequency of a prototype LC circuit using an unknown L with the internal tunable C and the variable oscillator. The Q of the L can also be measured. With known value L installed, an unknown C can be determined by the resonant f of the LC. A coil must be installed in the Type 260-A for the oscillator to work. 

Original selling price of the Type 260-A was $725.


Hewlett-Packard Company

4342A  Q-Meter

Boonton Radio Corporation was known for their Q-Meters. However, Hewlett-Packard bought Boonton in 1959. H-P still continued to have Boonton produce the Type 260-A for quite some time. Move ahead into the 1970s or so and you have the all solid-state HP 4342A Q-Meter. It accomplishes the same functions of determining unknown values of  L or C  in addition to measuring the Q of the particular resonant circuit under test. Since the oscillator can be tuned, resonate frequency of an LC combination can be measured. You can also "tune in" different values of  L or C in an "prototype" LC circuit (minus either L or C) and find the resonate frequency. 

I've used this Q-meter mainly in designing loop antennas but it does have a variety of other possibilities for the experimenter or homebrewer. Introduced in 1970 (at an original selling price of $1500,) it might be a little "too new" for being considered vintage equipment however it was found at an "electronics surplus" store by KE6LNI who subsequently gave it to me (he actually got two of them from the surplus store.)


General Radio Company

Type No. 1612-AL  RF Capacitance Meter

The GR 1612-AL was a device for measuring low value capacitance at a radio frequency of 1.0 megacycles. It employed a vacuum tube oscillator using a 117N7 which also provided the rectifier function for the power supply. To power the RF Capacitance Meter required 115vac. The circuit used a substitution method that required the user to "zero" the circuit and then connect the capacitor to be tested to the terminals marked "X." Then the large dial "uuf" was adjusted to rebalance the circuit and the capacitor value then read directly on the "uuf" dial scale. Note that "micro micro farads" are used which dates the 1612-AL to the late forties. By the mid-fifties, "pico farads" or "pf" had replaced the "uuf" term.

The "AL" in the type number indicates that this version is the "low-range" model that measures up to 100pf. There was also a 1612-A that had a maximum upper limit of 1000pf. General Radio advertised that the 1612-AL could measure capacitance down to 0.05pf and was suitable for measuring capacitance between tube socket terminals (RTMA spec TR-111.)

The 1612-AL sold for $170 in 1951. This RF Capacitance Meter came from W6AQU. 


Solar Manufacturing Corporation

Capacitor Analyzer Model CE

Capacitors are difficult to test since they require an AC signal to actually tell anything about their performance - other than if they are shorted. Capacitor analyzers generally allow the user to apply full working voltage to a capacitor to see if the dielectric can handle full voltage and to measure the current leakage of the capacitor. Then an AC voltage is used to measure the capacitance based on the 60hz line frequency and Xc at that frequency. Power factor (used more in AC circuits where the capacitor must be able to transfer power, motor starting capacitors or balancing L loads in distribution lines) can usually also be measured.

Solar Manufacturing Corp. was a major capacitor supplier during the forties, fifties and sixties so it seems logical that they should produce an instrument for checking capacitors. There are several different versions with the main difference being whether a meter is used for measurement or whether a cathode-ray tuning eye tube is used.

Other manufacturers built capacitor checkers and kits were also available from Heathkit and EICO.

Today, the hand-held digital capacitance checker is popular but these will only measure the capacitance value at a very low voltage. For vacuum tube equipment, there are a few other things that should be checked on a capacitor (like leakage current) before applying high voltage and these vintage capacitor analyzer units, when fully rebuilt, can provide those checks.


Vacuum Tube Testers

The ability to rapidly test a vacuum tube to assure that it will probably function in a circuit is fairly easy. What's difficult is to accurately measure any vacuum tube's transconductance and to have full confidence that the "tube under test" will function in any type of circuit at any frequency. All vacuum tube testers are compromises that combine manufacturability with "ease of use" along with some degree of accurate measurement of either cathode emission or, the more difficult to measure, transconductance. No tube tester is 100% accurate for all tube types or for all functions of that tube in a variety of circuits. No tube tester can find 100% of the faults that are possible in a tube under test. However, tube testers do a good job at sorting out tubes that are functional from those that have minor to catastrophic failures. The "final test" is always to see how the tube performs in the actual circuit that it is intended to be used in.

Sterling Manufacturing Co.

Vacuum Tube Reactivator R-403

Thoriated-tungsten filament vacuum tubes can become weak in emission if they are not operated for a long period of time. The thorium atoms that are in the hardened "paste" that is the coating on the pure tungsten wire filament tend to migrate to the lowest part of the coating. This reduces the filament emission. By heating the filament up to its normal operating temperature the thorium atoms will migrate back up to the surface of the paste (which increases the emission) but it can take a minimum of thirty minutes for the tube to regain its former operating gain. Some radio repairmen didn't have time to wait so the Sterling Reactivator allowed them to speed up the process. By elevating the filament voltage by two times for 30 seconds and then allowing a "burn in" period at 1.5X the filament voltage for 5 to 10 minutes, usually the tube would respond with a measurable increase in gain. The Sterling Reactivator does provide a "Test" function to monitor the results. This device can only be used with thoriated-tungsten filaments. Pure tungsten filaments will not reactivate. Also, DeForest tubes used a carbon based filament that will "burn out" if reactivation is attempted. Only 01A tubes and UV-199 tubes can be tested and reactivated with the Sterling Tube Reactivator.

Since the act of "reactivation" does involve some risk to a potentially good tube, I don't ever use this device. If you merely install the "weak" tube into the radio and operate the radio normally for about 30 minutes you will notice that the gain and sensitivity will have improved. Further operation will also show more improvement over time but the greatest gain increase will be in the first 30 minutes. Patience rewards you with safely reactivated tube. Also, once a tube is reactivated, it takes a very long time of "non-use" for the tube's emission to decrease again.


Radiophone Inc.

Preceptor Tube Tester - Model K

As tube testing progressed, sometimes unusual methods were attempted to make testing easier or faster. The idea of reading a "Bad-?-Good" scale was maybe too much for some technicians and actually reading the mutual conductance in uMhos on a meter scale was altogether unacceptable. Enter the "Preceptor Tube Tester." This tube tester showed the technician a lighted pattern in which the brightness of the lamps were compared to reference lighted bulbs to determine the acceptability of a tested tube. There is also a meter to read but the scale is only a centerline for setting the "line voltage." Sockets provided are four, five, six, seven (receiver tubes) and octal. This would date the Preceptor to the late thirties just before Loktal tube sockets appeared (1939.)

I don't have the manual for this tester to see what was expected as far as light patterns or how to determine the tube's performance by lamp brightness but one can assume that unless the tube being tested matched the specified pattern and/or the brighness, it was considered bad. Using the Preceptor Tube Tester must have provided the radio repairman with a way to sell lots of new tubes.

This Preceptor Tube Tester came from NU6AM.



Simpson Electric Company

Model No. 220 "Roto Ranger" Tube Tester

Simpson came out with their "Roto-Ranger" Voltmeter in the late-thirties. It featured an "scale-in-use" type of meter that used a rotating drum with several different scales printed on it. The scale viewed changes as the different switched voltage ranges were selected. That was followed with the Model No. 220 Roto-Ranger Tube Tester that featured an emissions-type tube tester with the additional functions of a DC voltmeter, DC ohm meter and capacitance meter.

The circuit of the Model No. 220 uses a triode tube (this one has a 201-A installed)  with the grid and plate connected together to act as a diode in order to rectify the AC that is routed to the meter so it will be able to read the cathode current of the tube under test. The cathode current that shows on the meter will be a result of the tested tube's emission and with the proper selected switches the scaling will be correct for the type of tube selected. The operator has to select "Diode," "Battery" or "Cathode" for the type of tube being tested along with the correct filament voltage and the specified tube function level (a potentiometer setting for proper scaling.) Various taps on the power transformer provide the different tube heater voltages and a single winding on the transformer is used for the plate voltage. There is also a winding on the transformer for the internal 201-A tube's filament. Other than tube emission, tube shorts can also be checked and there is a method for checking some tubes for gas. 

The rotating scale display is mounted in a cast metal housing that entirely enclosed the mechanism and requires major disassembly to access if there's an internal problem. Even with the fully enclosed housing for the rotating scales moisture seems to be able to enter and cause oxidizing damage to the scales. Tube sockets provided are four, five, six, large receiving tube seven pin, octal and loctal. The test leads for the voltmeter are brought out in the storage area above the tube tester panel. That this Roto Ranger Tube Tester has the loctal tube socket dates it to around 1939. There is a data book with this Model 220 that has additional tube information dated 1940. The chart in the lid is original to this Model 220 and has the same serial number as the tube tester, 1360.




Simpson Electric Company

Model 330 - Mutual Conductance Tube Tester

Simpson produced several models of tube testers from pre-WWII up into the 1950s. The Model 330 is the only Mutual Conductance tube tester the company produced. Introduced in 1946 for a selling price of $98.50, the Model 330 was rather expensive for the typical radio repairman. The purchase could be justified in that, with television coming on strong after WWII, the ability to test a tube and have a measurement of that tube's transconductance based on the tube manufacturer's specifications would be important. The measurement was actually a scale that readout in percentage of mutual conductance (aka transconductance) with 100% being the manufacturer's spec for a new tube. The Model 330 uses a 2.5kc oscillator for grid signal application and claims to test the tube in the manner that it operates in the radio or television circuit. The color-coded meter scale shows 0 to 140% on the top and sections of the scale are marked "Good," "Fair," "Weak" and "Replace." In the "Replace" section is an extra warning to the user - "Caution - May Cause Serious Overloading - Replace." - wow, that must have helped with hesitant customers that balked at buying new tubes for their set. The meter is a FS mechanical zero that is electrically driven to scaled zero when powered up.

The multitude of push buttons and rotary switch selectors assured the radio repairman that the Model 330 would always be ready to test the latest and most complicated tubes (with the latest test data hopefully coming from Simpson.) Many of the rotary 0-5 position switches are not even used for the then-popular tubes. When the test is completed the white push button can be depressed to reset all push buttons and all rotary switches back to the start position (which would be 0 or up.) Tube sockets provided are four, five, six, large seven, miniature seven, octal, loctal, noval, acorn and two odd-ball miniature five pin sockets.

The Model 330 has an impressive appearance with a stunning black bakelite panel with white-filled engraved nomenclature that probably accounted for many of the sales of the tube tester when new. Some Model 330s have a leatherette covered case while others have the solid oak case as shown in the photo right. This Model 330 was originally owned by "Modern Radio & TV," located at 538 So. Virginia St. in Reno, Nevada.


The Hickok Electrical Instrument Company

 Mutual Conductance "Cardmatic" Tube Testers

Without a doubt, Hickok produced some of the best tube testers. Hickok testers will actually operate the tube somewhat like it would be functioning in a circuit and thus be able to measure the mutual conductance (also called transconductance) of the tube under test. Hickok had their circuit patented so only a few other makes (that were usually licensed) will operate in a similar manner. These types of tube testers are called "dynamic mutual conductance tube testers" while most of the other types are called "emission testers." Emission testers will measure the ability of the tube to have gain by way of its cathode emission. Though somewhat related, cathode emission and transconductance are different measurements. If you can't measure (or calculate) the transconductance of the tube, then the tester is usually an emission tester. Some mutual conductance tube testers have meter scales that show percentage or an arbitrary scale that is referenced in the test data or roll-chart data for the minimum acceptable readings for a good tube. If it is necessary, usually the actual transconductance can be easily calculated from these meter readings.

Hickok built tube testers for several different types of users. For the radio repairman there was the 600 Series along with the 534. Ultimately, the 6000 was probably the last of the tube testers that was designed with the TV-Radio repairman  For the commercial users, Western Electric (Bell System) had Hickok supply them with a Cardmatic tube tester, the Western Electric KS-15874-L2 (shown to the right.) The Hickok Cardmatic tester used hole-punched plastic cards that were inserted into a "card reader" that actuated the tester and set-up the parameters of the test. Each tube type had at least one test card and multielement tubes would require additional cards to accomplish full testing. A large section of test cards would warrant the use of a separate carrying case just for the cards. The Military also ordered many Hickok-designed tube testers.

Shown in the photo above right is the Western Electric/Hickok KS-15874-L2 Cardmatic tube tester which was the Hickok Model 1234 Cardmatic specifically built for the Bell System. Included in the lid are calibration cards, self-test cards, blank cards and a hole punch to make a special card for testing a tube where a card is not available. The manuals cover all aspects of the KS-15874-L2, including how to punch cards and what the various holes do for setting up the tube test parameters. The round object above the hole punch is the mercury cell that is used in the calibration of the tube tester. Under the louvered panel are several controls that can be adjusted if necessary. Under the black screened cover are the vacuum tubes used in the circuit of the tube tester. The separate metal box contains a complete set of test cards for most vacuum tubes. Special test cards for Western Electric tubes are stored in the small pocket to the left of the tube sockets. This KS-15874-L2 was originally used at the Bell System Coastal Station KMI located at Point Reyes, California. In fact, they had two of them. This one was given to me by KE6LNI who worked for Western Electric.

Shown in the photo to the left is the civilian version Cardmatic, the Hickok 123A. This tester functions in a similar manner as the KS-15874-L2 and both testers can use the same cards for various tube tests. However, the 123A is not nearly as elaborate in construction or in features as the KS-15874-L2. The test card set is kept in the pocket at the front of the tester with binder rings keeping the cards in order in a reasonable size lot. If additional test cards became necessary then the user would have to purchase a storage box similar to the metal one shown with the KS-15874-L2. At an incredible 1960 selling price of $499.50, it seems that the 123A would have been too expensive for the typical Radio-TV repairman and more likely would have been found in a metrology laboratory or in the test department of an electronics firm. This particular 123A was a ham swap meet find that was bargain-priced at 2% of the original 123A's selling price. 

Hickok tube testers became the "standard" from the late-1940s up until vacuum tubes were no longer part of the electronics industry.

NOTE: Regarding HICKOK Tube Testers in General -  Nowadays, Hickok tube testers have once again become the standard for "used" and "NOS" vacuum tube dealers who must provide some assurance to their customers that the prospective tube to be purchased will function correctly when received. Hickoks are recognized for their ability to test most vacuum tubes accurately by measuring the tested tube's transconductance. However, one should remember that test results will probably vary somewhat from tube tester to tube tester. Also a point to consider is that there really isn't any tube tester that tests a specific tube exactly like it operates in radio or amplifier circuits. Even the best tube tester will not find all tube faults that are possible. The "final test" is always to see how the particular tube operates when in the intended circuit.


Military Tube Testers

Weston Electrical Instrument Company
for the U.S. Navy

Model OQ-2  Vacuum Tube Analyzer (Weston Model 788)

Weston supplied their Model 788 as the "OQ" to the Navy during WWII. It's nearly two feet tall and weighs 35 lbs. The contract is No. NX55/3-13581 and dates from 1942. The OQ was used at radio repair depots and in areas were radio design and testing were performed by the USN. The lid slides off of the hinges if desired. Inside the compartment at the top of the OQ are 12 patch cords and the AC power cord. All connections between the tube socket panel and the analyzer/power supply/meter panel are accomplished using the patch cords. The roll-chart assumes the operator knows the tube socket pin-outs but if needed all tube pin-out data is contained in the manual. Once the operator knows what the tube pin-outs are, the patch cords are inserted from pin jack receptacles marked 1 thru 8 on the socket panel and connected to the various pin jack receptacles marked to identify each tube element on the meter panel. The tube is then inserted into the proper socket, 4-pin, 5-pin, octal, etc. The tube heater voltage is roughly set by control A and adjusted using the fine adjustment to the correct voltage as read on the left-side meter. Control B should be set to Shorts Test initially. Control C is a variable pot and is set per the roll chart data. Control D is a multi-position switch that is also set per the roll chart data. These two controls (C and D) select the correct voltage divider resistors for proper tube element voltages. The Plate Voltage is an adjustable control that is set to read "100" on the right-side meter. Meter Reset should be fully CCW to start. First the tube is tested for shorts watching the neon lamp. If no shorts are indicated then the switch is set to TEST. Now the arced center meter will read some value of plate current. Using the Meter Reset control the plate current reading is adjusted to zero. Now the Gm TEST switch is actuated and the test value mutual-conductance percentage will read on the arced meter. Diodes and rectifiers only are emission tested. The OQ-2 is not a "quick" or "easy to use" tester and requires some extra data not found on the roll chart (all required data is in the manual however.) The OQ-2 is quite a visually impressive unit that's a lot of fun to use.

Weston-USN Model OQ-2 (1-CV-60072)  SN:724  (Weston Model 788)


The Hickok Electrical Instrument Co.

Shortly after WWII, the military decided to use the basic Hickok circuit for all of their tube testers. Even though the circuit might be Hickok, many other contractor companies built tube testers for the military. Each contractor company had to meet the specifications for the particular tube tester they were building. No matter what the contractor company is, the tube tester will have the same quality and will perform in the same manner. The military tube testers are generally portable and ruggedly built to withstand the rough treatment they would receive in a depot or when being taken to some remote location. The designs of some tube testers favor "simplicity of operation" which would usually speed-up the radio repair. By far the most popular military tube tester is the TV-7 series with the TV-7D/U being the most often seen. The TV-7D/U had the longest production life and had the largest number of contractors building these testers. Other popular military testers are the TV-2, TV-3, TV-10 and the WWII-era tube tester, the I-177.

I-177 - The I-177 (I-177B shown to the right) was based on the Hickok circuit for mutual conductance testing but, like most of the military tube testers, not all I-177 testers were built by Hickok. In fact, the I-177B shown to the right was built by Simpson Electric Company in 1951. The I-177 dates from near the end of WWII (1944) and went through a few updates allowing it to continue to be built up into the early 1950s. During WWII, Noval (aka 9-pin miniature tubes) had not been introduced yet, although seven-pin miniature tubes were being used during WWII. The sockets on the I-177 are four, five, six, receiver seven, octal, loctal, acorn, small five pin and seven-pin miniature. There was an adapter that was available after WWII, the MX-949, that allowed testing Noval based tubes and some other types. The MX-949 was connected with jumpers to the I-177. MX-949 adapters are somewhat difficult to find these days (read "expensive.") A nice feature in the test data book for the I-177 is that it contains a list of all of the military "VT" numbers converted to standard tube designations. WWII versions of the I-177 are usually painted olive drab while the later versions are usually in gray painted cases. This I-177B came from KG7DVA

TV- 7 Series of Tube Testers

The TV-7 is one of the most popular military Hickok-type tube testers. This is probably because of its ease of use, its accuracy, its small size and the large quantity of tube test data that is available. Shown in the photo left is the military TV-7B/U tube tester. Note that the lid contains the Test Data book with all of the settings for testing the listed tubes. The book here is for a TV-7/U. Later books (like for the TV-7D/U) will contain many more tube test set-ups. Pin straighteners and various adaptors were also installed in the lid. If your TV-7 has the older style book you can find additional tube set-ups for the TV-7 on the Internet for almost any vacuum tube (that will fit in the sockets provided.) Hickok didn't build many of the TV-7 tube testers. In fact, the original TV-7 and the TV-7B/U were the only contracts that Hickok was involved with. There were other contractor companies involved on all the versions but especially for the TV-7D (because of its longer production life.)

Calculating Transconductance from the TV-7 Meter Reading - The 0-120 scale used on the TV-7 tube testers requires using the test set up book to know whether the tube being tested is meeting minimum acceptable transconductance. Some other Hickok tube testers read out the transconductance directly. It's very easy to convert the TV-7 reading to transconductance in uMhos by knowing what the full scale settings actually are in umhos. Use the following table.

TV-7 Scale B = 3000uMhos FS -  use Meter Reading x 25 =  Transconductance uMhos

TV-7 Scale C = 6000uMhos FS -  use Meter Reading x 50 =  Transconductance uMhos

TV-7 Scale D = 15,000uMhos FS - use Meter Reading x 125 = Transconductance uMhos

TV-7 Scale E = 30,000uMhos FS - use Meter Reading x 250 = Transconductance uMhos

TV-7 Scale F = 60,000uMohs FS - use Meter Reading x 500 = Transconductance uMhos

Example:  If the tube under test reads 70 on the B scale then 70 x 25 = 1750 uMhos. If you want to know the minimum acceptable uMhos for the tube under test then multiply the "minimum acceptable" number listed in the test book by 25 (for scale B) for the actual minimum acceptable uMhos.  NOTE: Only the TV-7D has the "F" scale.

Troubleshooting and Repairing TV-7 Tube Testers

The TV-7 is a compact and densely packaged piece of equipment. Some parts of the circuitry are easy to access but other areas will require some disassembly to repair. You should have the military manual TM11-6625-274-35 for testing and troubleshooting procedures along with schematics for all versions of the TV-7. There is approximately 330 vac inside the circuitry around the rectifier tubes, so care must be taken in testing some areas of the TV-7. Also, there is no connection to chassis for plate and screen voltage returns. All B+ is referenced to the cathode circuitry. The manual will direct you how to test all parts of the TV-7 and there is a troubleshooting flow chart to go through during the testing process.  >>>

>>>  Once you've located the cause of the failure, the next problem is where to find replacement parts. Some of the parts are standard components and are relatively easy to find. Others will require some searching. The power transformer, for example, used to be available from Fair Radio Sales (and may still be.) Most unique parts are going to require a donor "parts set" TV-7 which will be difficult to find. Some parts can be built. For example, the diode rectifier pack for the "Line Test" function. You don't specifically need the original type of diode pack. Any pair of small silicon diodes that are rated at 300 piv will work. Luckily, most of the repairs are limited to the "Line Test" diode pack, worn out sockets, bad contacts (for a variety of reasons) and bad or wrong tubes. Occasionally, a bad power transformer will be the problem. With the military manual as your guide, it's pretty easy to find the problems. But, locating or building the replacement parts may prove difficult and time consuming.

Calibration of the TV-7 Tube Testers

The accuracy that the TV-7 provides the user is dependent on its circuitry providing the tube-under-test with specific DC and AC voltages and measuring the resulting current flow through the tube. All adjustments within the TV-7 are either locking potentiometers or wire-wound resistors with adjustable clamping slides on the TV-7A/U, TV-7B/U and the TV-7D/U versions. The older TV-7/U versions will have fixed resistors in some areas of adjustment that might need to be reselected for proper calibration. The TV-7B/U chassis is shown to the right.

Equipment Needed - You will need a Variac and an Isolation Transformer for the Simulated Tube Test. You will need a known accurate DVM. Remember, you are going to be measuring both DC and AC voltages and the accuracy of the calibration is dependent on how well-calibrated your DVM is. Metrology labs require that a measuring instrument must be ten times more accurate than the specified accuracy of the device being calibrated and many of the Metrology lab instruments will have their calibration tied to the National Institute of Standards. The point being, be sure your DVM is an accurate instrument before proceeding with the TV-7 calibration.

You will also need the following resistors:

(1) 10K,  (2) 12K,  (1) 100K,  (1) 375K  and  (1) 510K

Try to use 1% resistors for this calibration. Confirm the resistor values by measurement with a known accurate digital ohm meter. The two 12K resistors are connected in parallel (for 6K) for the SHUNT test-calibration.

A good vacuum tube - 6L6 is recommended - actual transconductance is not important but it should test good.

For the TV-7/U you might need an adjustable resistance decade box for selecting resistor values in some of the calibration steps. It won't be necessary for the "lettered versions," TV-7A, B or D.

Use TM11-6625-274-35 (the military TV-7 Field and Depot Maintenance Manual) for the calibration procedure. This is available as a free download PDF on the "BAMA edebris" website.

Tests Required - The tests performed will involve Bias Voltage Test, Plate Voltage and Line Level Test, Screen Voltage Test, Short Circuit Test, a Simulated Tube Test, Shunt Control Test and the Range C to Range B Ratio Test. Be sure to test the condition of the TV-7's  5Y3GT and 83 tubes on another tube tester before starting the TV-7's calibration. Be sure that your TV-7 is functioning correctly before performing the calibration.

As with many calibrations, you'll probably find that the TV-7 under test is already "in calibration" or maybe just slightly off. Sometimes you'll just be confirming that all of the adjustments are correct. If adjustment of one of the slides is necessary just loosen the set screw enough that the slide will easily move and then use a plastic tool or insulated rod to move the slide for the proper voltage and then retighten the set screw of the clamp. Don't over-tighten, just snug is enough. On the locking pots, loosen the lock nut with a 9/16" open end wrench. Then adjust the pot for the proper voltage measurement and, while still monitoring the voltage, tighten (just snug) the lock nut. Sometimes the pot shaft will change slightly with tightening, so be sure to monitor that this doesn't happen. Photos right show the adjustments.

The 6L6 Techno-myth - There's a myth that a "calibrated 6L6" is necessary for adjustment of the TV-7. The myth says that the 6L6 has to be a "new metal 6L6" that must have a "burn-in" of 30 hours on it. This is total nonsense. The 6L6 test is checking the range B (3000uMhos) to range C (6000uMhos) ratio and linearity of the scaling. It doesn't matter what the condition of the 6L6 is - it doesn't even have to be a 6L6. When the tube is set up for the test you must then adjust the Bias on the TV-7 for a full scale meter reading on Range B and then switch to Range C and note that the meter reading is exacting half scale (+/- 1/2 division.) The 6L6 was specified in the procedure because it was an easily available tube that was capable of being adjusted to read FS on Range B.

Other Notes - As with any calibration, if calibration is necessary and the adjustment doesn't change the measured voltage then there is something wrong in either the test set up or in the TV-7 circuit. In most cases, when you've gotten as far as doing the calibration procedure, the TV-7 is probably already fully functional and the problem will be in the hook-up of the test equipment or the resistors used for calibration.

If you've done receiver alignments or other calibrations and if you're familiar with reading and interpreting test procedure "lingo" and you have the proper instruments for calibration - then calibrating the TV-7 is pretty easy. Certainly the "lettered" versions , A, B and D, are the easiest while the "non-lettered" version is a bit more difficult.


TV-2B/U - The TV-2B/U is not a Hickok tube tester. It was designed by Air King, which was a division of Hytron - a famous tube builder at the time. Later, CBS bought Hytron and the tubes produced were labeled "CBS-Hytron." The TV-2B/U is a fairly late tube tester dating from 1958 and built up to 1962. This one was built by J. H. Keeney Co.,Inc. in 1961.

The TV-2 allowed the user to adjust the tube's operational parameters during testing to fully evaluate the tube's usability. Once the tube is operating in the tester, you can "fine tune" the various adjustments to exactly the levels you want. The design allowed the user to set up a tube test by referencing the tube data available in any standard tube manual. Each of the switches are labeled per the element of the tube and then the specific pin number is selectable. When a grid or plate cap is needed, "A" or "B" is offered as an optional connection via the Plate or Grid switch. The red clip lead is "A" and the black lead is "B." The white toggle switch is the "test" switch and it can be left in the "on" position (freeing up that hand) while the various voltages are adjusted to the levels desired or specified. The "Quality" meter is percent quality up to 150% with 100% being the manufacturer's specified transconductance for a new tube of type being tested.

If you're in a rush, the tube can be tested using the recommended set up shown on the roll-chart and the voltage levels specified will have red indicator lines on the meter scales for ease of adjustment. The TV-2B/U is a really fun tube tester to use - but not if you're in a hurry. Using the TV-2 can easily double your tube-testing time if compared to the TV-7.


Sylvania Drugstore Tube Tester - DIY TV Repair

Nearly everyone - if you're over 50 years old anyway - probably remembers seeing this type of tube tester in many types of stores years ago. Drugstores were popular places to find these DIY tube testers as were variety stores, sometimes even grocery stores. Most of us remember going along with an older family member (father or older brother was common) to test all of the tubes from the family television. This was an attempt to avoid calling the local TV repairman and trying to fix the TV yourself (DIY - do it yourself.) It was expected that the user of these types of tube testers really didn't have an in-depth knowledge of TV circuitry so the DIY tube testers were designed for "easy to understand" operation, fairly accurate testing of the tube's condition and, naturally, having a rather full supply of new tubes kept in roll-out shelves in the lower section behind the locked door. Storage trays in front on the Sylvania example shown are marked "Tubes to be Tested," "Good Tubes" and "Bad Tubes." These DIY Tube Testers relied on the probability that the most common failure in a vacuum tube television was going to be the Horizontal Output tube. Generally, the tube heater would go open and you'd loose the picture but still have sound. These types of tube testers would have no trouble finding the open heater and thus solve the problem for the "DIY TV repairman." More subtle problems might prove difficult since the tube tester is essentially an emission tester. Overall, the Drugstore Tube Testers were successful enough that the sales of new tubes more than paid for its space in the store. The last time I saw one of these testers actually set up and ready to use in an actual store was about 1980 in the Thrifty Drugstore in Carson City, Nevada.

This Drugstore Tube Tester wasn't located in a drugstore, however. It was located at Jerry's Furniture Store in Yerington, Nevada. Jerry Herman not only sold furniture, he repaired TVs. Everyone in town knew the tube tester was down at the furniture store. Jerry also knew that about half the time, the "DIY TV repairman" was going to find that all of the tubes tested okay but his TV still didn't work. Therefore, Jerry would get the job to find out what actually was wrong with the set. When Jerry retired, he moved to Virginia City, Nevada and lived there several years. The tube tester, along with a lot of other TV parts, radios, TVs and other things were stored in an airplane hanger at the Carson City Airport. Eventually, Jerry and his wife moved to Winnemucca, Nevada. Sometime later, Jerry's wife came to our museum (when it was open in Virginia City) and wanted to know if I'd be interested in "clearing out" all of the junk that was in the hanger since they needed to sell their airplane that was also there. I agreed and started on the clean up a couple of days later. I found out that airplane hangers are not weather-tight. Almost all of the wooden cabinet radios and TVs had been destroyed by rain and snow blowing into the hanger over the years. Fortunately, this tube tester was pushed into a corner that was out of the weather and it remained in excellent condition.

I had this tube tester located in the Western Historic Radio Museum for several years. I have to say that it generated the most comments of any of the exhibits - including the 1912 Dodd Wireless Spark Station (it was second.) "I remember those!" was the usual comment that was then followed by telling a story about how they had gone down to the store with their father and tested all of the tubes in the TV and had actually "repaired" the TV. I got this same story at least once or twice a day, so I can state positively that testing tubes at the local store tube tester was a popular method that was used by lots of people over many years for "DIY TV repair." I'd guess the popularity of the Drugstore Tube Testers ran from 1955 up to about 1980.



Clough-Brengle - "Graphoscope"  Model 126

The oscilloscope is essentially a "visual voltmeter" that allows the user to see a voltage level, that may be varying (AC,) over a specific period of time. Early oscilloscopes didn't have calibrated vertical amplifiers so you couldn't measure voltage levels. The 'scopes also didn't have a calibrated horizontal sweep rate so you couldn't measure time or frequency. That left the user with the ability to see characteristics of a waveform but not much else. As 'scopes evolved, many improvements were made to the vertical amplifiers allowing fairly accurate measurements of waveform's pk-pk value directly. Additionally, the sweep circuits became calibrated, allowing the user to measure frequency indirectly (f =1/t) since the sweep circuits were "time based" and selected time periods per division. These types of 'scopes had a graduated scale in front of the CRT that allowed "volts per division" vertical measurement and "seconds per division" horizontal measurement. Delayed sweep allowed a user to look at one section of a waveform and expand only that section for visual analysis. Selectable triggers allowed the user to have the display appear only on specific "trigger" events. By the late-1950s, 'scopes were becoming very important test instruments but their ability to see very fast events (high frequencies) was still somewhat limited. As 'scopes became "faster" other evolutions in technology brought on digital circuitry that required the user to be able to see very fast "pulses." Storage 'scopes allowed the user the ability to "hold" a display for comparison to other displays. Multiple channel vertical amplifiers allowed simultaneous comparison of several signals. Today, the 'scope is integrated into a computer display that allows the user even more abilities to see, measure and store different types of waveform voltages and frequencies (or time.)

Shown in the photo to the right is the Clough-Brengle "Graphoscope" Model 126. It is a very early model that dates from before WWII. Although there are many adjustments with a multitude of controls, the "Graphoscope" (due to its age of design,) doesn't have calibrated vertical amplification or calibrated horizontal sweep. The metal loops on the top were for a leather handle and the cabinet is dark green wrinkle finish - cool.  A swap meet find.


Allen B. DuMont Company

Model 164-E Oscillograph


Allen B. DuMont was one of the early builders of "oscillographs" (as his company called them.) DuMont was involved with Westinghouse and the DeForest tube factory in the 1920s. He got into early television when he bought out Jenkins' television lab in the late-twenties. DuMont developed the cathode-ray tuning eye tube and sold the patent to RCA. As DuMont developed his televison company, of which oscillograph production was only a small part, he partnered with Paramount Pictures to supply the much-needed money to further develop his television company. The partnership wasn't without disadvantages since it gave Paramount the power to elect their own people to positions within the DuMont company hierarchy. Another problem involved FCC regulations that prevented DuMont from expanding his television broadcasting. The FCC wouldn't allow DuMont to own more than four television stations because of the involvement of Paramount (movies.) Although the DuMont network did produce a few programs through its small network, it was very limited in its success. By the 1950s, the DuMont Company was sold off in various sections. Oscillographs were produced from the early thirties to the late fifties.

The Model 164-E was first offered in 1940 for about $58. It continued to be offered even after WWII with a 1949 price of around $127. Lots of controls and a little more versatility than the C-B Graphoscope shown above. Still, the 164-E is a pretty basic 'scope.

Donated by W7GNV.


Tektronix Corporation

 Tektronix was founded in 1946 by Howard Vollum and Melvin Murdock. Originally, the company was in Portland, Oregon but a land deal in 1957 allowed the company to build a new facility in Beaverton, Oregon, which is still their headquarters location. Tektronix produced advanced design oscilloscopes that used a time-based sweep circuit allowing accurate frequency and time measurements of displayed 'scope waveforms. Priced around $750, the first Tektronix models were about twice the cost of the competition. However, the competition didn't have the time-based sweep circuitry, so Tektronix eventually prevailed in the industrial electronics marketplace. Generally, the electronics industry wanted Hewlett-Packard for most of their test equipment except for 'scopes with Tektronix being the favored oscilloscope. By the 1990s, Tektronix was having many financial problems that eventually required selling off some of the company divisions. After Tektronix recovered they eventually became a somewhat attractive investment and, in 2007, Tektronix was purchased by Danaher Corporation. Today, Tektronix still provides the industry will precision measurement instruments but as a subsidiary of Danaher Corp.

Shown to the right is a Tektronix Type 535 - a lab quality, 15mc 'scope with lots of tubes and two large fans for cooling. It was introduced in 1953. This 535 belonged to my old radio collector friend Fred Winkler. Fred's company was called Tyoby Electronics and it was located in Mound House, Nevada (just outside Carson City.) Fred supplied the Reno-Carson City casinos with various types of Keno-number display boards. At the time the technology required individual drive lines for each of the eighty lamps that comprised the Keno number illumination within the "board." Although called a "board" a more accurate description would be "display housing." These Keno boards were usually about four feet square and about one foot deep. Several display boards were installed on walls all around the casino so, no matter where you were on the casino floor, you could always see the "picked numbers" for a current game. The installation using individual drive lines to the lamps coming from the Keno number selector keyboard was daunting. Fred's last boards were serial-digital "two wire" drive units that made installation much easier. This 535 'scope was always on Fred's workbench until he gave it to me (in about 1985.) I've used it many times but it's really huge, heavy and very noisy with the two fans blowing. Nice sharp trace though (after all, it's only a 15mc 'scope.)

Shown in the photo to the left is the Tektronix Type 561B - a lab 'scope from the late-1960s. The 561B is a real hybrid with completely solid-state regulated power supplies and combinations of discrete socket-mounted transistors, nuvistors and vacuum tubes in the vertical amplifiers and in the time base. The vertical amplifier shown is the 3A74 which has the ability to produce four traces from four separate inputs. The standard dual trace vertical amplifier is the 3A6 which I also have for this 'scope. The time base is the 3B4 with the ability to sweep down to .05usec per division (20mc.)

Around 2009, this 561B was in a "load" of test gear that I was given by a former Bently Nevada Corp. employee. He had obtained much of the test gear from various sources besides BNC over the years and had stored it all in a dusty garage. At least the garage was weather-tight but it certainly had its share of sawdust, insects, grease and sand, all of which ended up inside all of the test equipment. When the fellow delivered the load of test gear up to Virginia City (to the Western Historic Radio Museum) he specifically mentioned that the only piece of test equipment that he had "plugged-in to try it out" was this 561B. He said, "I heard a loud "snap" and then I couldn't get it to do anything. I guess I blew a transistor!"

I stored the 561B with the intent to "check it out" but I never got around to it while the museum was open and while we were living in Virginia City. With the move to Dayton, Nevada, I had the time to take a look at the 561B. It was filled with fine sawdust, sand, dirt and really just about anything that could fit through the small vent holes of the cabinet. I removed the vertical amp and the time base so I was just looking at the main frame of the 'scope. I began cleaning so I could see what damage might have happened from the "snap" that had been reported. A quick cleaning didn't reveal anything unusual. I checked the power supply board components along with testing and reforming the filter capacitors. I noticed that the board-mounted 150mA fuse for the +HV was blown. I decided to test the low voltage supplies without the +HV fuse installed. All supplies came up fine and within spec. A careful power up with a new fuse installed showed that I had +HV (+3KV) although adjustment was necessary since it was out of spec (>10% high.) I think what happened was the "plug-in and power-up" performed with all of the dirt and dust inside the 561B had caused an arc of the +HV to chassis and that blew the fuse. Thorough cleaning of the main frame, the vertical amp and the time base and installation of a new power cord got the entire 561B functioning fine. Well,...maybe a few noisy pots and switches but overall working pretty good. A really nice "sharp" trace.

Other Oscilloscopes

Shown in the photo to the left is a Heathkit "5 inch Push-Pull" Oscilloscope Model O-7 that is from the mid-fifties and was owned by Howard Gates Sr. who at one time was chief engineer for Detrola Radio Co. Gates also worked for Colonial Radio and for Warwick Mfg.Co. This 'scope was given to me by his son, Howard Gates Jr. Even though this Model O-7 is from the 1950s, still most 'scopes of that time won't have calibrated amplifiers or calibrated sweep. 

Shown in the photo to the right is USN OS-8C/U oscilloscope. These small 'scopes were easy to carry around and very light-weight. A multitude of controls are provided but the vertical and horizontal amplifiers are not calibrated. The sweep circuit is calibrated in frequency not time. There are civilian 'scopes that appear to be close copies of the OS-8 (Waterman is one) but in reality, who copied whom? The Waterman "Pocket Scope" seems to be an early civilian version that pre-dates the OS-8. This particular OS-8C/U belonged to Al Chin who used to work on the Reno Police radio equipment. It dates from the early-sixties.



The panadapter is a spectrum analyzer that was specifically designed to observe the passband of a superheterodyne radio receiver. Most early panadapters were designed around the common 455kc intermediate frequency of most radio receivers. By connecting the panadapter to the plate of the vacuum tube Mixer stage of a superheterodyne (use either a high value resistor or a very small value coupling capacitor to avoid damage to the panadapter's input stage,) one can observe the widest bandwidth of the spectrum based on the sweep design maximum of the panadapter and the receiver front end selectivity. Usually about +/- 50 kc bandwidth can be observed although this is adjustable with controls on the panadapter. The original purpose of the panadapter was to allow the user to "see" signals that couldn't be heard because they were outside the receiver's IF passband. The signals would appear as positive-going peaks on a base line trace. The user could then tune the receiver to investigate the observed signal. In surveillance situations the panadapter was a useful tool. Later versions of the panadaptor were called Spectrum Display Units by Nems-Clarke and were usually paired with their VHF receivers for observing missile telemetry. Today, many modern ham transceivers that interface with computers will provide types of panadapters that allow the user to "see" the characteristics of received signals or to observe signals within a specified passband with the information displayed on the computer monitor.

Shown above right is the USN RBU-2 Panoramic Radio Adapter from the WWII-era (built by Warwick Mfg. Corp.) The RBUs were used with the USN RBB and RBC receivers that used a 400kc IF. Many of the RBB and RBC receivers will have an additional tube circuit in the IF section. This is a buffer stage output to drive a panadapter. Panadapters were sometimes used to monitor a superhet receiver tuned to the 500kc emergency frequency allowing any signal near 500kc to be observed and tuned to if necessary. This RBU-2 came from KG7DVA.

Shown in the photo left is the Signal Corps BC-1031-C (built by New London Instrument Co.) designed for 455kc operation. The earlier versions of the BC-1031 are black wrinkle finish but the "C" is smooth finish satin black. Generally, the chassis is heavily coated with MFP which protects much of the components and the hardware from corrosion. Most BC-1031 units will work with original parts. Be sure to check everything over before applying power.

The RBU and the BC-1031 were not really designed for analyzing the characteristics of a signal. The intent was to use these panadapters to find signals that were outside the surveillance receiver's IF passband. Once seen, the operator could then tune to the signal (which was seen on the panadaptor screen as the "peak" traveling across the graduated scale to the center at which point the signal is heard in the receiver.) The operator could generally tell what kind of signal it was from the display - either CW or AM. Odd characteristics could also be spotted by the visual representation of the received signal. Detailed analysis of the modulation was difficult and not usually important.


Transmitter Modulation Monitors

National Co., Inc.


Hams used simple oscilloscopes to monitor modulation levels of their AM transmitters. National Co. offered oscilloscope monitors very early - in the mid-thirties. These monitors do not have calibrated vertical amplification or calibrated sweep. The intent was to use a sample signal from the transmitter modulator to drive the horizontal plates and to use an RF output sample to drive the vertical plates. The resulting display was a "trapezoid" if the modulation was near 100% negative and there was no distortion in the RF envelope or in the modulation. Since the voltages involved in sampling the audio modulation signal were fairly high usually an adjustable voltage divider circuit was used to adjust the input level to the monitor's horizontal plates for a good looking display. The RF input was sampled with a pick-up coil. Many transmitter and modulator problems could be easily spotted using these oscilloscope monitors.

Shown above are two National Co. modulation monitors from the late-thirties. The photo left shows the CPO with a 3" CRT. The CPO came from NU6AM. The photo right shows the CRM with 1" CRT. The CRM belonged to John Ridgway W3ON.

Sylvania Electric Products, Inc.

Direct-Reading Modulation Meter - Model X-7018

An oscilloscope-type of modulation monitor has the advantages of an instant, visual image to analyze but it is subject to interpretation by the user and requires experience to be a valuable tool. A simple, effective method of monitoring AM modulation percentage is to use a meter. Sylvania offered a very easy to use and accurate modulation meter that took advantage of their recent small packaging of the 1N34 germanium crystal diode. Prior modulation meters had used vacuum tube diodes that limited the carrier frequency response to about 30mc. With the 1N34, detection-rectification was possible to well over 55mc and could actually be relatively accurate at 145mc. In addition, no heater voltage and no plate voltage was required when using the 1N34. This allowed Sylvania to produce a modulation monitor in a very small package that only required that the user install a pick-up link-coil near a transmitter PA and to perform a few adjustments to end up with an accurate and easy to use modulation meter. The design appeared in the February 1947 issue of QST. 

A pick-up link of one or two turns of insulated wire with a suitable length of 75 Z ohm coax is installed in the transmitter final area and connected to the Mod Meter. With the transmitter tuned and in operation, the "METER READS" switch is placed in the RF position. The "PEAKS" switch can be in either POS. or NEG. The Mod Meter now reads the carrier level of the transmitter. The trimmer on the left side of the Mod Meter is adjusted to peak. The user then has to position the pick-up link to have the meter read "100." Link adjustment has to be done with transmitter power off. When link position results in the meter reading "100" with the transmitter set up on its operating frequency and at the desired output power then the "METER READS" toggle is switched to "AF" and the Mod Meter will read the modulation level of an AM signal. The "PEAKS" switch determines whether you measure the positive or negative going peaks. The trimmer allows for minor readjustments when changing transmitter frequency.

Donated by W6MIT 


General Radio Co.

AM Modulation Monitor - Model 731-A

AM Radio Broadcast stations were required to monitor the percent of modulation of their transmitter. Most modulation monitors were designed to comply with specific FCC regulations which stated the necessary requirements. The General Radio 731-A Modulation Monitor dates from about 1938. A variable capacitor tunes a pick-up inductor mounted near the tank coil of the broadcast transmitter. This input is then detected to drive the Carrier Meter. Regulated power supplies deliver reference voltages that compare the detected signal/lp filter output and give a percentage of modulation readout. Another circuit allows a variable setting of an Over Modulation warning lamp. Outputs for monitoring are accessed via a Jones Plug on the rear panel. The operator can select Positive or Negative Peaks for readout on the modulation meter. It's interesting that the Over-Modulation warning lamp is an incandescent lamp which would be rather slow to illuminate on over-modulation peaks. More modern Over-Modulation indicators used a "fast responding" neon lamp for a warning indicator.

GR731A.jpg (21241 bytes)

Lampkin Laboratories, Inc.

Type 205  FM Modulation Meter

Measuring the modulation level of an FM transmitter requires special equipment. Many times it was a "maintenance" check that would require portable test equipment since the user would be checking different transmitters in different locations. Lampkin supplied the Type 205 FM Modulation Meter for the technicians that were checking and maintaining "business" and "service" transmitters, most of which were FM transmitters. This Type 205 belonged to Al Chin and his dymo-lables are still on the tuning dial bezel. Al Chin maintained the police transmitters for the city of Reno up to about 1970.



Digital Voltage and Frequency Measuring Instruments

Non-Linear Systems, Inc.

Model 451 - "Digital" DC Voltmeter

Non-Linear Systems, Inc. was started by Andrew Kay in 1952. The original location was in Del Mar, California but later the company moved to San Diego. NLS is credited with creating the first digital voltmeter in 1954. Later, in the 1980s, Kay started Kaypro to produce home computers. Kay died in 2014 at the age of 95. NLS is still in business in San Diego.

This isn't the "first" digital voltmeter from NLS but it's an early one that uses vacuum tubes and incandescent lamps (probably dates from the late-fifties.) The 451 uses various analog comparator circuits to create a "selector circuit" that operates stepper-motors that search for a "null" and when the stepper-motors stop the circuit will turn on one of ten incandescent lamps that are arranged above each of the four Digit Assemblies. Each Digit Assembly consists of ten plastic pieces with numbers engraved into the plastic that are stacked "front to back." When one plastic piece is "edge lighted" the engraved number appears to "glow." There are four levels of ten numbers each allowing the voltmeter to readout the input voltage "+/-0000" as a "digital" display. Note in the photograph to the right that the numerals appear to have depth running from front to back. There are 43 lamps total used in the digital display. The incandescent lamps are spring-loaded within their holders in the Digit Assembly which is held in place with the lamps contacting the printed circuit board pads. The lamps are "wheat-germ" types similar to the #328 lamps used in the R-390A receiver. The entire upper part of the 451 chassis is a horizontally-mounted circuit board that has downward facing sockets that hold the stepper-motor driver boards. The entire area is sealed with foam rubber pads for sound-proofing of the stepper-motor action. The noise created by the stepper-motors resulted in these NLS DVM being referred to as the "box of snakes." In the lower portion of the 451 is the power supply and the stepper-motor chopper circuitry. Input to the 451 is via screw terminals located on the rear of the chassis. A cable from the terminals is then routed out through the back panel and then to whatever was going to be measured. The Calibrate switch on the front panel is spring-loaded and, when held in position, the pot located behind the hole in the panel above the switch was adjusted so the display read whatever was written into the blank portion of the label.

Currently, the Model 451 has not been restored. It was given to me by my old radio collector friend Fred Winkler around 1988. At the time, Non-Linear Systems offered a data packet on the Model 451, which I ordered. To my surprise when the packet arrived I found that Non-Linear Systems had apparently reassigned the same "451" number to a modern small panel-mount digital voltmeter. I've never been able to find any info on this early NLS Model 451. 


Westport Electric
El Segundo, CALIF

 WE-140  Frequency Meter & Counter

This is an early digital frequency counter that doesn't provide the user with actual "numbers" in the readout. Instead the Nixie tube displays, called dekatrons, are a series of ten "dots" that are arranged like a "clock face" As the counter is sampling the frequency the "dots" appear to "spin" around the clock until sampling is completed at which time the "dots" lock in the finished position. The user then has to read each digit somewhat like a "clock face." The Westport Electric counter is for low frequency applications as it's maximum readout is 9999 Hz. Probably dates from the late-1950s.

In this close-up photo the WE-140 is shown in self-test. If it were actually reading an input frequency it would be 9993 hz. Note that the "units" and "tens" dekatrons appear to be a lighter color than the "100s" and 1000s." This is because a "faster" dekatron is used in the "units" and "tens" position since these "dots" do change rapidly. The "faster" dekatrons used a slight amount of hydrogen mixed with the neon to "speed up" the response.

General Radio Company

1192-B  Digital Frequency Counter Using Numerical Nixie Tube Display

The commonly found Nixie tube was produced by Bouroughs and required a specific numerical decoder IC that was either built into the special socket base or it could also be supplied as an externally mounted (on a PC board) decoder IC. The Nixie tube itself requires a fairly high voltage (~+40vdc) to ionize the gas and produce the orange glow around the selected numeral. Although Nixie displays are beautiful they do have a longevity issue in that extended periods of time showing the same number will migrate a deposit of metal material to the inside front of the glass causing a pattern that "mirrors" the number with a reduction in clarity of the display. In severe cases, the numerical display will be difficult to read at all. This problem is found more in Nixie tube-equipped industrial monitors that display the same reading for very long periods of time and are operated 24/7. Digital Frequency Counters are seldom found with "silvered" Nixie tubes. Shown is the General Radio 1192-B Counter (measuring the output from an HP-606B set to 3974 kc.)


Test Bench Power Supplies

    Lambda Electronics Corporation  -  Model 25 Regulated Power Supply

Hewlett-Packard Company  -  Model 712B Power Supply

Back when vacuum tubes dominated the electronic industry and designing or testing a vacuum tube prototype device required some "real juice," there were power supplies available to perform the task.

Lambda Electronics Corporation can be traced back to around 1948 building high-quality power supplies for various uses. Several mergers over the years resulted in the company being called Nemic-Lambda beginning in the late seventies and with the latest merger (in 2008) being called TDK-Lambda.

The Lambda Model 25 is an adjustable B and fixed A power supply. The B+ is regulated and adjustable from +200vdc up to +325vdc at 100mA. The A supply is 6.3vac at 3A. This type of power supply can power up a 8 or 9 tube receiver. The National HRO, for instance, requires 6.3vac at 3A and +230vac at 70mA. The Lambda 25 can provide this with ease and additionally, the B+ is regulated where the original National B+ power supply wasn't. Also, these types of supplies are ideal for reforming electrolytic capacitors. A separate current meter will be necessary along with some method to limit current as a safety measure. The Lambda 25 was available in either a table model or in a rack mount configuration (Model 28.) The table model is shown.

HP-712B Power Supply - The H-P 712B is the ultimate in a vacuum tube-based, adjustable B+ and fixed high-current A+ and adjustable C- bias supply. The 712B is massive with plenty of current capability in the B+ which can be adjusted from  0  to +500vdc. Current available is 200mA. The A supply is 6.3vac at 10A and the C- bias supply is adjustable from 0 up to -150vdc at  30mA along with a fixed -300vac at 3mA. A variac on the primary of the B+ transformer is how the B+ voltage output is adjusted using the large knob between the meters. Regulation is by way of four 6L6GB tubes. The 712B was available in either a cabinet table model or in the rack mount configuration. The rack mount version is shown in the photos below.

This 712B is awaiting restoration. As can be seen by the photo to the lower right, the 712B was stored for decades in a garage where all kinds of dirt, dust, insects and other contaminates drifted down through the holes in the top cover (this HP 712B came from the same place as the Tektronix Type 561B oscilloscope profiled in a section above.) Sometimes this type of dirt can have a protecting effect leaving the metal finish underneath in perfect condition. Fortunately, that is the case with this 712B (yes, I have cleaned the chassis since the photo.) Also, since this type of equipment was used in test facilities where the operator or calibration tech wasn't the owner, sticker residue and acceptance stamps are all over the front panel. Sometimes these can be removed but often times it's better to just leave the residue alone since removal will probably damage the panel paint. At least this piece of test gear wasn't "plugged in and turned on to see if it worked." 



Variable Voltage Transformers - Variac, Powerstat

 Variac - General Radio Co.,  Powerstat - Superior Electric Co.

The ability to adjust the AC line voltage has several advantages and the device needed to accomplish this is the Variac. Sometimes referred to as a Powerstat, which is actually the product name from Superior Electric Company (now owned by Danaher Corporation who also owns Tektronix) or officially as a Variable Voltage Transformer (and sometimes as an adjustable auto-transformer.) These are all various names for the same device that we'll call a variac. Variacs come in many sizes from very small panel mounted types for adjusting AC voltage for a specific function in an electronic circuit up to very large variacs for adjusting the AC line voltage to power large pieces of equipment. Variacs are available for single phase AC or three-phase AC. You can get variacs that are "stacked" for simultaneously adjusting more than one AC output. Variacs can be wired to output voltages higher than the line voltage. Whenever the requirement is to adjust the level of the AC line voltage or to bring the level of the AC line voltage up slowly, a variac is the device that's normally employed.

The variac's winding is connected across the AC line with Neutral at one end and Line (Hot) usually at the next to the last tap. The output is taken from an adjustable contacting rotor arm that uses carbon brushes to make contact against the individual turns of the main winding. The return end of the output is referenced to the Neutral end of the main winding. Due to transformer action it is possible to actually adjust the rotor to gain more voltage than the AC line depending on where the Line is connected on the winding - next to the last tap for "higher than AC line output" is typical. With 120vac to the main winding you can usually adjust the rotor to achieve about 140vac maximum output. Ratings are usually in amps or KVA and the average test-bench variac is typically rated at about 10 amps.

When using a variac there are a couple of important things to remember. First, there is no isolation from the AC line when using a variac. If it's necessary to have the device you're powering (and testing) isolated from the line then you'll have to use an Isolation Transformer between the variac and the device under test. Second, never switch the variac ON with a load with  the rotor adjusted to any level other than zero volts. Always start with the variac set to zero, then switch on the AC and then adjust the variac rotor to the AC voltage level that's needed. When powering down the variac, again return the rotor to zero before switching off the AC input voltage to the variac. Full voltage switching with a load connected (either ON or OFF) will cause burn spots on the surface of the windings and will eventually cause damage to the brushes and winding surface.

Shown in the photo upper right is the General Radio Type 100-Q 18amp Variac. This unit dates from the 1940s. From KG7DVA

Shown in the photo left is the Powerstat Type 116 made by Superior Electric Company. This variac is rated at 7.5 amps and features a handy AC outlet. This style of work bench type variac is still being made although this particular one dates from the 1940s. From W3ON

Shown in the photo right is a later style (early fifties) General Radio variac, the Type V 20M which is rated at 20 amps. The 1951 GR selling price was $55. I purchased this V 20M from Bently Nevada's inactive stock.

When I was working a Bently Nevada Corporation (1973 to 1997,) from time to time I would purchase various types of used equipment that was in "inactive stores." This was fairly easy to accomplish only requiring some paperwork and payment. Things began to change in the early 1990s when all employee purchases then had to go thru the Purchasing Department. About that time, I spotted an old 1940s General Radio 100-Q 18 Amp Variac over in "inactive stock." When I asked the Purchasing Dept about buying the GR Variac I was told, "We'll have to look at what those are selling for in the catalog." To which I replied, "Good luck, that variac is from the late-1940s. It won't be in any current GenRad catalogs." The purchaser then said, "Well, then we'll have to find something like it and see what the current replacement cost would be." I could see this purchaser thought a 50 year old variac had no depreciation so I gave up trying to buy it. Six months later I was looking around out in the back "scrap yard" and I saw the very same GR Variac "tossed" in a large metal 55 gallon oil drum (filled with pieces of scrap aluminum and other metal parts) ready to go to Reno Salvage. What a waste, but that's bureaucracies, I guess.

Miscellaneous Test or Measuring Instruments

General Radio Company

Type No. 761-A  Vibration Meter

Excessive vibration in rotating machinery is generally an indication of bearing wear or of inbalance. The ability to measure a machine's vibration and then log that data to keep track of progressive bearing wear can reduce "down time" by allowing the user to plan maintenance. The easiest way to measure vibration is to use a seismic-measuring device in contact with the machine case. This method is actually measuring a combination of shaft vibration due to bearing wear and machine case vibration that could also be affected by mechanical resonances. In any event, if the data is logged and a slow, steady increase in case vibration noted then bearing wear would be indicated. The GR-761-A Vibration Meter uses a handheld transducer that is a pizo-electric device (Rochelle Salts crystal element) that outputs a varying voltage depending on the movement of the "stinger" that is contacting the machine case. This voltage is proportional to the velocity of the case movement but using phase-shifting circuitry, the velocity measurement can be converted to show acceleration or to show displacement (vibration.)

The GR-761 was intended for measuring the vibration of almost anything that might have unwanted vibrating movement. This could be building frames, support beams or other structures that had resonances or other types of unwanted movement. The ability to measure vibrations could then allow the user to find the cause of the unwanted movements. Rotating machinery and case vibrations on reciprocating machinery were other possibilities. Any place that the "stinger-tip" could be pressed against could be measured for vibration. Output is in micro-inches. Push buttons allow for selecting Velocity, Acceleration or Displacement and also for scaling. Power is supplied by a single multi-tap battery. An AC receptacle is provided for using the AC line as a calibration source (not as a power source.) Selling price in 1951 was $490.


General Radio Company

Type 1390-A   Random-Noise Generator

One would think that there's enough noise easily available everywhere so why have a "random noise generator?" Since the output of the Random-Noise Generator is so broad in the frequency spectrum, it becomes a useful instrument to test responses of loudspeakers or microphones. Room acoustics or filter responses. Anywhere that a very broad frequency spectrum of intentional electronic noise would provide means to measure responses without the need for sweeping frequencies or testing a specific frequency responses at many points along the spectrum. The 1390-A uses a 6D4 tube as the noise generator and due the gas discharge nature of the tube and the fact that it is surrounded by a magnetic field, it produces a wide spectrum of "white noise," which is basically a "hiss-like" static. Two 6AQ5 tubes are used as a two-stage amplifier with filters between the stages. The OUTPUT control adjusts the level of noise along with the HIGH-LOW switch. The RANGE selects the filters to provide noise within this upper limit specified by the switch position. Selenium rectifiers are in the power supply and a 3-4 ballast tube is also used. Output levels are around 1.0v P-P with no load. Output Z is around 800 ohms. The output could be used to drive other equipment. Sold for $275 in the early fifties.


General Radio Company

Model 1558-BP   Octave-Band Noise Analyzer


General Radio built test instruments for sound applications in addition to RF instruments. Shown is an instrument for measuring various types of sound - actually noise, which would be a term that implied "unwanted" or "irritating" sound. Since these types of sound generally have specific audio bandwidths or might even have narrow bandwidth peaks, it would be necessary to identify where in the sound spectrum the noise was happening. The 1558-BP uses various filters that can be roughly tuned in octaves to allow the user to find where in the sound spectrum the noise was loudest and to measure the sound level of the noise in DB. A dynamic microphone was used for the input under normal conditions where the sound or noise was rather loud. Less powerful noise would require an accessory sound level meter or other device to drive the 1558-BP. The phone jack below the Cannon mic input is for a sound level meter.

The 1558-BP is battery operated and uses rechargeable batteries. The charger input is the Jones' plug receptacle on the lower front panel.

Some uses for this device would be measuring noise levels around aircraft or vehicles. Analysis of environmental noises in offices or factories. Studies of environmental noise as related to hearing loss or damage. As one can see from the applications, it's not surprising that this 1558-BP was property of the Utah State Department of Social Services - Environmental Dept.

This is a rather late production unit from 1970. Note that the case is the General Radio style that allows the instrument to be placed in any convenient position for use and then the lid can "wrap-around" and be closed up providing a cover over the front panel and a carrying handle. Also, these late-production instruments have gray knobs and panels and generally use much more plastic in the dials and meter cases than the earlier GR instruments.


The Goldak Company

Model U-238 Geiger Counter

I didn't know whether to include this instrument or not. It's not really "test gear" but it is interesting and it is vacuum tube circuitry so at least it's vintage. Prospecting for uranium was somewhat popular in the early 1950s and Nevada was the perfect place to go looking. Maybe not rich in uranium but with wide-open spaces nobody was going to bother you as you wandered around the desert checking out different rocks and deposits with your little "hand-held" geiger counter with your headset on. Battery operated using two D cells for the filaments and two 45vdc B batteries in series for the plates of the two 1U5 vacuum tubes. Also, a 1B85 "pick up tube" and a CK1038 (appears to be a neon bulb) are in the circuit. The holes in the bottom and side of the case are to allow the uranium radiation to hit the "pick up tube" and cause "clicking" in the phones and some meter movement. All great fun and at the very least it was great exercise and got you out into the Nevada desert sun.

This U-238 belonged to John Ridgway W3ON and I don't think that he ever found any uranium with this instrument. I also got a couple of books from John on uranium prospecting and on this instrument. I haven't powered up the U-238 yet - but it's tempting.


C. G. CONN Ltd.

Strobo Tuner  Model ST4

Here's another interesting test instrument that's not really for testing electronic devices but does employ vacuum tubes in its circuitry and it is vintage. It's a CONN Strobo Tuner, a device for precisely setting temperament when tuning keyboard instruments, especially pianos.

The twelve tone scale that western music uses doesn't necessarily have specific frequencies for specific notes and related intervals if string instruments are being played. The string performer is constantly adjusting what is called the "comma" while playing his instrument which allows him (or her) to play perfect fifths and perfect thirds, regardless of the key or key changes within the piece that is being performed. Unfortunately, the tuning of keyboard instruments is fixed and can't be adjusted while being played. Various archaic methods of tuning keyboard instruments usually gave the performer the ability to play "in tune" in three major keys and the related minor keys. Pythagorean tuning was popular in the early seventeenth century and allowed the keyboardist to play with perfect fifths which sounded very pleasant but, without retuning to a different "base" key, the instrument could only play "in tune" in three major and three related minor keys. All of these various early keyboard tuning methods seriously limited the music composer's ability to add variety by way of changing keys within the piece (called modulation.)

Around the time of J. S. Bach, "Equal Temperament" was introduced and was popularized by Bach. Equal Temperament spread a pre-set error within the tuning that was hardly noticeable. By slightly flattening the fifths and sharpening the thirds, a performer (or composer) could play in any key without retuning the instrument. Bach composed "The Well-Tempered Clavichord" as a demonstration piece that featured 24 preludes composed in the 12 major and 12 related minor keys to show that Equal Temperament worked and allowed greater variety in how music would be composed and performed in the future.

So much for the history of how keyboard instruments were tuned. Since tuning a piano or other keyboard instrument required the tuner to judge just how much to flatten the fifths, all keyboard tuning was slightly different depending on the ability and the method used by the tuner. >>>

>>>  Enter the CONN Strobo Tuner. By using various frequency oscillators that beat (heterodyne) against the incoming piano note (picked up by the microphone) a rotating strobe wheel is observed through the central window in the panel. The pattern appears to rotate in one direction if the note is flat and the other direction if sharp. When the note is in tune, the strobe pattern appears stationary. Although seven octaves are available on the strobe scaling, usually only the "temperament" was set on the central octaves and the remaining tuning completed "by ear." To calibrate, a pitch fork was used and the "Sharp - Flat" trimmer adjusted for a stationary pattern. Then the Strobo Tuner was calibrated for all the other frequencies for the twelve tones necessary for setting temperament. The ST4 shown is from the late-fifties or early-sixties.

Since the Strobo Tuner is loaded with paper capacitors to set the frequency of each of the twelve oscillator "tones" it's likely that most of these vintage Strobo Tuners can't be relied upon for accurate setting of temperament when operated "as-is." The crystal microphones generally aren't functional either since rough treatment was the norm for these tuners. If you plan on using a vintage Strobo Tuner, verify the oscillator frequencies for all twelve tones. Don't haphazardly replace all of the capacitors and expect the frequencies to be correct - you have to check them. To fully rebuild a vintage Strobo Tuner will require accurate test equipment to verify correct operation afterwards. Not to worry though, modern solid-state Strobo Tuners are available.

Radio Boulevard, Western Historic Radio Museum, Henry Rogers  ©  2014



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Rebuilding the Collins 51J Series Receivers
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