Radio Boulevard

Western Historic Radio Museum


Vintage Test Equipment - Part 2


Gear Profiled in Vintage Test Equipment - Part 2

     ~  Diagometers, Radio Test Kits  ~

     ~  RF Signal Generators  ~

     ~  Heterodyne Frequency Meters, Deviation Freq Meter, Audio Freq Meter   ~

     ~  Wave Meters, Grid Dip Oscillators  ~

     ~  Audio Oscillators, Vacuum Tube Fork  ~


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 one of the largest Standard RF Signal Generators 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 < 1.0V 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 450 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. When in good condition and recently serviced, the 1001-A is a very useful and excellent RF Signal Generator, mainly because it has the capability of providing stable signals as low a 5.0kc. Much lower in frequency than most RF signal generators.


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 nearly always 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 (also the URM-25D.) These high quality RF signal generators all use rotating turrets that carry the coils for each tuning range for both the Oscillator and the Output Amplifier.

Usually these types of instruments use separate RF Oscillator and RF Output sections and screen modulate the RF Output with a separate audio oscillator. Regulated power supplies are provided for stability. A separate sealed attenuator that is highly accurate and mechanically precise device is included. These generators will have full measuring capabilities of the output and modulation level. However, the HP-606 was probably one third the cost of the  GR 805-C or the URM-25D. It certainly weights less-than half as much and is half the size of the GR 805-C and although somewhat larger than the URM-25D, the weight is probably about the same.

Shown in the top 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 ($10) 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 65mc 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 KDWC.

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 KDWC

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 right 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 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 180 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.


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