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Old 10th Jun 2008, 7:31 pm   #8
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Racal RA17L BFO behaviour

Hi -
There's a lot of data in that last Post of yours; for the time being I'll limit this reply to three issues.

The thought has crossed my mind that if you have recently acquired this radio and perhaps don't know too much about its history, the coil may have been taken out at some time and re-fitted - and re-fitted incorrectly - so that the cathode tap is closest to the 'top' of the coil, instead of the 'earthy' end. Under these conditions, I would expect the cct. to behave exactly as you have described; too much positive feedback and the valve being over-driven, etc., etc.

The white insulation material is simply Racal's method of keeping damp / moisture out of the coil.

As regards the inability to resolve ssb transmissions, (and assuming that the BFO is on freq. at 100 KHz and that the IF amp. does not have a poor response curve), the usual cause of this with BFO injection to an AM / envelope detector technique (as used here) is simply insufficient BFO injection (via C215). At present, I can't find my records to tell me what level to expect - but off-hand, 2.7 v peak-peak sounds far too small. I would have expected something like 20 v p-p. The small level of injection that you have could be due to the substitute coil you are using. Which takes us back to my first paragraph - as above.

Al / Skywave.
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