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Old 9th Jun 2008, 11:12 pm   #6
Skywave
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Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Racal RA17L BFO behaviour

Paolo -

Well, you seem to have a strange fault there. It sounds like the valve is being badly over-driven due to incorrect biassing.

The CV4014 is correct. Equivs. are CV138 and EF91.

I don't know if you have the manual to hand, but the d.c. voltages at the valve electrodes should be:
Anode 180 v dc;
Screen 210 v dc;
Cathode 0 v dc.

You seem to have elimininated the major components in the cct. - including the valve.
I assume that you know the history of this set and can verify that the BFO cct. hasn't been modified in some way - easy to check, since the cct. is quite basic.
Apart from measuring the dc voltages (as above) to see if that throws any light on the problem, then the next logical thing must be to suspect the BFO coil itself.

But let's see what others have to say on this before we seriously entertain going down that route.

Incidentally, I see that the BFO O/P cap. in the '17L is 47pF. In the forerunner (RA-17) the value is 10pF. I suppose this cap could be faulty - see what the waveform looks like when this cap. is disconnected from the rest of the unit. Also make sure that the valve screening can is in situ. (clutching at straws now )

Al / Skywave.
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