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[JGreenslade]

Hi,

Whilst in the process of re-capping a collectable discrete Disco Mixer (circa 1970), I came across an unusual style of cap on the mix amp (please see attachment), and unless I’m overestimating the problem, modern “equivalents” seem hard to come by; they either have a straight DC rating and no AC rating, or are bipolar...

Correct me if I’m wrong, but I’m *assuming* the Frako cap (C1 / C11) is rated for 100v DC, with the ability to withstand up to 35v reverse bias?

Most of the caps had become leaky (particularly those in the output-stages – the readings were pretty funny to say the least) and the client wants the mixer to work for another 20 years without any hassle, so a change of all electrolytics is in order. So far, all electros have been replaced bar the unusual one...

Even though modern caps are much smaller than their counterparts from 30 yrs ago, it does seem strange to replace this 1” long cap with a modern Japanese 10u 100v DC cap... But having said that, I’m weary of fitting bipolar caps just for the sake of it – we’ve witnessed a few horror stories at this group where people have stuffed bipolars in place of old electros...

I should add that the client has specified uber-costly Black Gate caps (the mixer is entirely cap-coupled with no transformers or chokes); BGs are supposed to last 20yrs + and one would like to think they're pretty rugged - at least they should be for that price...

Maybe I should replace with a modern cap, drive a +22dBm signal into all inputs to work the buss hard and put a temp probe on the cap? If it stays cool, all will be fine?

The holiday season has started early in the UK, and both the people I would usually ask are on holiday, so it would be much appreciated if anyone here could assist – that way I can clear the bench!

Many thanks in advance.

Justin  

[Andy Peters]

thermionic wrote on Fri, 30 June 2006 07:56

Whilst in the process of re-capping a collectable discrete Disco Mixer (circa 1970), I came across an unusual style of cap on the mix amp (please see attachment), and unless I’m overestimating the problem, modern “equivalents” seem hard to come by; they either have a straight DC rating and no AC rating, or are bipolar...

The schematic has a "+" sign by the FETs' gates, leading one to assume the cap is polarized.

Quote:

Correct me if I’m wrong, but I’m *assuming* the Frako cap (C1 / C11) is rated for 100v DC, with the ability to withstand up to 35v reverse bias?

No, it's not the reverse voltage rating, which for electrolytics is very small indeed (maybe a couple of volts, max).  Reverse bias of 35 V on an electrolytic will destroy the part in short order.

If the 35 VAC rating is rms, then (do the math) it's about 99 Vpk-pk.  Round it off to 100 V

-a

[JGreenslade]

Thanks Andy.

I did note the polarity on the schematic, and ordered a 100v DC rated cap when I first saw it; it's just that I've never seen a cap listed in a schematic with both DC and AC ratings before (the cap itself also has this printed on it - as you can compute the AC rating from the DC rating, why print both ), and with 1970 being somewhat before my time, wondered if something unusual had been fitted.

I now feel slightly daft asking such a question, but I guess it's better to be safe than sorry.

BTW, check out the attached photo (not the cap in question btw, just an example).

Justin