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Default Very poor electros ...

Had a Rega Elex hifi amp fetch up on the bench today. It was low on one
channel compared to the other. On a 'scope, both channels looked a bit
scruffy at the output, as well. Both channels were the same level and
reasonably clean at the volume control. It's a fully discrete power amp
south of the volume control, and nothing was obviously wrong in the bad
channel. So just for sport, I picked up the ESR meter with a view to giving
all the caps a quick read. The first one that I put the meter across, was
quite close to the driver transistors, and it went over 60 ohms. Ah-ha
thought I. Got it ! I checked the cap on the opposite side of the circuit
near to the complementary driver, and it too had a high ESR. Putting this
down to them being near to components that probably ran pretty warm, I
didn't think a lot more about it, and just went ahead and replaced them.
That restored the level of the 'bad' channel - to TWICE the level of the
'good' channel. At that point, the alarm bells started to ring. There are
only three values of cap used - 2.2 uF, 10 uF and 22 uF. Curiously, they are
all marked with the same "C" numbers on the board, i.e. all of the 10 u's
are called "C8", the 22u's are "C10" and the 2.2u's are "C9".

Anyway, I started to measure them all, and every single one, in both
channels, is either high ESR or completely open. Over 20 caps in all. I've
pulled them all out, and re-measured them out of circuit, and the state of
them is confirmed. I hadn't got enough in stock, so I've got some more
coming tomorrow. Most of them are not located near to hot-running
components, so I'm at a bit of a loss to explain how that many caps all go
bad. It will be interesting to see how much better the amp performs when all
of the caps are replaced, and whether the scruffiness of the output waveform
will be improved.

Arfa