Revox A77 Dolby - Capacitor Reversed on Record Processor Board

[Humber888]

I am continuing to restore a basket case A77 MkIII Dolby machine and recently sourced the record and replay Dolby boards. Starting on the record processor board, I changed all the preset pots as some had already lost their wipers and then changed the 10uF tantalums for long-life, low ESR, aluminium electrolytics as the tants' ESRs were all over the place (from 2R up to 16R). Strangely the 3.3uF tants all look healthy - maybe a different manufacturer. Anyway, in checking my handiwork I found that one capacitor I had replaced seemed the wrong way round when checked against the circuit and layout diagrams. I thought I had triple checked everything when I replaced the capacitors, so looked at the same Dolby board in my good machine; only to find this was mounted the same way round as my reworked board. So, it appears that both boards were originally fitted with reversed tantalum capacitors in this one position.

This capacitor is part of the calibration oscillator, used to set up record levels and is fitted between the emitters of two transistors in the oscillator. The circuit is attached and hopefully readable - the bottom left part is the relevant bit. The capacitor in question is 10uF, C30, and it was not obvious to me what d.c. voltage existed across it. So I did a few experiments on the working card with a DVM across C30 and also checked both ends on a 'scope. I found that there was about 0.3Vdc reverse bias across it according to the (average-ish reading) DVM and the 'scope showed about 6.6Vdc on both ends with a 2V peak positive going only half sinewave on top. I also ran up the repaired board and got a similar result, other than the reverse bias was 0.45Vdc. Oscillator output from both boards is 11V p-p sinewave, 800Hz at the wiper of P103r.

Finally my question! I think less than 0.5V reverse bias on a tantalum is probably just about viable, especially as the circuit is only powered during calibration. Since the board in my working machine has worked OK since 1973, I don't propose to do anything to it at present, but I think it prudent to put a new tantalum into the repaired board rather than stay with the new aluminium electrolytic and also fit it so that its polarity matches the circuit diagram. But, have I misunderstood my measurements? It just seems odd that Revox would make a mistake fitting this capacitor on both cards. Your views oh mighty experts?

Mike

ps: I forgot to check the phase relationship of the half sinewaves on the ends of C30 to see whether they were in-phase or not.
pps: To be clear, the circuit and layout diagrams, if followed, would apparantly result in a forward biased capacitor on both boards if my measurements make sense.

[mitajohn]

The + of C30 as seen on PCB is connected to the emitter of Q11.

[Humber888]

John,

Do you mean that on an actual PCB that you have looked at, the +ve of C30 goes to Q11 emitter? If so, that is very useful information. Both of the actual PCBs I have here had the -ve of C30 going to Q11 emitter. This does not agree with either the circuit diagram or the layout in the A77 service manual.

Would anyone like to comment on whether a tantalum capacitor is better at coping with reverse bias than an aluminium electrolytic, and how much it might stand?

Cheers.

Mike

[mitajohn]

Do you mean that on an actual PCB that you have looked at, the +ve of C30 goes to Q11 emitter?

Yes, as well as on the PCB pic in the SM. May be it is because of component tolerances that in some builds were in the opposite connection, but your "problem" can be easily solved by replacing the cap with a non polarized one of the same value and voltage.

[Humber888]

I doubt that Revox would choose the orientation of the capacitor on test - especially since the two boards I have with C30 the 'wrong way round' are reverse biased. It just seems surprising that the error would appear on both boards. Anyway, I have fitted a new 10uF tantalum in accordance with the polarity shown on the circuit/layout diagrams. I doubt I would find a non-polarised 10uF of the same size as an electrolytic - the PCB is quite crowded in that area.

Nobody has picked up on the question of how much reverse bias a tantalum or aluminium electrlytic can stand, so I may raise the question separately in the Components and Circuits part of the forum.

Mike

[Radio Wrangler]

It used to be reckoned that 10% reverse voltage was allowable, but that was back before tant failure rates shot through the roof for normally biased parts. 10% was allowed at HP back in the day.

David

[Humber888]

Thanks for that David. The original tantalum was 16V rated. The capacitance measured fine but ESR was high - mind you so were all the other 10uF tants on the board - ESR up to 16 Ohms. Since the board dates to about 1974, it looks like 0.5V reverse bias occasionally had not done too much harm. I've bunged in a 25V replacement tant, assuming it would be happier than an aluminium type where the dc bias is a bit ill-defined. It measures under 2 Ohms ESR and is hopefully the right way round.

Mike

[mitajohn]

Have you checked the polarization of the Tants according to the Revox attached notice?

[Humber888]

All the tantalums are marked with the capacitance and with a + sign on a plain background - so no problems telling orientation.

[mitajohn]

OK. I do not know what is going on in your boards. I measured mine, 6.76VDC on Q11 emitter and almost 6.2VDC on Q10 emitter.

[Humber888]

John, that sounds about right depending on how your DMM 'averages' the dc with positive half sinewave on top. My DMM gave about 6.6V on the emitter of Q11 and about 0.3V or 0.45V lower on Q10 collector (board 1 or board 2). The sinewave at the wiper of P103r was about 11V p-p. The oscillator worked on both boards, just that C30 was fitted the wrong way round in both cases!

Mike

[mitajohn]

...on Q10 collector

You do mean the emitter, because the collector of Q10 is at ~21VDC.

[Humber888]

Brain damage! Yes I mean emitter in both cases. Hence, if you put a DMM across C30 i.e. between both emitters, you get about 0.3V for board 1 and 0.45V for board 2 with Q11 emitter more positive.