Setchell-Carlson model 524 receiver: the BC-1206-C

I found one of these tiny WWII A-N radio range navigation receivers on a famous web site for on-line auctions. It looked in nice overall conditions, complete and unmodified, both externally and internally. It was untested, but for me as a collector of vintage radios this was not a big resistor. The price looked fair for this kind of unit so I decided to buy it, despite of the relevant cost of shipment from the U.S.A.

The Setchell-Carlson model 524 was an A-N radio range navigation receiver
Once it arrived in my hands, of course I started to inspect it and I found it was really in good conditions as it promised to be. It had the serial no. 5888 on the front plate.

The Setchell-Carlson model 524 a.k.a BC-1206-C.
The short wire in the foreground is the antenna connection, the longer wire
from the back of the unit is the power supply (+ 28 VDC) connection

Internal view with tubes. From left to right: 28D7, 14R7, 14H7, 14J7, 14H7

Internal view with the tuning capacitor

The back of the unit with the +28 VDC and ground connections. The two capacitors and
the two coils form an input filter on the power supply voltage.

View from the bottom. All components and wires look original.
Only the white wire on the right is more recent (may be a repair).

Generally speaking, the construction tecnique is rather simple and low cost, compared with bigger WWII receivers like the venerable BC-348 or the BC-312, just to mention two. Only tuning and volume controls, a few cheap connectors, a very simple (and rather rough) tuning mechanism. Probably there was no provision for real maintenance and no needs for an higher accuracy.

I found a very nice description of the role of BC-1206-C during WWII in this post of "The US Militaria Forum": US Navy marked Setchell Carlson BC-1206 CM2 radio range receiver.

Forum user MIFlyer wrote: "In the 1940’s the standard aircraft radio navaid system in the U.S. was the AN Range.   The AN Range predated the widespread use of Automatic Direction Finding receivers and used the 200-400 KHZ aircraft navigation band.  Use of the system only required an ordinary AM receiver tuned to the band.  Pilots would tune in the station that best corresponded with their destination, listen to the signal, and hear either the Morse code signal “A” which indicated they were to the left of the desired course, the Morse code signal “N” that indicated they were to the right of the desired course, or a continuous tone, which indicated they were right on course.  Everyone with an aircraft sophisticated enough to have a radio used the AN Range, and so many light aircraft were equipped with only receivers for the 200-400 frequency that most control towers were set up to transmit on 278 KHZ for the purpose of giving landing and takeoff clearances to aircraft.  For this reason, when the USAAF adopted the SCR-274N  “Command Set” receivers and transmitters early in WWII, a standard equipment set consisted of a BC-453 receiver for 190-550 KHZ, a BC-454 receiver covering 3-6 MHZ and a BC-455 for 6-9.1 MHZ, together with a couple of transmitters, enabling coverage of both the AN Range as well as the standard control tower frequencies and other required military communications.

When the USAAF got to Europe, it found that the RAF had adopted VHF for fighter aircraft communications, using a crystal controlled set known as the TR1143.  The Americans had to be compatible with the British when it came to fighters, and VHF gave far superior short range communications anyway, so the U.S. built the British set as the SCR-522, and adopted it as the standard radio for fighters, at least in Europe.

VHF was a big advancement but it caused a bit of a dilemma, especially for fighters operating in the U.S., which had to still use the AN Range for navigation.  Overseas, low frequency beacons were less available and a lost fighter pilot would call for DF steer from a ground station.  But fighters flown in the U.S., at least, had to have the low frequency receiving capability.  Problem was, the SCR-522, while no more bulky than the SCR-274N receivers, transmitters, and modulator, still took up virtually all of the available room in the aircraft.  Also, the fighters deployed to Europe would not necessarily require the low frequency capability, so an easy add-on capability was desirable to keep things as standard as possible.

The answer to this problem were the two cutest aircraft radios ever built; the Detrola Model 438 and the BC-1206.

Both the Detrola and the BC-1206 were designed to operate directly from 24VDC, without a dynamotor, and to be set up so that they could fit into a standard aircraft 3 inch instrument panel hole.  They were small enough to be mounted directly in the cockpit with no more than a power lead and an antenna connection.  Using their headphone jack, the sets could be plugged directly into the same audio circuit used by the SCR-522, so switching between radios was not required.  They could be installed or removed within minutes without affecting the VHF installation.

The Detrola went into the later model P-38, P-51, P-47, and some P-63’s, fitting right into the cockpit, possible because the remarkably small receivers were only about twice the size of the control boxes used for the larger radios.  The P-51 and P-47 had the little set right next to the pilot’s seat, facing upward at an angle, while on the P-38 it was to the right of the pilot’s seat, almost resting on the floor, facing up.  On the P-61 it was in a rather strange installation behind pilot’s right shoulder, the dial facing forward.  The P-63 manual I have describes the Detrola as a “portable” installation, even though it is bolted down under the main radio panel at the bottom of the instrument panel and the P-38 manual says the Detrola “may still be installed.”  Presumably this indicates the possibility that the set was yanked out when the aircraft was deployed overseas.   Interestingly enough, none of these installations used the set’s ability to be installed in an instrument panel hole.  All installations used long wire antennas; on the P-51D, the Detrola antenna is the long wire coming through the hole in the top of the bubble canopy and stringing back to the tail.

In contrast to the Detrola use and installations, the only manual I have found that mentions the BC-1206 shows it installed in a side instrument panel hole in a P-80A, the early model Shooting Star jet fighter.    

The two sets fulfilled an identical function and were clearly made to identical specs, but are actually quite different electronically.  I have always assumed the Detrola and BC-1206 were just related models of the same radio but recently found out they are very different.  The Detrola Model 438 was made by the Detrola Company, and employs five standard WWII type octal tubes, including a VT150 oscillator/mixer and a couple of 6L6’s for audio output.  The BC-1206 was built by Setchell Carlson and uses five 14 volt loctal tubes like the 14A7 and 14J7 and a 28D7 output tube.  Both have an unusual antenna spring loaded lead socket on the left lower side near the front, use a combined volume/on/off switch, and have a ¼ inch headphone jack on the front. While the tuning knob on the Detrola simply operates a geared pointer against a frequency dial painted on the face of the set, the BC-1206 has a window over a dial to show the selected frequency.

While the Detrola was used extensively in the U.S., I don’t know if it actually went to combat.  However, the period photos of P-51’s in Europe show the long wire antenna, even though an SCR-522 usually can be seen behind the pilot, so one would presume those wire antennas were hooked to something 

Postwar, the Detrola and BC-1206 were replaced in refitted WWII aircraft and the newer jets by the BC-453 and/or the new ADFs such as the ARN-6 and ARN-7.  No doubt the larger sets were much better receivers, - but they were not half as cute!"

A good and simple description of the AN radio range navigation system can be read here:
Detrola 43B - Low-Frequency Radio Range Receiver

Well, now of course I was curious about discovering wheter my BC-1206-C was still able to work or not. So I quickly inspected the circuit looking for anything suspect. There was a white wire in the filament power supply circuit which was apparently newer the all other wires around it (i.e. PVC or teflon insulation instead of cloth insulation), anyway the wiring resulted to be functionally the same (not exactly the same) as in the service manual I had found on the web. Also, I checked that there were no short circuits between the +28 VDC wire and the chassis, so I decided to give it a try at 12 VDC, just to see what would have happened.
The power supply current at 12 VDC resulted to be a reassuring 440 mA. But the biggest surprise came after connecting a few meters of random wire to the antenna input and quickly surfing the limited tuning range between 200 kc and 400 kc: suddenly the id of the local NDB (PIS at 379 kc) clearly came out of the LS-166/U loudspeaker that I had connected to the audio output! Not that bad as a first power-on test.

Now it was time to give it the full +28 VDC power supply voltage. I did not have a suitable power supply, so I connected the 12 VDC power supply in series with a 12 VDC battery to get a good +24 VDC for my test.
Measured power consumption resulted to be (approx) 700 mA at 25 VDC.

Local noise (from man made electric sources) was terrible during this test. May be this problem could be mitigated with a better antenna (during test I had attached to the antenna input simply a few meters of thin insulated wire lying on the floor).
At the lower end of the 200-400 kc tuning range, the receiver detects few european longwave broadcasting stations. The level of signals is good but reception is unclear, may be due to passband limitations of the radio, which was intended for the AM reception of Morse encoded signals, not for voice or music. Anyway, I noticed that the readability became good when the unit was powered by a 12 VDC battery only, despite of the fact that the audio output level was much lower in that case, of course.
At the lower end, in addition to the local NDB (PIS 379 kHz), I could hear the signal from GBG 426 kHz (Gleichenberg for Graz, Austria), as documented in the short video below: