03-09-2025, 10:06 PM
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. Hard to guess the magnitude though - but it can't be too much, I'd have thought? 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'm assuming they missed the minus sign from the voltage on the base of VT2.
I think I'd take some measurements and make a decision about whether a non-polar cap is really needed. After all, they can withstand an amount of reverse bias - around a volt or so is usually OK, especially if it's only temporary. That said, Hacker made a mistake with the silk screen markings (if I recall correctly - I'd check but that involves climbing up in the attic) with the Hunter, meaning the FM discriminator cap was incorrectly installed in all sets. It saw several volts of the wrong polarity, yet the sets seem unbothered by this. I used to replace them with a new part correctly fitted, but as it made no obvious difference to the performance, I gave up worrying about it. They were always the blue Philips axial types. The current available was tiny, so no explosions or other drama, but I assume they gradually re-formed themselves with the polarity reversed. I should take one out and run some tests one day - the ones I did replace gave good results on a tester, but as these were paying jobs at the time, I never thought to test them for polarity.
Regarding non-polar electrolytics, I'm not aware of them being less reliable than equivalent polarised parts.. I use them in low level audio circuits from time to time with no issues at all. They survive pretty well in passive crossovers too, where they have to deal with rather more current than in a pre-amp or similar. I'm ignoring the vintage parts here - stuff from the '80s onwards are pretty good in my experience. They often have higher ESR than film caps, of course, and that is something the designer has to account for at the design stage - replacing electrolytics with film parts can change the frequency response by a dB or two, depending on the circuit.
As I have them in stock, I would probably use one of these: https://uk.rs-online.com/web/p/aluminium...rs/7270486
They're good for 2000 hours at 85 degrees C, which is the same as most standard grade polarised caps. I use them for lots of things, never had a problem with them.
I think I'd take some measurements and make a decision about whether a non-polar cap is really needed. After all, they can withstand an amount of reverse bias - around a volt or so is usually OK, especially if it's only temporary. That said, Hacker made a mistake with the silk screen markings (if I recall correctly - I'd check but that involves climbing up in the attic) with the Hunter, meaning the FM discriminator cap was incorrectly installed in all sets. It saw several volts of the wrong polarity, yet the sets seem unbothered by this. I used to replace them with a new part correctly fitted, but as it made no obvious difference to the performance, I gave up worrying about it. They were always the blue Philips axial types. The current available was tiny, so no explosions or other drama, but I assume they gradually re-formed themselves with the polarity reversed. I should take one out and run some tests one day - the ones I did replace gave good results on a tester, but as these were paying jobs at the time, I never thought to test them for polarity.
Regarding non-polar electrolytics, I'm not aware of them being less reliable than equivalent polarised parts.. I use them in low level audio circuits from time to time with no issues at all. They survive pretty well in passive crossovers too, where they have to deal with rather more current than in a pre-amp or similar. I'm ignoring the vintage parts here - stuff from the '80s onwards are pretty good in my experience. They often have higher ESR than film caps, of course, and that is something the designer has to account for at the design stage - replacing electrolytics with film parts can change the frequency response by a dB or two, depending on the circuit.
As I have them in stock, I would probably use one of these: https://uk.rs-online.com/web/p/aluminium...rs/7270486
They're good for 2000 hours at 85 degrees C, which is the same as most standard grade polarised caps. I use them for lots of things, never had a problem with them.
