04-09-2025, 09:09 AM
(03-09-2025, 10:06 PM)Mark Hennessy Wrote: It looks like the left hand side of C11 will go positive rather than negative when there's a signal, based on the detector diode, in order to reduce Ic in VT2.
Yes I'd agree with that - strictly, it would be 'positive-going' in the presence of a signal (ie become less negative).
(03-09-2025, 10:06 PM)Mark Hennessy Wrote: As has been said, the voltages shown are for no-signal conditions, when the voltage across the cap is the other way around - albeit by a tiny amount.
I can't see that the polarity would reverse, though - if it did, then the transistor would be biased 'off' and couldn't amplify linearly - so there would be no detector output to generate the AVC control voltage. Of course, the signal level could be large enough to overcome the bias, but I discounted that because then the modulation envelope would be very distorted.
(03-09-2025, 10:06 PM)Mark Hennessy Wrote: I'm assuming they missed the minus sign from the voltage on the base of VT2.
Good spot - I hadn't noticed that! The other voltages are prefixed with a '-' sign (they would all be expected to be negative of course).
(03-09-2025, 11:41 AM)ppppenguin Wrote: I have some 2.2uF 50V Wima plastic dielectric caps. About 0.5" high and about 0.25" square. This shows that you can get a fair number of uF of plastic cap in a small package.
They're good capacitors! It's what I also had in mind when suggesting polyester - you might find a slightly cheaper, epoxy-dipped version rather than the Wima cased type, looks more the part. Either way, polarity then becomes immaterial.
Though if you do go down this route, we'd still like to know about the actual voltage and polarity measurements!
