The RA1B Receiver

  The design of this model of the US-made aircraft radio receiver dates from around 1938 and was used in WW2.

 As with most short wave receivers from WW2 this example has been modified for amateur use. You can readily see some obvious modifications viz. a missing connector (lower right) and next to this a modern "Plessey Plug" used for feeding power to the set which originally had an external dynamotor power unit. Next to the new plug is a hole used originally for on/off. There's a modern-looking headphone or loudspeaker jack socket and upper left you can see a modern Belling Lee aerial socket and an added green lamp. There's provision for remote mechanical operation of the receiver via the protrusions under the tuning and wavechange knobs.

 You can see from the picture below that the set has six wavebands and a pretty well standard layout with a tunable RF stage and mixer. The 3-gang tuning condenser has a larger oscillator section and it carries gearing to enable slow motion tuning. All eight valves are visible with all bar the audio output valve being screened to avoid instability. The IF is around 1.6Mc/s rather than the usual 465Kc/s which helps reduce image reception.


 The underside shows what is clearly American design with lots of characteristic features making it look entirely different from British equipment of similar vintage. There's also something you can't see and that is the characteristic smell of US made radio equipment. It's a sort of earthy smell from something used in the manufacture.


 A close up of the tuning arrangements


  Here's a close up of the wavechange switch gearing. The design is very clever because it incorporates a method of not only switching the set to its 6 wavebands but allows for setting the tuning cursor to be offset for each band so that the tuning scale can be adjusted to line up with an accurate external frequency.

 A picture of the back end of the receiver showing six of the eight valve holders. The decoupling condensers use rubber seals which are perishing. This problem may have started relatively recently and might be a consequence of too much ozone in the air. The problem was common when I used to repair VCRs when those located near the sea had problems with their rubber drive belts. Often a drive belt was reduced to a pool of rubber stuck to the chassis. These metal cased "Sprague" condensers often leak oil and sometimes are no better, leakwise, than their UK wax-covered tubular equivalents.I like the open construction making it easy to fault find and replace bad components


 Below an excerpt from a 1941 Bendix brochure (click to see a PDF)

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