Radio Boulevard
Western Historic Radio Museum
 

Radio Equipment from
D.F. Station B/C. No.2
WWII British
Mobile High Frequency Radio Direction Finder
 

 Reception Set R106 - National Company, Inc.

 Reception Set R.107 - Radio Transmission Equipment, Ltd.
 

Detailed Description of D.F. Station B/C. No.2. and How the Equipment was Used
How to Determine a True Direction Bearing using Aural Null with Loop and Sense Antennas
Description of Reception Set R106, Refurbishing R106 SN:P861-SN:52
Description of Reception Set R.107, Refurbishing R.107 Mk.1/1 SN:2112
Performance Review of an Operating R106, Performance Review of an Operating R.107
Comparison of the R106 to the R.107

by: Henry Rogers WA7YBS Radio Boulevard-WHRM

Reception Set R.107 with Wireless Set No.12 transmitter installed in a WW2 Morris 15cwt Truck photo from: fareham-darc.co.uk/g8kzo

D.F. Station B/C. No.2
 

In England, D.F. stations were called "Huff-Duff" which was sort of an acronym from "HF-DF" or "High Frequency Direction Finding." These stations were simple in design and they were easy to operate using the Aural Null method of direction finding. This employed a loop antenna in combination with a sense antenna for producing a specific type of reception loop pattern that was accurate enough for locating enemy radio transmitters behind enemy lines or locating transmitter signals that were from spies or collaborators operating clandestine within a city or small town sending reports to their military superiors via radio. Finding the location of clandestine types of transmitters were within the capabilities of a mobile D.F. station because the station could be moved to an area close to where the suspected clandestine station or individual spy transmitter was operating, such as various field locations adjacent to or nearby a town. The ability to easily move the D.F. station was essential to allow taking multiple bearings from different field locations to accurately determine where the enemy station was operating from. These mobile D.F. Stations were very basic in their function and operation. Obviously, there were much more sophisticated D.F. systems in use during WWII but the simplicity of the equipment and the ease of operation was the advantage of using these types of mobile Aural Null D.F. stations to determine enemy radio locations quickly and accurately. 

The normal procedure was for very large "fixed location" D.F. Stations (with much more sophisticated antennas and equipment) to D.F. from a long distance (sometimes hundreds of miles away.) Then, using bearings taken by several other similarly large D.F. stations, also long distances away, the multiple bearings from each station could be taken and then plotted to allow triangulation to determine the suspected station's location within a mile or two. It was then up to the mobile D.F. Stations to get in close for quickly taking multiple accurate bearings from several field locations to allow triangulation for determining the location within a few hundred feet. If a more accurate location measurement was necessary, small portable D.F. receivers were sometimes strapped to the chest of the D.F. operator (hidden under a large coat) and "walked" into the area to determine exactly where the spies or the clandestine station was located. This could be inside a building or sometimes even the particular room inside a building. If the transmitter was behind enemy lines then the mobile D.F. Station would take bearings from as close as was practical and then the triangulation would be close enough for either shelling or bombing the area to eliminate the enemy transmitter.

As a result of this "multiple purpose" ability of a mobile D.F. Station they were sometimes used near enemy lines searching for enemy radio communication stations that could then be disrupted or destroyed by shelling or bombing. Or, they could also be used far from enemy lines searching for clandestine radio stations within cities or towns operated by spies or collaborators who, when discovered, were usually captured, arrested and jailed. They might also be used for enemy signal intercept. The key to their success was that the equipment and methods were simple to operate. The complete station was easy to move around to different locations. Enemy transmitter locations could be quickly determined and the end result was a rapid and successful elimination of the enemy's radio transmitting capability.

Drawing of the D.F. Station B/C. No.2
The interior space was quite limited with the hut being about 8 feet long and about 6 feet wide. Height would allow an average height man (1940 Englander) to stand, maybe 5'7"
from the website: https://www.qsl.net/pe1ngz/army/vehicle/dftrailer/html  
A recent check and this website is still online but the photos don't load.

This type of D.F. is called "Aural Null" and implies that the operator was listening to the receiver output for the signal-loop null using headphones and that was certainly the primary method used for discerning the signal-loop null. Sometimes indicating meters were used for a more accurate null measurement. The R106 S-meter could be used for a D.F. indicator (the null.) This would have required the AVC to be used and would only provide carrier level indications in the AM-Voice mode. The R.107 didn't have a Carrier Level meter so, if an external indicator would have been needed, it might have been possible to use an audio output meter to indicate the null but it would have depended on the type of signal. The modulated carrier output of an AM-Voice signal would have the audio meter needle "bouncing" following the modulation and that would make finding a minimum null difficult. The same problem would happen when CW was received since the signal plus BFO would result in the same "bouncing" audio meter indication. Only a constant carrier signal would have provided a stable audio signal that would allow finding the null via an audio meter indication. This type of signal would only occur during "tune up" or during pauses in voice modulation. It's possible that the Telephone Adaptor No.2 was provided with external sensitive audio response meters that had damped-response circuitry to act as D.F. indicators (this device is mostly hidden by the R.107 in the drawing.) It might be possible that the Telephone Adaptor No.2 could have provided some type of usable null indicator since the typical Audio Output Meter really wouldn't have worked. Note that the Reception Set R106 has a cord running from the receiver's headphone output jack to the Telephone Adaptor No.2. In the HRO circuit, the headphone jack is capacitor-coupled from the first audio amplifier plate so the audio signal isn't particularly "high level." However, for rapid D.F. bearing measurement a meter indication wasn't actually necessary. Since the loop-null is very sharp (very narrow) it can easily be discerned quite accurately by just listening.

When transporting the hut to the field location, the two Loop antennas were stowed inside the hut. Aerial Loop D.F. "B" No. 3 and Aerial Loop D.F. "C" No. 3. were installed in compartments by the wheel wells of the trailer. The implication of the drawing is that Aerial Loop D.F. "A" No. 3 is installed on the roof of the hut since it's not shown. That would be a total of three Loop antennas available depending on the frequency range of interest. The "Sense Antenna" was a vertical antenna comprised of the four rods shown as "Antenna Rods D" that mounted to the wall of the hut. If the drawing is studied carefully, it can be seen that there's a chair bolted to the floor behind the Telephone Adaptor No.2. This second chair provides for two radio operators to be in the hut simultaneously. Since all of the connections for the R.107 are on the front panel, it's obvious that the drawing shows the R.107 from the rear. This also implies that what looks like a window on the rear wall is actually another access door. Without a door on both the front and back how would each operator get to their respective positions? The drawing indicates that it's showing the D.F. hut from the "FRONT" but that designation might be dependent on which operator you were talking to.

The ability to change loops would allow a greater frequency coverage for the entire B/C No.2 station although its specified use was D.F. for 480kc up to 10mc. However, the R.107 tuning range didn't go below 1.2mc therefore the R106 had to be used for frequencies below 1.2mc. This would have necessitated having HRO coil set E and HRO coil set F to allow frequency coverage of 480kc to 900kc on coil set F and 900kc to 2.0mc on coil set E. Between the two receivers, the specified frequency coverage of 480kc to 10mc would be easily accomplished. Actually, if all of the R106 coil sets were available, coil sets JD (1.8mc to 4.0mc,) JC (4.0mc to 7.3mc) and JB (7.0mc to 14.4mc) would allow tuning to be extended up to 14mc. Coil set JA tuned 14mc to 30mc, although that wasn't in the D.F. Station B/C No.2 frequency coverage specifications. It's possible that while one receiver was operating in the D.F. mode the second receiver was tuned to receive orders or deployment details from the military command and that's why there's a separate antenna and an extended frequency coverage on the R106. However, it's also quite likely that one receiver was used for D.F. while the other receiver was used for enemy signal intercept. Use of the second receiver may have been variable depending on the installation and the task that had to be performed. From the drawing that implies the view is from the "Front" of the hut and the fact that the "Aerial Loop Control Gear Brake" is adjacent to the operating position of the R106, it appears that the R106 was the primary D.F. receiver and the R.107 was for intercept or for receiving  communications. No transmitting capability is shown in the drawing. If the towing vehicle had radio T/R capabilities, then any two-way communications with D.F. Headquarters could have used that set up.

In the upper left rear part of the hut are two lamps. One is shown as "Battery Lamp" and the other is shown as "Mains Lamp." The implication is that the D.F. hut ran on DC voltage when deployed in the field but it was possible to use the "AC Mains" as a power source if it was available. However, it looks like the radio equipment was normally set up for DC operation only. To switch the R.107 to AC operation would require pulling the chassis from its cabinet to move the power supply switch from "DC" to "AC." To convert the R106 to AC would require operating the R106 from a different power supply. Possibly the Supply Unit Rectifier was the AC operated power supply for the R106 and the Power Unit Vibratory No.2 was the DC operated power supply (more details on R106 power options are in the R106 section description further down this write-up.) It would seem that the AC Mains operation might be for convenience when troubleshooting or changing set ups at a depot and then DC operation was normal for power in the field. The length of operational time, of course, would depend on the current draw from the storage batteries. No charging system is shown but it certainly had to be possible to charge the batteries, or even operate the D.F. hut using an external motor-driven DC generator. It might have also been possible to power the hut in the field using some sort of motor-driven AC alternator-generator.

It's interesting that the storage battery is shown as "Battery Sec. Port. Six Volt 100/125AH" with the emphasis on "Sec Port" implying a "secondary" and "portable" use for the battery, even though it's shown as the power source for the Reception Set R-106 when operated with the Power Unit Vibratory No.2. However, since that six volt storage battery is the "secondary" where's the "primary" battery? It could be out-of-view in the drawing and located in the corner near the R.107. It would have to supply 12 volts and probably consisted of two six volt storage batteries connected in series.

Another piece of equipment not shown is a radio transmitter for communication of bearings for triangulation information or other observations to the D.F. Headquarters. Usually, these were small battery operated "handie-talkie" types of transceivers if the distance involved wasn't very far. It could be that the towing vehicle would have had T/R communications equipment installed for that purpose and the range of that type of communications equipment would be far greater than the "handie-talkie" type. It's certainly possible that other items like maps, set-up compasses, parts, tools and other necessities might have been in the vehicle that towed the D.F. Station B/C. No. 2 to its operating location.

There were several other pieces of auxiliary equipment such as satchels signals, various lamps and window shades that were part of the D.F. hut but it's obvious that not all of the parts and pieces could be shown in just one drawing.

Wrap-up and Disclaimer - All of these descriptions are somewhat speculative and are based on the analysis of the D.F. Station B/C No.2 drawing and examination of an actual R106 receiver and an actual R.107 receiver. Some procedures or purposes of the support equipment are assumptions based on other types of "aural null" D.F. equipment and how that equipment was used. Some D.F. equipment that was used for comparison were "field" D.F. measuring devices while others were for airborne "search and rescue" operations. Certainly some errors in how the support equipment in D.F. Station B/C No.2 worked are to be expected since I don't have the actual support equipment in front of me to examine. However, most "aural null" D.F. stations worked on the same principles and most of the assumptions will be found to be essentially accurate or, at least, plausible. Estimates are that about 200 of the D.F. Station B/C. No.2 mobile trailer-huts were built during WWII. It's thought that none have survived completely intact although there are on-going efforts in Europe to restore a couple that have survived in less-than-complete condition.

The desired signal was tuned in with the receiver operating just on the "Sense" antenna which was Omni-directional. To determine the station's bearing, the Coupler B/C was then switched to Loop Only (Bi-directional.) The signal was now tuned for a null, that is, the minimum strength signal response. The Loop Compass now indicated a "null" in two directions (on each side of the Loop.) This was a bi-directional response since the "null" would happen twice in the Loop's rotation. Only one of the two "nulls" was the correct bearing however. To determine which one was correct the Coupling Unit B/C. was switched to "True Direction" which combined the Sense antenna with the Loop antenna creating a "Cardioid pattern." The Loop was rotated +/-90º and the Loop position slightly rotated for a minimum signal or "null" (the cardioid pattern places the ONE "null" inline with the loop axis which is why the loop has to be repositioned +/-90º.) However, if the "null" couldn't be found, then the Loop was rotated an additional 180º and the operator again searched for the "null." Some D.F. systems will have the operator adjust a "null" control that further adjusts the gain of the Sense antenna for equal response to the Loop for more accurately positioning the Loop for minimum null. When the accurate "null" position was found then the bearing shown on the loop compass in degrees azimuth was noted. BUT, now the azimuth bearing indicated was referenced to Magnetic North, so at this point the operator would have to add or subtract the magnetic variance from the Loop Compass indication to end up with "True Direction." Sometimes the Magnetic Variance can be significant (and it varies all over the Earth but it is usually shown on DF and navigation charts.) As an example, my location in Dayton Valley, Nevada has a Magnetic Variance of -18ºE, meaning that -18º has to be subtracted from the Magnetic North indicated bearing to obtain "True Direction. A Magnetic Variance to the +West would have to be added to the Magnetic North bearing for True Direction. Although this all sounds rather complicated, once the radio operators had a little experience, intercepting the signal, finding the null, taking the bearing and determining the True Direction could easily be determined in less than 15-20 seconds. Speed and accuracy were essential because enemy transmissions were typically very brief, since the enemy generally knew or suspected, their transmissions were being monitored and their location was being determined. Usually, multiple bearings had to be taken due to the brief nature of the transmissions involved. Then, for pin-pointing the enemy transmitter, the D.F. Station had to be moved to a different location and another set of bearings taken. The bearings would usually be radio transmitted to the communications center to allow triangulating the bearings on a chart. The point of intersection of the two or more bearings would be the enemy transmitter location. Of course, for rapid determination of the enemy transmitter location, multiple mobile D.F. Stations could be used in widely separated locations and their bearing information transmitted via radio to D.F. headquarters for rapid triangulation and determining the enemy transmitter location as quickly as possible.

The drawings below show the loop response "basic patterns" involved in determining "Bi-directional" and Uni-lateral "True Direction."
 

IN D.F. POSITION BI-DIRECTIONAL - TWO NULLS OFF EACH SIDE OF LOOP
ASSUME THAT LOOP AXIS SHOWN IS POINTING 0º/180º AZIMUTH
TWO NULLS AT 270º W AND 90º E BUT THE COMPASS READS 0º (LOOP AXIS)

SWITCH TO "UNI-LATERAL" AND ROTATE THE LOOP -90º SO THAT NOW THE LOOP AXIS IS 270º/90º - THE NULL IS NOW OFF OF ONE END OF THE LOOP ONLY - THE LOOP IS SLIGHTLY ROTATED FOR THE LOWEST "NULL" - THE COMPASS READS 270ºW THEREFORE THE CARDIOID "NULL" IS POINTING AT SIGNAL SOURCE

TO CHECK, THE LOOP IS THEN ROTATED 180º - THE CARDIOID NULL IS NOW POINTING 180º AWAY FROM THE SIGNAL.
SLIGHTLY ROTATE THE LOOP AND THE SIGNAL REMAINS AT A CONSTANT STRENGTH AND DOESN'T SHOW A "NULL."
 ROTATE THE LOOP BACK 180º TO CONFIRM THAT THE NULL IS AT 270º W - THE "TRUE DIRECTION" OF THE
SIGNAL IS READ ON THE COMPASS AS 270º W. THEN MAGNETIC VARIANCE IS ADDED OR SUBTRACTED TO OBTAIN "TRUE DIRECTION."

Some D.F. inaccuracies could happen since it was possible that the enemy might use counter-measures to confuse the D.F. operations by using two different transmitters located at separate locations performing "split-transmissions" where one station would transmit for a few seconds then the second station would transmit for a few seconds until the message was completed. Sometimes a directional antenna was used to force skywave propagation which would reduce the accuracy of the D.F. determination. Depending on the frequency and using a directional antenna the signal could be reflected off of buildings or parts of the terrain to give false indications of the signal's origin. Using two separate transmitter locations and "split-transmissions" had its drawbacks because there was always the possibility that the D.F. operators were skilled at their task and both locations could be determined and both locations would then be destroyed. Another counter-measure was that the enemy would use a mobile transmitter that made taking meaningfully accurate bearings almost impossible.
 

The Radio Gear used in the WWII British D.F. Station B/C. No.2
 

Reception Set R106, Z.A.22906 SN:52  - National Company, Inc. - HRO-M  SN: P861

General Description - The Reception Set R106 was a standard HRO Senior (M) that was intended for Lend-Lease deployment to England during WWII. This R106 shown was somewhat modified when it got to England. This was to allow the Coupler Unit B/C to connect to the antenna input on the side of the HRO. Unfortunately, this adapter was removed sometime in the past, leaving just the mounting holes and a relief-notch. The non-original "round knob" is for an antenna trimmer air variable that was definitely a British modification. Additionally, three of the coil sets have British-incorporated modifications to the Ant/1stRF inductance.

When I acquired P 861, the matching serial number coils for the receiver were stowed in a canvas bag and included the F, E, JD, JC, JB and JA coil sets. These are all general coverage-only (no amateur bandspread capability) coil sets that were typical for the WWII HRO. Since this was the R106 version intended for DC operation, no "dog house" power supply was included. There is an U.S. Army Signal Corps acceptance stamp on the back of the receiver but the receiver has many characteristics of British use and modifications, including the use of "cheese head" screws in the locations that were reworked. The data plate indicates "D.F. No.2" designating that this R106 was definitely part of a D.F. Station B/C No.2 Mobile DF Hut. Also, the ID number of Z.A.22906 and the serial number 52 are on the engraved data plate.

Reception Set R106  SN: 52   National Company HRO-M  SN: P861
stenciled "G.39" on the receiver and on the coils and with "SER.No 52" on the data plate and "52" on coil sets JD, JC, JB and JA.      2022 photo

The top of the chassis of the R106, showing its excellent condition with no corrosion. It's unknown if the red paint on the gearbox cap had any significance.         2025 photo

Coil Sets - Although, the R106 could tune from 50kc to 425kc and 485kc up to 30mc (nine coil sets required) the absolutely necessary frequency coverage required was 480kc up to 1.2mc exclusively. From 1.2mc up to 10mc was shared with the Reception Set R.107. The hut drawing doesn't show any indication for HRO coil set storage although it seems likely that the coil sets would have been stowed somewhere in the hut. Since the R.107 didn't tune below 1.2mc, the R106 was probably mainly used with coil sets E and F. But, since there was the "Open Aerial" available for additional monitoring, perhaps there was a need for the other coils sets, JD, JC, JB and JA. Additionally, the coil set modifications are to coil sets JD, JC and JB that provided 1.8mc to 14.5mc indicating these were intended coil sets to be used. It can be seen in the drawing of D.F. Station B/C No.2 that the table does have a drawer on the left side which could have been used for coil set storage. This R106 had six original coil sets with it that provided a frequency range from 480kc up to 30mc. Coil sets JD, JC, JB and JA are matching serial numbers P861 to the receiver. These coil sets also have "52" ink stamped on the frame further "matching" the receiver. Coil sets E and F are not serialized, but Coil set E has "39" stenciled on the front panel although the stenciling doesn't match the other coil sets. Coil set F doesn't have any stenciling. Coil sets JD, JC, JB and JA all have "G.39" stenciled on the coil front panel (same as the receiver.)

The Coil Sets for R106 SN: P861/52
Coils marked G.39 are matching serial numbers. Coil "39" is the E coil set and the unmarked coil set is F.      2025 photo

- IMPORTANT NOTE -
Serious Error in Signal Corps & British R106 Manuals
Regarding Disassembly or Reassembly of the PW-D Micrometer Dial

The Signal Corps R-106 manual and the British manual for the R106 both have a significant error with their detailed description of how to remove, disassemble and reassemble the PW-D micrometer dial. In both manuals the procedure is totally wrong and completely contradicts the the correct National Co. instructions for PW-D disassembly and reassembly.

Using either the Signal Corps or British procedures will cause problems rather than allowing successful completion of any PW-D adjustment task. Additionally, there was a supplement to the SC manual with a PW-D instruction sheet with the same incorrect procedure. How these completely erroneous instructions ever got into the manuals is a mystery since the correct instructions were easily available from many National Company publications.

DO NOT USE the Signal Corps R-106 or the British R106 instructions when working on the PW-D. The correct method of removal, disassembly and reassembly is in Part 3 of my "National HRO" write-up, "Servicing the PW-D" here on this website. This same correct procedure can also found in many of the National Company HRO manuals.

Reception Set R106 showing the excellent original condition underneath. Note the hole in the coil bay bottom to allow access to the modified ANT/1RF inductor in coils sets JD, JC and JB.    2025 photo

Close-up of the Antenna Trimmer Mod    2025 photo

Shown is coil assembly D1, the Ant/RF LC for 1.8mc to 4.0mc. The inductor is a replacement with the L adjustment in the vertical position.    2025 photo

Other antenna-related mods were the two small mounting holes and a small notch above the antenna/ground terminals access port that indicate some type of antenna input cable support was used. Actually, when used as a DF receiver an adapter plate was installed to allow cables and connectors from the Coupling Unit B/C to mate with the receiver (the adapter plate is somewhat visible in the D.F. Station B/C No.2 drawing.)

The four-pin plug on the power cable was changed to a British-made plug that has a fiberboard base with shorter pins (two large and two small, standard spacing) and then there's a metal cover over the back of the plug protecting the wires of the cable. Despite looking quite different from the typical National Company installed plug, this British plug does fit into the four pin socket of the standard National "Dog House" power supply. The British plug can be seen in the photo below, "Reception Set R106 as found in 2022."

>>>   Close inspection of the AF Gain pot repair indicated that it wasn't a depot job and additionally the potentiometer shaft was hopelessly stuck (even applying heat and oil couldn't break loose the stuck shaft.) A good condition vintage HRO replacement pot was installed.  Oily dirt was all over the chassis. This was easy to remove using WD-40 as a cleaning solvent. After cleaning, Glass Plus was used to remove the WD-40 residue. The cabinet was also greasy and needed the same treatment. Knobs were reconditioned using WD-40 as the cleaning agent. Each coil panel was cleaned. Inspection of the wiring under the chassis revealed that the speaker terminal wiring had been moved to the BSW (remote standby) terminals and the two wires that had been connected to the BSW terminals were shorted together using an insulated twist "wire nut."  The actual speaker pin jack was broken and repair might require replacing the entire terminal board and that's riveted to the chassis. A good way to repair this is to use the old style pin jacks used in battery sets. These type have a threaded barrel and mount with a nut. Removal of the fiber board wouldn't be necessary. I have to find a pair for the repair (the pin jacks were installed Sept 27, 2022 - see update below.)  >>>

The R106 was reassembled without any further issues. I've tried to keep P 861 as an original example that has only been cleaned with most of the mechanical problems repaired. (Initial refurbishment completed: April 28, 2022)

UPDATE: SPKR Jacks - Sept 27, 2022 - I found a pair of matching pin jack type of receptacles for the SPKR repair. The installation required removal of the broken original jacks, enlarging the mounting hole to 5/16" diameter and mounting the pin jacks. The wires that were connected to the BSW terminals were actually the SPKR wires, so those two wires had to be moved to the new pin jack terminals. The two wires that were connected together with the "wire nut" were actually the BSW wires and these two wires were moved to the BSW terminals. This completed the repair of this area of the R106 receiver. I'm sure this old modification wasn't military in origin and was more than likely a ham-mod done post-war. Now, for normal operation (as with almost all HRO receivers,) a jumper has to be connected across the BSW terminals. When the receiver is to be remotely placed in standby, the jumper can be removed and the BSW terminals can be connected to a T-R relay. Photo right shows the repair.

The SPKR pin jacks installed in Sept 2022

R106 Testing and Operation - 2025

Since I was going to try to operate the R106 on its original parts, there were a few things to check prior to applying power. I checked the DCR from B+ to chassis and got 36K ohms, so no hard shorts. I gave a quick test of the two electrolytic capacitors but those are low voltage cathode bypass caps and they checked about as expected,...okay. I looked around the storage area and found a Model 697 "Doghouse" power supply. I remembered using it a while back and it was okay. A little more digging and I found a National loudspeaker Type R-115 that was intended for a WWII HRO receiver. I checked the audio transformer and its DCR on the plate side was 330 ohms indicating that it was probably an original 7000Z winding. I used a line bucking transformer to make sure the AC line voltage would be around 115vac.

Since all of the accessory equipment had been checked before, the only "unknown" was the R106. I'm sure I didn't test the tubes but I decided to just "see what happens." I had coil set JD installed. I connected the Collinear Array antenna. The R106 came up with no problems. I heard normal background noise and no hum.

R106 in Operational Testing - Of course, 1.8mc to 4.0mc during the day won't have anything receivable, so I installed the JB coil that covered 7.0mc up to 14.5mc. I tuned around and didn't hear anything. I checked the frequency graph and 40M is around 20 to 40 on the PW-D. I tuned down there and heard a couple of loud SSB signals. BFO turned on, AVC turned off and then reduce the RF gain and the SSB signals demodulated fine. 10mc WWV came in strong around 230 on the dial. Tuned in a few 20M hams at 490 on the PW-D. The alignment on coil set JB seems fine. The Brit mod Antenna Trimmer works just fine and peaks the signals as it should.   >>>

>>>  HRO receivers will hold alignment very well since all the adjustments are air variable capacitors and some of the inductances are trimmed with a moveable wire loop while others use a moveable metal disk inside the coil form (HFO coils.) All designed to maintain alignment despite rough handling or long term storage. What "kills" everything in receivers is poor storage conditions with lots of humidity or especially salt-air moisture found in coastal areas causing corrosion everywhere. Then there's insect ingression, rodent ingression, storage temperature extremes with sub-freezing winters and boiling-hot summers all taking their toll year after year. But, P861 had obviously been stored inside all of its existence. It doesn't have any corrosion, everything looks excellent underneath - not even so much as a spider web. So, even though I'm running the R106 on its original, 80+ year old paper dielectric capacitors, it works very well. Of course, the manufacturer of the paper dielectric capacitors is an important factor. These capacitors were all Sprague types, one of the best of the time.

More testing and more time in operation will be necessary for alignment checks on all of the coils. I've only tested coils sets JD, JB and E. The highest frequency coil, JA, is always a tough one to keep in alignment.

Checking Alignment using National's Method  - Since the performance indicated that the R106 didn't need alignment, I thought I'd do the "National Alignment Check" that is usually in all of the HRO manuals. The alignment check used the outer rotor plate of each tuning condenser section. By using an insulated tool, like a pencil eraser, one could push the outer plate slightly out or slightly in, thereby either reducing C or increasing C of the tuned circuit. If there was a decrease in receiver output in either direction the assumption would be that the LC is in alignment. But, if there was an increase in the receiver output when manipulating the rotor plates, then the LC was out of adjustment. It was a very easy method (on the HRO) to determine if an alignment was necessary but one had to be gentle,...only a slight movement is necessary for the indications. This method works for the two RF stages and the Mixer stage. The HFO check is how accurately the PW-D tracks in comparison to the graph. The operation of the Crystal Filter will indicate how accurately the IF is adjusted.

An original SPEAKER pin jack piece installed with fake rivets   2025 photo

Reception Set R106 - Performance

Immediate Gratification - I was completely surprised at the incredible level of performance that P-861 has since it is operating on all original Sprague paper dielectric capacitors and all original Centralab resistors. The tubes are the tubes that were in the receiver when I got it. I did have to correct several mechanical problems but the electronics and the alignment are "vintage" and seem to be "right on."

Sensitivity - The R106 has lots of sensitivity on 20M using the JB coil set. This coil set also tunes 40M where the normal daytime stations were heard. With the JC coil set installed 40M is at the top of the tuning range and 80M is at the bottom. This is a nighttime coil set. JD is 80M at the upper end and 160M at the lower end. JA is the only original matching serial number coil set that wasn't modified with a different Ant/RF coil. To my surprise, the JA coil sets works very well on 20M at the lower end of the range and, as I tuned higher, I heard WWV 20mc coming in quite strong. Tuning higher still, the 27mc CB band hooligans were also quite strong. I heard a few data type signals on the 10M band but no voice or CW. Absolutely impressive performance all the way to the top of the frequency coverage. The E coil set and the F coil set mostly cover the AM-BC band. These coil sets are good for testing since there's nothing interesting (or even listenable) being broadcast on AM-BC in this area of the USA. When there were a lot of NDBs on the air, the low end of the F coil set allowed tuning a few NDBs that operated around 510kc up to 525kc. The R106 operates just about like a "rebuilt" HRO that's just been aligned. A little "scratchy" now and then, but a totally pleasant surprise.

Tuning - The PW-D Micrometer dial provides very smooth tuning, it's a little "heavy feeling" but there's no backlash. The tuned frequency is determined by correlating the Micrometer reading with the graph on the front panel of the coil set. It's not accurate as far as directly indicating the tuned frequency but it wasn't intended to do that. If the tuned frequency had to be accurately determined, a Heterodyne Frequency Meter was required. However, the PW-D is hard to beat for accurate reset capability.

Frequency Drift - Like all HRO Senior receivers, the R106 drifts for about ten minutes, then slows down considerably and is very stable after about 25 minutes of operation. In General Coverage, that is, with "J" series coil sets, the frequency drift is much less noticeable than using the pre-WWII HRO coil sets that had Band Spread capability.

Selectivity - Only the Crystal Filter is available to narrow the 456kc IF bandwidth. The R106 has the early style National Crystal Filter with a variable-C Phasing control and a variable-C Selectivity control. The early Crystal Filter takes some getting used to. Also, the Selectivity control has to be peaked, even if the filter isn't being used.

Noise Filters - The R106 doesn't have a noise limiter or an audio filter. Either the RF Gain had to be reduced or the Crystal Filter employed to reduce sensitivity or bandwidth to cope with pulse-type noise.

Beat Frequency Oscillator - The BFO shouldn't be set up as described in the National or Military manuals. At the time, the BFO was for CW and the original set up was fine for CW. Nowadays, to demodulate SSB correctly, the BFO should be set to 456kc at "6" on the BFO scale. This allows setting the BFO above or below 456kc for USB or LSB demodulation. The original setting of "9" only allowed one side of 456kc to be tuned.
 

For much more detail on the design, history and use of the National Company HRO Receiver, go to "National Company HRO Receiver - the Cream of the Crop" - a four-part article that presents just about everything one would need to know about the marvelous HRO receiver, from its beginning design in 1934 up to the last vacuum-tube version in the late-1950s. Use the navigation Home-Index link below.

Reception Set R.107 Mk.1/1 - Z.A. 25266 - TPH   SN: 1221

Reception Set R.107 Mk.1/1  Z.A. 25266   SN: 1221          2025 photo

Tubes Used - The R.107 was a nine tube set (8 tubes plus rectifier tube) but only three types of tubes were used in the set. Four CV1053 tubes, four CV1055 tubes and one 6X5 rectifier tube. The British tube numbering had several different designations for the same tube type. The CV1053 = EF39 = ARP34, is a pentode tube with standard octal base. The CV1055 = EBC33 = AR21, is a duplex diode/triode tube with a standard octal base. The pentodes are used as the RF Amplifier, the Mixer and the two IF amplifiers. The duplex diode/triodes are used for the HFO, the BFO, the Detector/AVC (the only place where the diodes are actually used) and the Audio Output tube. Only the triodes are used in the HFO, BFO and Audio Output functions and the diodes are not connected. All of the receiving tubes have a metalized, conductive-paint coating that provides the RF shielding by way of a thin copper wire wrapped around the top of the bakelite base. This wire is covered in the metalized-paint coating and the other end of the wire is connected internally within the tube base to pin 1. The DC resistance of the conductive paint coating to base pin 1 is very low. When the receivers were built, all of the tube sockets had pin 1 wired to chassis, thereby grounding the painted shielding when the tube was installed. The 6X5, being the power supply rectifier, has no conductive coating. This "later version" R.107 has the "CV" designators installed for tube location indicators.

Antenna Requirements - The Reception Set R.107 tuned from 1.2mc up to 17.5mc in three bands. The tuning featured a two-speed combination direct-drive and a reduction vernier drive for "slow-motion" tuning. The R.107 circuit used a single preselection front end, two 465kc IF amplifiers with two selectable IF bandwidths and tertiary-coupling (using two series-connected IF transformers) for the second IF amplifier input transformer providing steep slopes for good selectivity. Wide bandwidth was about 7kc and Narrow bandwidth was about 3kc. Sensitivity was rated at <1uv for CW and 2uv to 6uv for Voice. The Aerial inputs are "Open Aerial" which bypasses the primary winding on the Antenna coils and is connected to the first RF amplifier grid LCs (this connection is to a tuned parallel LC on the grid - not RF transformers.) This would be for non-tuned relatively short length antennas. The "Dipole Feeder" input is two terminals that are routed to the primary windings on the tuned Antenna LCs using actual RF transformers ahead of the RF amplifier grid input. This was intended for balanced feed lines such as used on dipole antennas with an expected impedance of around 70Z to 100Z (twisted line was popular around 1940 for dipole feedline with an expected impedance of 100Z.) Unbalanced line (50Z or 75Z coax) can be used by connecting one of the Dipole Feeder terminals to the Earth terminal (chassis ground) and using the other terminal for the center conductor. One has to test which terminal to ground works best. The manual states that the right terminal is "earthy" but performance is what actually determines which connection works best. With my R.107, the left terminal connected to Earth works noticeably better with the matched Collinear Array antenna (the manual does say to "try" both connection set ups for best performance.)

Other Features - The R.107 had features like a "Crash Limiter" that was a noise limiter to reduce the intense pulse noise from lightning crashes, a narrow CW audio filter (900hz peak) that could be switched in if needed, a 600Z line audio output, dual parallel Lo-Z phone jacks and a built-in 4" diameter Goodman Industries loudspeaker that could be switched off if necessary. The TEL OUTPUT control only adjusts the level of the two Lo-Z jacks. The 600Z line audio output is from a dedicated winding on the audio transformer and the output is dependent on the settings of the RF and AF gain controls and the signal level being received. Additionally, the associated transmitter's sidetone signal could be routed through the R.107, if desired. An unusual feature is the black bakelite circular housing at the upper center-left of the panel. This was for holding a 2" diameter pocket watch as a time indicator (probably for the radio operator's pocket watch - the British military were issued watches that had "GSTP" engraved on the back - "General Service Time Piece.") The yellow "T" indicates this receiver was "Tropicalized" (a lacquer coating on areas that could corrode, screws, components and soldering. Sometimes the lacquer would contain a fungicide and was then usually referred to as "MFP" for "Moisture and Fungus Proofing.") The top of the case also has a "T" indicating the same thing. The small metal tag has "R/TPH 1221" and in several places within the chassis are many red ink inspection stamps indicate "TPH" leading one to conclude that "TPH" was the contractor for this receiver. I can't find any British Radio company that used those initials during WWII. However, it's interesting that Ferguson Radio Company was a Canadian company that supplied R.107 receivers to the Royal Army during WWII. It might be possible that "TPH" was another Canadian company. Another possibility is the the "R" that proceeds "TPH 1221" on the serial number tag indicates a rebuild or refurbishment of this receiver and the red ink inspection stamps relate to the rebuild. At this point, I can't really find any definitive information on what "TPH" indicates.

Onboard Troubleshooting, Muting, Sidetone - One interesting feature was the small testing pin-board on the upper part of the front panel. This feature allowed the operator to check (while the receiver was in operation) the cathode current of each of the vacuum tubes via measured IR drops across a 3K test resistor. The manual gives the expected voltages that should be measured when the receiver is operating correctly. Also, the two large pin outlets on the testing pin-board provided access to the tube filament voltage (for a small lamp, probably for visual troubleshooting purposes.) Even more troubleshooting information was in the manual for all of the voltages and resistances that could be measured on the four "tag boards" that were wired together (tag bds. A to B and tag bds. C to D) to interconnect the three modules that comprise the receiver. The other connector is the Sidetone and Muting inputs. The upper large pin is Muting in and one should know that B+ is on this pin when the receiver is on and the pin exposed. By using an external Antenna Relay that has a set of NO contact that switches to NC upon transmit and having those contacts connected to the upper large pin and the middle small pin, the B+ will be routed through dropping resistors to actuate the receiver's internal Muting relay which routes the transmitter sidetone to the CT of the 600Z winding of the audio transformer and grounds the grid input of the audio output tube. The lower large pin is for Sidetone which is usually provided by some types of transmitters as a keyed audio tone that allows monitoring the CW sending. Some transmitters also had the Sidetone provide an output in the Voice mode also. The R.107 Muting and Sidetone functions were intended to operate with specific British Army transmitters like the Wireless Set No.12 or the Wireless Set No.33.

Physical Details - The three modules are the RF section, the IF/AF section and the Power Supply section. Each module was removable from the main frame and front panel assembly. This modular approach was to ease repairs in the depot for quick turn-around although module removal is far from easy requiring substantial disassembly and unsoldering. The testing pin-board and the tag board information was provided to hopefully allow easy repairs to be accomplished in the field. The later versions of the R.107 had storage inside for five spare tubes and a spare vibrator. The chassis mounted into a large steel case with double carrying handles (side or top.) There were four threaded bosses in each corner of the front panel that were for mounting the cabinet front cover. When the front cover was removed, it could be mounted to the back of the cabinet using the similar threaded bosses located there. The entire weight of the R.107 was approximately 95 pounds and it measured 24"W x 13"H x 17"D. The removable front cover that could be screwed to the back of the cabinet for storage but if the front cover wasn't mounted front or back, the case would be  about 14" in depth.

Reception Set R.107 Mk.1/1 - Refurbishment

I wanted to bring the R.107 up on a "soft start" initially. The seller had powered the receiver and checked it out but all of the tubes have been in and out and, of course, the R.107 had travelled from the East Coast to Nevada. "Soft start" is just increasing the AC input from 0 to 120vac in about ten seconds,...nothing radical. I usually "soft start" the equipment for maybe the first few times until I'm fairly confident that the circuits will survive the regular power up. Of course, being a rectified power supply with a vacuum tube rectifier, the B+ "soft starts" anyway since the rectifier tube doesn't conduct for several seconds and then increases within four or five more seconds to full rectified voltage.

I had a 15 foot long wire on the floor for the antenna. At first I just heard the normal operational noises through the loudspeaker. BFO was operational. AF and RF gain controls functioned, just no signals. This was during the day, so the upper end of Band 3 and all of Band 2 didn't have any signals anyway. The lower part of Band 3 had a few AM-BC stations coming in with the local (well, 25 miles away) AM station being fairly strong. I heard a few heterodynes on Band 1 but nothing strong. So, the R.107 was fairly operational. I'm sure it needs alignment and it needs to be connected to a better antenna (how true.)

Reception Set R.107  Top of the Chassis. Note the spare tubes and spare vibrator mounting. This feature isn't on early versions of the R.107 (Z.A. 3220.)    2025 photo

1. The long screw that is on the lower part of the front panel and passes under the chassis to thread into a welded bolt on the back of the cabinet is broken. This appears to be an old break, so lucky the long screw didn't come out and get lost. I'll have to repair the long screw by making a new end and then attaching it to the end of the screw. The length has to be correct for the new part so that the screw will pull the bottom of the front panel into the cabinet. I used a .25" opening metal coupler with a "cut to length" 1/4x20 shoulder bolt with the head cut off. I had to "cut to length" the long bolt so that the coupler could rotate when tightening or loosening the screw and not interfere with anything under the chassis. The long screw has to go through the cross-strut of the main frame. The long screw with the coupler has to be assembled installed under the chassis because of the holes that it protrudes through. The welded nut on the back of the case was extremely close to 1/4x20 threads but I ran a 1/4x20 tap to make sure the repaired long screw would thread in easily. It would be possible for the coupler set screws to loosen and then the long screw wouldn't be extractable and the rear piece might not allow the chassis to be removed without threading the screw inward from the rear. To prevent this, I drilled two shallow holes in the screw shafts to allow the coupler set screws to protrude into these holes. Now, even if the set screws loosened considerably, the long screw would still be able to rotate and unthread. There are two other short front panel screws that are in good condition.

2. The socket for holding the spare vibrator was missing a screw and nut. This made it difficult to plug the spare vibrator into its socket. The British use "cheese headed" screws which are very similar to Fillister head screws. I installed a Fillister head 6-32, lockwasher and nut to secure the socket. The spare vibrator now mounts securely. If the primary vibrator isn't in use it's mounted upside down (pins up.)

4. Tuning condenser was shorting in the upper third of rotation. Problem was due to a large "flake" of the conductive metalized paint from the HFO tube that had fallen onto the stator HFO section of the tuning condenser. When the rotor contacted this conductive paint it caused the HFO to stop working (with significant "scratchiness" coming out of the loudspeaker.) Removing the flake was somewhat difficult because it was extremely brittle and fragile so it would break apart whenever it was gripped with the long needle-nose pliers in order to remove it. After about three tries, I gripped the middle part of the flake and it was able to remove it intact. I brushed the stator with a soft medium-size paint brush to remove any other small flakes.

5. The Antenna Trimmer seems to not adjust the air variable correctly. The problem was the scale mounted to the rear of the knob was very loose (three screws on back of knob.) I removed the knob and saw that the trimmer is a half-shaft and so is the knob. The scale has to be set correctly and then the back screws tightened. Then the knob set screw can be snugged up.

Under the R.107 Chassis showing the three modules that make up the receiver and the main frame that supports the modules. Incidentally, note that this photograph has been "rotated" to make it appear that the chassis resting on its back. When out of the cabinet, the R.107 should never be placed on the back of the chassis or possible damage to the "tag boards" could result.     2025 photo

7. Left side grab handle was loose and the left side panel was also loose. The side panel has to be dismounted to be able to tighten the bolts that mount the grab handle. The two bolts were tightened, the side panel remounted and its screws tightened. Two nuts on the BFO knob skirt clamp and two nuts on the Tuning knob skirt clamp are missing. These missing nuts are 6BA (British Association standard for small screw/nut sizes and threads.) Clamps still operate even with the nuts missing. The nuts are easily available from many sources, such as eBay (all sellers are in the UK but their prices are cheap and they ship quickly.)

8. Found the Band 2 RF trimmer capacitor had its mounting screws almost all the way unscrewed. I had to remove the RF section shield cover to have access to tighten the screws. Also found that the locking nut on Band 3 Ant L adjustment had backed all the way out and needed to be run down to actually "lock" the L adjustment. Found that the C trimmer for the Ant coil on Band 2 was stuck. I had to use heat from a small soldering iron and DeOxit to break the adjustment loose. I checked all of the front end adjustments to make sure everything was moveable prior to doing an alignment.

Showing the four gang Tuning Condenser, the RF Amplifier tube shield and the Tag Boards A and B. The yellow lacquer is the "Tropicalized" protective coating.        2025 photo

10. Dipole Feeder right side terminal was loose and couldn't be tightened. I took this spring loaded terminal apart and the two bakelite halves must fit together and when they are correct there's just enough clearance for the thickness of the front panel metal gauge used (plus paint.) The embossed projections of each half piece seemed to be too high and when fit together the clearance was greater than the front panel metal gauge. I used a small flat file to remove just a slight amount of material from each half. This material removal was sufficient to allow the terminal halves to be snugged up (not over-tightened) and not allow any movement of the terminal when fully mounted.

With the small knob removed, you should be able to rotate the tuning shaft with your fingers. Rotate the shaft until you feel it begin to "grab" and at that point, slightly loosen just one of the front screws trying the shaft to see if the "grabbing" is gone. If that screw doesn't change anything return it to the position it was and go on to the next screw. There are three screws on the backside too. These are accessible without removing anything. So, try them - one at a time - to see if the "grabbing" stops. Sometimes it takes two loosened screws to achieve adjustment. Never move the screws more than one-quarter of a turn. If the screws are too loose then some back-lash will result. It's a compromise between smooth, low-resistance tuning and excessive back-lash.

It's possible that if the grease is dried up then the vernier mechanism might have to be removed in order to thoroughly clean and lubricate it. It's fairly easy to remove but it can't be totally disassembled without destroying it. The mechanism can be soaked in a degreasing solution. Then the new grease pushed in through the screw holes. This should clear-up any "sticking" or roughness. There's a write-up online from Allan's Virtual Radio Museum (in the R107 Overhaul section) that describes the procedure. You should be somewhat familiar with the R.107 tuning mechanism when reading this write-up in order to understand what the author is describing (British vernacular is liberally used throughout the write-up.)

This is a close-up of the bellows coupler between the tuning condenser and the dial drive mechanism. The rod that's clamped to the shaft is the dial pointer. The right-angle rod is the tuning dial "limit" stop. Note the copper plate that's mounted to the vernier mechanism. The three screws mounting that plate are the adjustments for smooth slow-motion operation. The external large ring mounted with two screws holds the assembly in place. Note the grease - that's a good sign.    2025 photo

Conductive paint is easy to find and not usually expensive. The most expensive is nickel-based and the cheapest is carbon-based (there's also copper-based.) For tube shielding the carbon works fine. I bought 2oz. off of eBay for $11 plus shipping for $15 total. I only used about 5% of the 2oz. to paint six tubes, so only a small amount is needed. Before starting, I used a yellow paint pen to write the tube identification on the bakelite base. Clean off any residual old conductive paint using a razor blade and clean the glass envelope with Glass Plus. Scrape clean the copper wire that's wrapped around the top of the bakelite base using a razor blade (it should have several places that are bare metal for good contact.) I had to practice on one tube to get the proper brush stroke technique to get good coverage and a fairly smooth finish. The conductive paint I used dried very quickly (and it didn't smell and it was water-soluble.) I tested the resistance between pin 1 and the paint measuring about 25 ohms, which is very conductive. I proceeded to apply the conductive paint to all of the tubes that needed it. The tubes were reinstalled into the receiver. I also repainted the shielding on the spare tubes that needed it.  

The conductive paint I used was black. You can paint another color, like red or silver, over the conductive paint so the tube looks more authentic. Some of the British restorers actually use the proper color paint over the conductive paint and then make decals to apply the proper logos and ID on the correct color paint. These restored tubes look authentic and their installation into any chassis greatly enhances the appearance. I'm thinking about doing my tubes in a similar manner, but, for the time being, I just left mine black (see photo to the right.)

Improved Performance After Tube Painting - One wouldn't think things would change doing this conductive paint on the tubes,...but they did,...and for the better! I guess the conductive shielding does slightly change the capacitance in the circuit for various reasons. The overall gain was greatly improved. I suppose that some feedback or mutual coupling due to the lack of shielding was causing losses in the receiver's front end. Also, due to this capacitance change, the receiver had to be realigned in the RF-front end section, especially the HFO section. About a 500kc change in the tracking on Band 1 and slightly less on Band 2 and Band 1. Since the HFO had to be readjusted, so did the Mixer, RF and Antenna stages although not to the extent that the HFO had to be readjusted. It's hard to believe the difference in performance now. With fully shielded tubes, the R.107 is seeming to easily meet its specifications for sensitivity.

The upshot is, although the R.107 will function okay with "unshielded" tubes, having tubes with fully intact paint-on shielding will greatly improve receiver performance.

 CV1053 with new conductive paint applied. Note the connecting wire around the top of the bakelite base and, of course, the conductive paint doesn't contact the grid cap.
2025 photo

Showing Tag Boards C and D that interconnect the IF-AF module to the PS module
2025 photo

Once VTVMs became common place, most alignment instructions then started to reference the Diode Load as the measurement point. Using the Diode Load provided a negative voltage that was at the diode plate load R of the detector tube and the VTVM's super-high input isolation didn't "load down" the diode load voltage. This meant that the AVC didn't have to be on, the signal generator output could be an unmodulated waveform and the Diode Load voltage would become more negative as the signal level increased and less negative as the signal level decreased. It's perfect for nice quiet and very accurate alignments. Of course, the R.107 manual wouldn't specify this method since the equipment needed was not easily available in 1940. To access the Diode Load point in the R.107, use the manual's photo of the IF/AF module underside. Locate the junction of R2.E and R12.B (they are just the two resistors mounted on a fiber component board near the RF Gain potentiometer.) Connect the VTVM to this junction, set the VTVM scaling to negative voltage of about -10vdc FS, reference the VTVM common lead to the R.107 chassis. Be sure to turn off the R.107 AVC. Adjust the signal generator output level or the RF gain control for about a -6vdc to -7vdc voltage reading during the various adjustments. As the circuits are aligned the output level of the signal generator will have to be reduced. Minor adjustments can also be accomplished with the receiver's RF gain control but generally it's best to reduce the signal generator's output so the minimum amount of "signal" is used for alignment accuracy.

IF Alignment - The IF operates at 465kc. There are four transformers each with two adjustment trimmers on the side of the shield-can. IF transformers T.I. B and T.I. C are connected in a tertiary manner to provide increased selectivity but the adjustments are still for peak output at 465kc. The T.I. A and T.I. B are adjusted from the rear of the chassis with the trimmers on the back side of the transformers. Access to T.I. C and T.I. D, with their forward-facing trimmers, require removal of the small rectangular plates mounted to the front panel with two screws each. There are four access plates plus one access plate for the BFO adjustment. All of these plates really don't need to be removed. Just remove the upper screw and loosen the lower screw, then the plate will rotate down on the lower screw and that allows access to the alignment hole. The output from the RF signal generator is connected to the grid cap of the Mixer tube through a .1uf capacitor. The generator is set to 465kc. Set the Bandwidth to Narrow. Adjust the generator output for an indication on the VTVM. Begin at T.I. A and adjust its two trimmers. Then move forward adjusting each transformer's two trimmers until T.I. D has been adjusted. Set the BFO control to its center line and turn on the BFO. Through the front panel access hole adjust the BFO trimmer for zero-beat. This completes the IF section alignment. Return the alignment cover plates to their vertical position, install the top screw and "snug" both screws ("snug" means don't over-tighten.)

Showing the built-in Goodman Industries loudspeaker with its cloth covering. Also, the round electrolytic capacitor is a vintage British replacement. The unshielded tube in the foreground is the 6X5G rectifier tube. Obvious is the vintage repair to the back of the panel where the "Watch Holder" is mounted.    2025 photo