The BC-312 is a 9 valve communications receiver, a superheterodyne design using single conversion. It was made in the USA, and designed at Fort Monmouth, in the mid 1930s. It was intended for ground use, either individually or with a transmitter. It can receive AM (amplitude modulation) or CW (continuous wave) or MCW (modulated continuous wave) signals, and used the LS3 loudspeaker. It looks similar to the BC-348 airborne receiver, having a wedge shaped dial, but the radio is much larger, heavier, and very different inside. The BC-312 weighs 48 pounds.
Figure 1: BC-312
The receiver covers 1.5 to 18 mHz, in 6 ranges. They are……
Band A: 1.5 – 3.0 mHz
Band B: 3.0 – 5.0 mHz,
Band C: 5.0 – 8.0 mHz,
Band D: 8.0 – 11.0 mHz
Band E: 11.0 – 14.0 mHz
Band F: 14.0 – 18.0 mHz.
It has two RF amplifiers (Radio Frequency) using VT-86 (6K7) valves, a frequency changer using VT-87 (6L7) with a separate oscillator using a VT-65 (6C5), two IF amplifiers (Intermediate Frequency) using VT-86 (6K7), a detector and audio pre-amplifier using a VT-88 (6R7), an audio power amplifier using a VT-66 (6L6) or 12A6. There is also a crystal filter and a BFO (Beat Frequency Oscillator) using a VT-65 (6C5). The internal power supply can be a 12 or 24 volt dynamotor or a 115 volt AC supply. The IF is at 470 kHz. The receiver is very nicely made, with several castings and a steel case, and so is very heavy. It is also very stable and nice to use with the vernier dial. The front panel is distinctive as it has a large connector pointing downwards, not outwards as is usual. This is for power input, audio output, or connection to a transmitter. The receiver can then be a transmitter remote control, as it has a key, a microphone jack, and a Send Receive switch.
There are many models in the BC-312 family, 30 variations in all, denoted by a number with a letter suffix. They all look the same, but a close inspection reveals the differences. They can easily be divided into two groups, the HF versions consisting of 22 models, and the MF versions consisting of 8 models. The HF frequency coverage is 1.5 – 18 mHz, and the MF frequency coverage is 150 – 1500 kHz. The HF versions can be further divided into two categories, the DC powered models, all called the BC-312, and the AC powered models, all called the BC-342. Similarly the MF versions can be divided into the DC powered models, all called the BC-314, and the AC powered versions, all called the BC-344. Strictly speaking then, there are only 13 models of the BC-312, 9 models of the BC-342, 6 models of the BC-314, and 2 models of the BC-344.
The power supply is modular and can be easily replaced, the DC dynamotor supplies
are either 12V or 28V DC. The early supply is a DM-17 and the later model is a DM-21.
The 115V AC supply is an RA-20. The two 28V models are denoted by an X, and use a 12A6
instead of a 6F6 as the audio power amplifier. One of the early uses was in vehicles,
so a noise antenna was fitted and there was a separate connector for the noise antenna input.
A control allowed the noise to be cancelled out, which is useful for ignition noise.
This model is obvious as it has an extra connector and two snap covers on the lower
left hand side. Most models were fitted with a crystal filter, and the phasing control
was at the top right hand side. The other models had a dial light dimming control in this position.
Some later models had raised lettering as opposed to engraved lettering,
and some had a watch holder near the nameplate.
Other models were basically the same, and the model number indicated the number of audio jacks,
minor electrical and mechanical differences, and the identification of the manufacturer.
A table from the BC-312 manual is below.
To summarise the differences:
frequency coverage (HF or MF)
power supply (12V DC or 24V DC or 115V AC)
noise balancing (fitted or not)
filter (crystal filter or dial light control)
audio output tube (6F6 or 12A6)
oscillator heater (fitted or not)
metal used (aluminium or steel)
capacitors (mica or paper and ceramic)
muting relay (antenna or antenna and audio)
audio jacks (3 second audio, or 2 second audio and one first audio)
BC-312N Farnsworth Radio and Television Corporation
SCR OUTFITS AND USES
The receiver was often used by itself.
If it was used with a transmitter, it was then called an SCR station.
SCR-245 a BC-312 and BC-223 transmitter for communication in jeeps and light tanks
SCR-177B a BC-312 and BC-314 with the BC-191 transmitter for a portable station
SCR-188A a BC-312 and BC-314 with the BC-191 transmitter for a portable station
SCR-193 a BC-312 and BC-314 with the BC-191 transmitter for a portable station
SCR-197 a BC-342 with the BC-325 transmitter for a portable station
SCR-399 a BC-342 with the BC-610 transmitter for a mobile truck station
SCR-499 fixed version of the SCR-399
The receiver has all the controls on the front panel. It has a wedge shaped dial window with the nameplate underneath it. The dial window is removable so that the dial lamps can be replaced. There are five knurled screws that hold the receiver in the case. The screws are all labeled LOCK and have a directional arrow to indicate the screw tightening direction. The screws do not screw into the case, but have a clip or stub that is tightened in a slot in the case. The screws rotate through about 10 turns, and stop at both extremes, and are captive. Do not over tighten them. Do not unscrew with too much force, as when they stop, they are disengaged.
On the left hand side of the dial window is the ALIGN INPUT knob which is the antenna trimming capacitor. Next to this is a connector labeled SIG. ANT. and underneath it is a press terminal labeled ALT. SIG. ANT. for a wire connection to the antenna. They are joined together. Below this is a handle. At the bottom is a press terminal labeled GND. The models with the noise suppression capability have an extra connector labeled NOISE ANT. and two preset adjustments, under snap covers. They are labeled NOISE BALANCE, and NOISE ADJUST. Between the handle and the nameplate is a big knob labeled BAND CHANGE. It has 6 positions for the bands.
Figure 2: BC-312 with noise antenna input
On the right hand side of the dial window is the CRYSTAL PHASING knob, which adjusts the crystal filter. It has an OUT position. Some models use this front panel location for the DIAL LIGHT adjust knob which is for dimming the dial lamps. It has an OFF position. Next to this on right is the volume control labeled VOL. and it has a graduated skirt from 0 to 9. It is a dual potentiometer, and is an audio volume control for AVC operation, and an RF gain control when in the MVC position. Next to this are the three BFO (Beat Frequency Oscillator) controls, a toggle switch for turning it on and off labeled C.W. OSC. OFF ON, a note control knob labeled CW OSC ADJUST, and a screw out blank, which allows screwdriver access to the pitch trimmer. Below this are two frequency tuning knobs. One has a crank and is labeled FAST TUNING and above it, is a geared down tuning knob labeled VERNIER. It has a lock. Next to this are two fuse holders, labeled LAMPS and DYN-FIL. At the bottom is another fuse holder containing a spare fuse, labeled SPARE. There is a three position main control switch which is labeled OFF-MVC-AVC, to turn the radio on, and to turn the AVC on if required. Next to this is the main connector, and this has a right angled bracket that allows it to face downwards. It is an SO-94 connector, which is for DC power input, audio output, and controls lines for a transmitter. There is a toggle switch labeled REC. and SEND which is for controlling the transmitter. In a vertical line at the lower right hand side are five phone jacks each with a snap cover. The top two are for transmitter control and are labeled KEY and MICRO for a microphone. The other three are for audio output, either for headphones, labeled PHONES, or for speaker operation, labeled SPEAKER. Depending on the model, the audio may come from a transformer in the audio preamplifier (first audio stage), or another transformer in the audio power amplifier (second audio stage).
The receiver will work with whip or long wire antennas. The normal connection is to the ALT.SIG.ANT binding post. If a shielded antenna lead-in wire is used, connect it to the shielded SIG.ANT. input. The manual refers to this as a “shielded binding post” not a coaxial connector. The antenna input is unusual. It looks as though it will mate with a PL-259, but it will not. The PL-259, N, and TWINAX connectors all have the same diameter and same threaded ferrule, and are interchangeable with each other (only the ferrule). The BC-312 shielded connector is slightly larger, but has the same thread. Inside the body is a normal push type binding post, with a paxolin sleeve, for a normal wire connection. The body is only for the earthed screen. There is a blind cap, that can be used to cover the input when it is not required.
Figure 3: PL-259, N, TWINAX connectors
Figure 4: shielded connector in pieces
Figure 5: shielded connector assembled
Figure 6: Mechanical Drawing (antenna)
Figure 7: Mechanical Drawing (speaker transformer)
Figure 8: Mechanical Drawing (capacitor cases)
If a transmitter is used, then the antenna will come via the transmitter, and can be connected to either input. A ground wire can be connected to the GND. binding post. Both the antenna binding posts are connected together, and go through a series trimming capacitor to a tap on the tuned grid circuit of the first RF amplifier. The trimming capacitor (ALIGN INPUT) can be used to peak the signal, allowing for any detuning by the antenna, and helping with resonance. There is an overload protector to ground. In early models it is a neon lamp type LM1, but later was a bigger LM4 with a series capacitor. There is a muting relay that shorts the antenna to ground.
The models with the noise suppression circuit, have an extra antenna connector. This is for connection to a “noise antenna”. The manual states that this should be installed alongside the vehicle engine, and is intended to pick up ignition noise, but not radio signals. There are three mutually coupled coils in the antenna circuit. The noise connector goes to one of them, and this is electrostatically shielded from the others. There is a coupling adjustment, called NOISE BALANCE, which is protected by a snap cover. This is screwdriver adjustable, and determines the amount of noise introduced. The other coil is electromagnetically shielded, and also has a coupling adjustment. This is called NOISE ADJUST, which is also protected by a snap cover. This determines the noise phase relationship, and is screwdriver adjusted for minimum noise. There is a paragraph in the 1946 manual stating that these parts are no longer available, and there is a procedure for removing them if they fail.
Figure 9: BC-312 Noise Circuit
The first and second RF amplifiers are both VT86 (6K7) metal pentodes. The anode coils are a single winding with a tap. There is a different coil switched in for each band, and the unused coils are shorted out. The coils have a trimmer capacitor but do not have an adjustable slug for alignment. Beware as the tuning capacitor has high voltage on it for bands A, B and C. Each screen has its own voltage divider from the high voltage. AVC voltage is applied to the grids of both RF amplifiers. The cathodes are connected to ground with a normal resistor and capacitor combination. When switched to MVC, the AVC grid bias is removed by grounding the AVC line, and the cathodes are connected to a voltage divider from the high voltage supply, with the RF gain control as the bottom leg.
The mixer is a VT87 (6L7) and the separate oscillator is a VT65 (6C5), both metal tubes. The tuning coils for both, are similar to the RF tuning circuits, a single winding with a tap, with high voltage on the tuning capacitor for bands A, B and C. The tuning capacitor is a four gang variable capacitor. There is a FAST TUNING knob and a VERNIER tuning knob, with several anti-backlash reduction gears. There is AVC on the mixer grid, but no RF gain control of the mixer when switched to MVC. It uses cathode bias. The oscillator is an electron coupled type (Hartley). There is a different cathode resistor switched in for bands A, B and C. The mixer injection is taken from the cathode, and applied to grid three of the mixer. The oscillator anode is at RF ground. The oscillator is in its own screened box, and some models had a heater for stability.
The anode of the mixer goes to an IF transformer. There are two IF amplifiers using VT86 (6K7) tubes at 470 kHz. The models without a crystal filter have normal IF transformers but the first IF secondary has a tap for the grid input. AVC is applied to the grid of both stages, and is removed for MVC operation. The cathode of the first IF amplifier, is grounded for AVC operation, but goes to the RF gain control for MVC operation. The cathode of the second IF amplifier, always has cathode bias using a parallel resistor and capacitor to ground. The models with a crystal filter are the same, except the first IF transformer has a 470 kHz crystal inside it, a PHASING control, and an OFF switch which shorts out the crystal.
The second detector, AVC and first audio amplifier uses a VT88 (6R7) tube. This has a normal AM detector using a diode in the tube, and delayed AVC from the cathode. The VOL control is a normal potentiometer, and is applied to the triode grid. This is switched out for MVC operation. The second diode is used for AVC rectification, and comes from the last IF stage anode via a capacitor. The AVC voltage is grounded for MVC operation. There is a transformer in the anode circuit of the audio preamplifier, and the secondary goes to a phone jack on the front panel. The secondary is tap selectable from 4000 or 250 ohms. On some models this is not used, and the phone jack is rewired to the power output amplifier. The transformer is used, in this case, as transmitter side tone injection, and it is wired to the main connector. There is normal capacitor resistor coupling to the grid of the power amplifier, also called the second audio amplifier. It uses a VT66 (6F6), or in the 24 volt versions, a 12A6. This has a transformer output which is connected to the other audio front panel jacks.
The CW oscillator is a VT-65 (6C5) triode connected as an electron coupled type (Hartley), similar to the mixer injection oscillator. The cathode goes to a tap on the single coil, the anode is at RF ground. There is a coarse tuning capacitor behind a screw out plug. The fine adjusting capacitor comes out to the CW OSC ADJUST knob. There is an ON OFF switch. The output from the cathode goes through a LPF (low pass filter) to remove harmonics, before being capacitor coupled to the detector.
Figure 10: BC-312 circuit
The tube heaters are 6 volts, and are connected in a series parallel string. There is a compensating resistor used for the audio power amplifier. The heater arrangements are different when used on 24 volts, and also when the 12A6 is used. The dial lamps are behind a removable escutchion and are bayonet types, either 6 or 12 volt in series. Some models that do not have the crystal filter, and instead have a 75 ohm potentiometer to the right of the dial, to enable dimming. This circuit is completely independent of the filament circuit an even has its own fuse, labeled LAMPS. It also comes out the the ring of the Speaker jack, so that it can power an electrodynamic speaker.
The power supply is in a self contained box, and is removable and easily changed. Any supply, 12 VDC, 24 VDC or 115 VAC can be fitted. The box hinges out to allow access to the circuitry for maintenance. The AC supply is slightly larger than the DC supplies. The RA-20 AC supply uses a rear connector for the 115VAC input. The AC supply is a normal transformer and rectifier power supply using a VT97 (5W4). It has its own toggle switch and fuse on the back, but also uses the OFF-MVC-AVC switch on the front. The DM-17 or DM-21 DC supplies are a dynamotor type and have a built in FL-6 noise filter. There is a tagstrip behind the front panel near the main connector. It appears to have a lot of unnecessary connections on it, however, these allow the connection of different power supplies and voltages, without modifying the radio.
The front panel connector is a large SO-94 type, but is unusual in that it faces down, not out, as is common on other receivers. All the remote control signals appear here. The receiver can be used for remote control of a transmitter, so that all control is at a single operator position. It has a SEND REC. toggle switch on the front panel which energies a receiver muting relay. This relay grounds the receiver antenna, and on some models the audio preamplifier grid. This relay is powered from the transmitter. There is a microphone jack (MICRO.), and a KEY jack which go to the transmitter. The microphone can have a PTT (Press To Talk) button that can activate the muting relay, as can the KEY jack. The headphone jacks and the key jack are normal sized, but the microphone and speaker jacks are a smaller diameter. If using the BC-312 as a stand alone receiver, then only the DC supply input, or audio output need be used. If it is a BC-342, then the AC supply comes in the back, so front input is not used. If using the headphone or speaker jacks, then it is not necessary to use the front connector at all.
Figure 11: BC-312 SO-94 plug
The manual recommends period test equipment, an I-72 signal generator, an I-56 output meter, and a BC-221 frequency meter. It also recommends a dummy antenna for HF (50 pF capacitor) and for LF (250 pF capacitor). It also talks about voltmeter loading, and the voltage charts in the manual are for a 1000 ohms per volt meter. It gives an example of how to allow for this, if your meter is not the same. It has trouble shooting charts, resistance diagrams, and dynamotor maintenance procedures. Note that the BC-312 circuit uses the M symbol to mean 1000, not the K symbol, so the cathode resistors in the audio amplifier are 2M, meaning 2000 ohms.
The chassis is made of aluminium and has boxes to enclose the oscillator, BFO, and RF coils, and all are plated or passivated. Some models were tropicalised. The IF transformers are in pressed cases, with ribs on top to protect the adjusting slugs. The capacitors and transformers are in metal cases (bathtubs or small boxes), with the values stamped into them. The two RF amplifiers and mixer are on a slightly tilted subchassis, to allow acess to the underside of their sockets. There is screen printing of the circuit component designation. Most had metal tubes but some BC-342 had some glass tubes in the IF amplifier, and audio preamplifier. There is a picture in the manual, Figure 42 showing three glass tubes. The front panel is very thick, as the removable power supply hinges from it. The front panel is removable, but not easily. There are several wires screwed to it, and the main tuning knob has an adjustable eccentric (to adjust the meshing) which is a chore to remove. The dial pointer has 10 screws just to hold it on the back of the panel. The panel has machined relief on the back to allow for the shallow depth phones jacks. To the contrary, there is a spacer for the fuses, as they need a thicker panel. There is screen printing on the back of the panel to identify components, and half the printing is upside down, for when the chassis is inverted.
Figure 12: BC-312 back panel
There are also some pins interference fitted, as stops for some of the controls. Some panels are painted black wrinkle, but there are a few that have a different finish, that looks like a sort of like a cracked mud pattern (crackle). The panel lettering is engraved and filled with white paint. The main tuning capacitor has three gangs for the mixer and RF, then a gap, and the fourth gang inside the oscillator box. There is a precision spring loaded gear set, with a worm drive, for the tuning. It has a 25:1 coarse tuning and a 100:1 vernier tuning. The dial has a movable mask that only shows the frequency band in use. There are many Nylock nuts used, some riveted to the chassis. The carry handles look like cupboard handles, and are in several styles. The manual specifies Moisture and Fungus Proofing (MFP) when the radio is returned to the depot for service, and it contains instruction for performing this task. The Micamould capacitors were often changed as well. If your radio is MFP treated, then the Micamoulds may have already been replaced. Certainly check the audio coupling capacitor, audio preamplifier plate, to power amplifier grid. It is located on the top of the chassis, on a little fibre board, behind the VOL. control. There is a video on Youtube that shows how to remove the coil boxes and change the capacitors, however, it recommends changing all capacitors. My policy is to only change faulty components, with period components, and attempt to keep the inside looking as original as possible. Of course this can embarrass you when 60 year old components fail.
The case is made from steel and is very heavy. It has a gasget around the edge, but this is normally brittle or missing. There are six small slots in the case edge that the front panel LOCKs engage with. The locks are captive, knurled, and have a direction arrow on the front panel. Inside the case is a circuit diagram. There are several screened ventilation holes. There is an oval shaped cutout on the back, that is normally covered by a plate. When the AC power supply is fitted, the plate is removed and screwed to the back of the case, for storage. The AC supply matches the cutout, and the AC fuse and ON/OFF switch poke through. The AC power cord plugs directly into the supply input receptical. The receiver is mounted on a FT-162 shock mount, which is a simple piece of metal with four rubber mounts that engage with slide clips on the case bottom. Some receivers will have four holes drilled in the top or side of the case, where an FT-178 mount was attached to steady the receiver in vehicles. The mechanical design is rugged, complicated, not easy to disassemble, but has a look of quality.
Figure 13: BC-312 FT-162 mount
Figure 14: BC-312M with FT-178 top mount
Figure 15: BC-312 with holes
The receiver had been heavily modified, modern parts added, and many original parts had been removed and lost. The main modifications were in the audio, detector, AVC, IF, heater and power supply sections. I located the HT input, the heater supply, and the audio output. I powered it up, using 6 volts for the heaters, and had plenty of audio output, lots of uncontrollable feedback, and other instability. It seemed to receive stations, but was uncontrollable. I tried to figure it out, but the wiring did not make much sense. There was a 6GW8 miniature audio valve (a triode pentode) wired into where the 6F6 should be. There was too much audio gain. I eventually gave up. I photocopied a circuit from the manual, started tracing out the circuitry, replacing wire by wire, while marking each one off the circuit with a red pen. I became confused, when I found original wiring that did not match the circuit. I got three BC-312 circuits from different manuals and used parts of each, one for the heater wiring, one for the crystal filter, and one for the audio stage.
I rewired the audio preamplifier as per the manual, discarding any plastic wire, and using period cotton covered wire. The 6GW8 triode was temporarily bypassed. The pentode section was wired as per the 6F6. I rewired the AVC and RF gain control. I discarded the separate RF gain control, which was mounted in a fuse holder hole, as luckily the dual RF/AUDIO gain control was still fitted, just disconnected. I removed the ON/OFF toggle switch and reconnected the OFF/MVC/AVC switch. I rewired the IF amplifiers using the correct type of wire, period resistors, and as the manual indicated. I powered it up, and of course all the controls were at full and it was tuned to a station. It blew me out of the room and scared me silly. After backing off the audio, it worked great, which makes me wonder why it was it modified at all. I removed the 9 pin miniature socket and the 6GW8, fitted a ceramic octal socket, plugged in a metal VT-66 (6F6), and rewired the heaters for 12 volts. The audio power amplifier was rewired as per the manual.
The muting relay was reconnected, and a neon located for the antenna protection. The missing jacks and snap covers were taken from a parts donor. The antenna connector also came from here. A copy of the audio output box was made, and the modern transformer was hidden inside. Some matching fuse holders were found and wired in. The missing SO-94 and elbow were obtained a few years ago for this project, and now they have been added. The front panel and case were repainted.
Figure 16: BC-312 crackle paint
I checked the IF alignment and it was a little off. The manual has a strange method of alignment. It basically says in Paragraph 84, set the signal generator to 470 kHz, and peak all IF transformers, that is all it says. It then covers the RF alignment. Well that is fine, but what about the crystal filter, all it said was turn it off. Reading on, and finding the last paragraph, Paragraph 89, it finally talks about the crystal filter. It tells you how to adjust it, then says realign the IFs, then realign the oscillator, and RF stages. Seems like a duplication of effort.
I normally use a sweep generator to make sure the IF transformers are set to same frequency as the crystal filter, just in case the crystal is a little off frequency. I decided to use the method in the manual, as I had not seen a good method described before. It says inject the 470 kHz signal into the mixer grid, turn on the BFO and set it for a beat note, then adjust the PHASING control until you get a minimum audio output. Change the signal generator frequency up and down, until you find an output peak, this is the crystal. It may require increasing the signal generator gain, and adjusting the BFO note. Once the crystal frequency is located, turn off the BFO, turn on the signal generator modulation, and peak the IF transformers. This method actually seems to work. It indicated that the crystal was 50 Hz high. Then it says re-peak the IF transformers. After alignment, the sensitivity at the mixer grid at 470 kHz was 22 uV for a 10 dB signal to noise.
The method for aligning the RF stages is also unusual. It requires connecting a signal generator to the mixer grid, then injecting a signal into the oscillator compartment (received frequency plus 470 kHz), by placing an insulated wire near the stator plates of the oscillator tuning capacitor. Then adjust the receiver oscillator for zero beat. You then remove the signal generator, seal up the oscillator box, then readjust the oscillator for maximum audio output, and lock the trimmer. Next, move the input signal to the antenna input, using the dummy antenna, and peak the RF stages. Paragraph 87 gives spot frequencies for aligning each band, but only mentions the high frequency end, as there is only one adjustment per band, a trimmer for the high frequency end. In the very last paragraph (the one covering the crystal filter), is the first mention of tracking. There is no padder or slug for the low frequency end of each band. If the tracking is out, you have to open the oscillator box, and physically move a wire in each oscillator coil. It says move the single turn “into” (HF model) or iron core “out of” (MF model) the coil, increasing the frequency. Do the opposite to decrease the frequency.
Figure 17: BC-312 oscillator coil movable wire
After alignment, the sensitivity was very good, and the results given,
are for a 15 dB signal to noise ratio:
Band A was 2 uV at 2.0 mHz
Band B was 0.5 uV at 4.9 mHz
Band C was 1.6uV at 7.85 mHz
Band D was 2.5 uV at 10.8 mHz
Band E was 1 uV at 13.75 mHz
Band F was 2.2 uV at 17.7 mHz
The power supply was opened with the intention of greasing the dynamotor bearings, but inspection revealed more work was required. I also found that the bearings can be greased without opening the box, as there are two plugs, one on each end, that screw out and give access. It is a simple matter to load them with grease, and screw the plugs back in. I had two DC supplies, and each showed some signs of work. On one supply, the dynamotor mounting on one end had been broken, and replaced. It was originally a circular alloy casting. It had been replaced with a circle of strong wire, with lugs soldered to it for the dynamotor screws and the mounting screws. It was done in a very professional way. It also had the LV supply negative lead changed to a plastic sleeved wire. The insides of the filter box were original. I replaced the electrolytic capacitor, checked the chokes and the other capacitors, and replaced the plastic sleeving with woven spaghetti.
The other supply had been reworked in a sloppy and untidy way. The insides had a new electrolytic capacitor, plastic wire and polycarbonate capacitors. I tidied up the wiring, replaced the plastic wires with cotton covered wire, and replaced the modern capacitors with old style ones (after they were checked). But there was a problem! The legend on the tag board had the colours of the wires scratched into it, and the A+ was changed to A-, which was strange. The wiring was also reversed. Upon closer examination, it appeared that the black LV negative lead from the dynamotor, was too short to reach the GND tag on the tag strip. So the two LV wires were swapped, the yellow positive wire going to GND, and the short black negative wire to the unlabeled closer lug. This was fine, as the dynamotor would happily rotate either direction, but the HV wires needed to be reversed also, the HV positive was now coming out of the green wire and the HV negative voltage was on the RED wire. The tag board was scratched to reflect this. I wondered why this was necessary. At first, I thought it was a factory manufacturing error, and it had been solved in the field by two methods, swapping the dynamotor wires, and in the other case by lengthening the LV negative wire. I read the manual carefully, and found a sketch of the physical wiring which revealed the truth. It showed the black LV negative wire, NOT going to the GND lug (as was logical), but instead going to an earthed mounting screw on the case. It was long enough for this. See the photo of the correct wiring. So to put things back to normal, on the professionally repaired power supply, I removed the long plastic wire, and put in a piece of cotton covered wire with lug, to the earth screw. On the sloppily repaired power supply I added new cotton wires, set them up the correct way, and scratched the A- to be A+ as it should be. The dynamotor now rotates in the correct direction, and the wires are connected correctly. So this inconsistency has caused two people (and myself) confusion. I wonder how many others have traveled the same path.
Figure 18: BC-312 power supply
Figure 19: BC-312 power supply filter
The radio is very nice to use, is easy to operate, and is very sensitive. The tuning has several split gears and has a good feel with no backlash, and the fast and slow tuning is good. The controls are laid out nicely. It is very well made and has a look of quality. There is also a 1930s feel about the whole mechanical construction. The mechanical design could do with some revision, to enable easier disassembly, and less complicated construction. It would be better if the front panel was not a structural member, then it would then be easier to remove. The power supply could be attached to the main chassis. All the tuning gears could be attached to the main front assembly, as all but one are. It would be cheaper to have generic knobs, and not have them individually engraved. The legends could be on the front panel. There would only need to be two knob sizes used, a large size for the band change and tuning, and a small size for all other functions. The two slip clutches on each tuning knob are both different designs, and a single design would simplify things.
You would be lucky to find and unmodified radio. The common changes are the addition of an “S” meter, a mains power supply, and individual RF and AF gain controls. Some have had all their capacitors changed, whether they needed it or not. Most radios will have the front connector and hood removed, the shielded binding post replaced with an SO-239 coaxial connector, and some or all of the phone jacks and snap covers removed. Sometimes the valves are replaced with miniature valves.
Reading the manual, the repetition of the alignment steps, and the tracking added like an afterthought, indicates that the manual may have been revised several times, and little attention has been given to flow or clarity. It must have been a successful radio as it has had many successive models, and even has an MF version. It could be that the airborne BC-348 receiver has inherited a lot of the characteristics, but not the weight.
My radio came from New Gineau after the war. The owner said that when the Americans left, they abandoned everything, and he just walked up and carried it away, back to Australia.
TM 11-850 Radio Receivers BC-312, -A, -C, -D, -E, -F, -G, -J, -L, -M, -N, -HX, and –NX, September 1946
Instruction Manual for Radio Set SCR-245 –GX, -JX, -MX, -NX, -PX
Ray Robinson VK2NO