My MS‑10 has been behaving a little poorly recently, with the EG release time fluctuating to a gradually growing degree. So I’m going to have a look at it. First, have a look at the service manual as this failure seems likely to be a misbehaving capacitor. And yes, there’s a 6µ8 Tantalum cap (C35) on the release stage, so that’s the main suspect. I hear some bad things about Tantalums, but I’ve never had one fail before. Anyhow, I did have the thing apart for cleaning when I got it but didn’t take the PCBs out as it involves a fair bit of unscrewing things. So this is my first look at the relevant board (KLM-126c).

(Picture taken after repair.)

Checking C35 in-circuit, difficult to be certain but nothing obvious wrong.

But out of circuit it tests well. The next most likely failure is probably the op amp C11 which takes the signal from C35. It’s a TL071CP. This type of chips are also apparently unreliable, but I don’t recall a failed TL071 in anything I’ve used previously (and I’ve had a few). I don’t have any in stock, so pause for a few days. I did note that compared with the other ICs on this board, the legs and maybe the top on this one seem rather dark-stained.

While waiting I did some checks on the other capacitors in-circuit. The only ones that give cause for concern are C33, the negative rail smoothing cap, which is a little high in DF but not out of tolerance. More of a note for the future at this point. And then there’s C14, the only other Tantalum cap on board, where the DF seems very high in-circuit, though I’m not actually sure what the original spec for these was. I was going to test it out of circuit but as I was desoldering it fell out and disappeared into the undergrowth so I replaced it with an Aluminium cap.

New TL071s have arrived. I’ve taken IC11 out, initially just for testing but I realised that the dark staining was in fact soot, which indicates something. I’ve put a turned-pin socket in its place, added a new chip and put everything back together, sans knobs and most of the nuts. That seems to have fixed it.

While I was at it, I checked the output from the voltage regulators. They’re both reasonable.

Having had it running for a while I decided to check whether there are any other TL071s on the board, and there is one, IC4, in the VCO. Both are TL081 on the schematic.

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