RA1B - but not the usual circuit

[GMB]

The RA1B I am working on has been ham-modified but incompletely it seems (judged by obvious errors and components going nowhere).

BUT there is another issue. It is clear that the original set was not as appears in the many circuits we find online.

So does anyone know of a later circuit? The clear difference is that the detector circuit is quite different, and easily recognised as all the circuits I have seen show the two diode anodes joined together. In my case this is clearly not the case. This part of the circuit looks original too.

That was not the only difference. The layout of the central tag strip is not the same as the one I have seen in the manuals I am finding online. Again the different arangement looks original.

So has anyone seen this? Have you got a different circuit for it?

[David G4EBT]

Originally Posted by GMB

The RA1B I am working on has been ham-modified but incompletely it seems (judged by obvious errors and components going nowhere).

BUT there is another issue. It is clear that the original set was not as appears in the many circuits we find online.

So does anyone know of a later circuit? The clear difference is that the detector circuit is quite different, and easily recognised as all the circuits I have seen show the two diode anodes joined together. In my case this is clearly not the case. This part of the circuit looks original too.

I assume when you say 'the two diodes in the detector circuit', you mean the detector diode and the AVC diode. Don't know about the RA1B, but in the RA1, the anodes of those two OA81 diodes ((D1 Detector, D2 AVC) aren't connected together - they have a 15pF capacitor (C51 in the RA1 circuit) between them.

There isn't a manual or circuit of the RA1B in the Forum Heathkit Archive, but there is the 20 page partial RA1 manual.

Might be no use at all, but it might give a few pointers.

https://www.vintage-radio.info/heathkit/

Good luck in your endeavours with the RA1B.

[ms660]

Originally Posted by David G4EBT

I assume when you say 'the two diodes in the detector circuit', you mean the detector diode and the AVC diode. Don't know about the RA1B, but in the RA1, the anodes of those two OA81 diodes ((D1 Detector, D2 AVC) aren't connected together - they have a 15pF capacitor (C51 in the RA1 circuit) between them.

There isn't a manual or circuit of the RA1B in the Forum Heathkit Archive, but there is the 20 page partial RA1 manual.

Bendix ?

Lawrence.

[GMB]

OK, I was unaware of the ambiguity.
I am talking about the Bendix RA1B (valve receiver, US aircraft, for the use in)

The detector/AVC are one diode, but the valve has two diodes so in the famous original form they wire the two anodes together. But not in mine.

[GMB]

OK I have solved the mystery. I stumbled upon a Bendix official upgrade notice dated July 1944 which documents a series of modifications that results in the version I have. So I presume that they started making them like that from that date as mine only looks patched by later repairs.

The update was to improve the AVC to prevent the set being swamped by a very strong signal.

So does anyone know when they stopped making this model? This modification seems to pin down the date quite well. Is there a later manual that shows how it was then built?

[Dave757]

Hi

Manual for the Bendix RA1-B at BAMA Archive.

Kind regards
Dave G0ELJ

[GMB]

Originally Posted by Dave757

Manual for the Bendix RA1-B at BAMA Archive

That's the one you see everywhere, from 1941 or 1942.

It is not clear if they issued an updated manual after the upgrade. It would be dated second half of 1944 at the earliest. I have yet to find one.