GEC BC506 Non electrolytic capacitor question

[Danndad]

Can anybody tell me why GEC put a non elec cap in this position (C11) please?

[Craig]

I think that is the symbol for a non-polar electrolytic.

[Danndad]

I understand that Craig but why?

[Kalee20]

My first thought was that maybe the DC voltage across it might reverse - though looking at the circuit snippet you have given, and assuming the wires going off the left-hand side just go to an IFT secondary, this will not be the case as it will always have the emitter-base DC voltage across it (about 0.12V using the voltages marked, which would be right for germanium).

It's easy enough to prove - if you need to replace this capacitor, just use a conventional electrolytic, connect it positive to base, and tack a couple of leads to it to connect to an external DVM so you can monitor the voltage across it as you scan the bands looking for something to enjoy.

Of course, these days you might find a low-voltage polyester or ceramic capacitor small enough to fit. Just check the temperature coefficient if it is ceramic!

[Mike Watterson]

No-one uses bipolar unless they need to, because they are bulky and more expensive, and less reliable.

It's decoupling the AGC. Fed from positive detector via 8.2k Ohms. One side has emitter voltage of -0.68V or more or maybe zero depending on signal level. It feeds the PXA101 base on the AGC side via IFT, so base can be -0.8V. But as signal rises the base voltage drops.

The temperature co-efficient and voltage rating won't matter on a miniature ceramic.

The 8 uF seems large. I'd try a 1uF ceramic, which is a shorter time constant. Or maybe 2uF, though the 8.2K feedback resistor is small.

If using a polarised electrolytic, then the negative goes to IFT and positive to emitter. But it's likely to go leaky on strong signals. A strange place to put the "earthy" end of AGC response time / filter cap. But I wouldn't use a polarised cap. They have a polarised 8 uF on the volume control, so it's a deliberate decision.

R&TVS 1962-1963 pages 170 - 171

I have the earlier BC505.

Edit:
Note C18 8uF polarity and voltages are with NO signal.

[ppppenguin]

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.

[Danndad]

Thanks for all the replies/explanations.
Craig is right in that it is a non polarised electrolytic, I initially said it wasn't.....
I had an issue with the radio drawing double the stated amount of idle current on the sheet. 1 red/black plessey had turned into a resistor so was replaced and that reduced the current considerably but will probably be better to replace all those plessey electrolytic.
Danndad.

[Mark Hennessy]

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.

[Kalee20]

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).

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.

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).

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!

[Mark Hennessy]

I'm fairly happy that the polarity must reverse - not least because if it didn't, they wouldn't have gone to the expense of a non-polarised part

With no-signal conditions, we know that there is -0.8V at the left of the cap. The other end is at -0.68V. So the positive end is on the right.

With the signal present, the voltage on the left can only be taken "down" the page, i.e. made more positive. As mentioned, I can't predict by how much, but that's the only direction of travel possible.

Of course, VT2's emitter voltage also falls, but that can only fall to 0V worst-case. So if the AGC can pull the left of the cap more positive than 0V, then there's your reversal. I considered doing some sums to see how likely this is, but I keep coming back to my opening statement - it might happen because they fitted the non-polar cap.

Yes, that will likely put the transistor into class C. But it's feeding an IFT, and that's followed by another IFT. So I suspect that will remove a lot of the harmonics, and even if not, how problematic would they be in this set with a very basic audio side? Others will have more insight than me, I suspect, but certainly, I've seen AGC circuits that work this way before.

It's interesting that they fitted another non-polar cap across the LS. You'd think they could have used a regular cap ahead of C22, but perhaps that didn't work for them? Or perhaps there just wasn't the price differential with non-polar caps back then, so there was no reason not to use them? If so, perhaps C11 was non-polar "just in case", which rather undermines my argument!

That's why I said it needs to be tested. If there is a reversal, and the signal-present conditions results in a greater potential than the rarely used no-signal condition (which is only 0.12V apparently), then I'd be comfortable using a polarised part installed appropriately for realistic use. No modern electrolytic will mind 0.12V of reverse bias.