In article , Don Pearce
wrote:
On Tue, 17 Nov 2009 15:27:33 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article , Don Pearce
wrote:
I've downloaded a PDF of the service manual for $8. The offending
power amps are strange (to me) in that they have 3 HT rails. +/- 35v
for most and +60v for the output pair.


Could be class H. This is a power saving scheme that uses two power
rails - one low for normal listening, and a higher one that gets
added on when the power requirement is higher.

Right - but if you removed the 60 volts you'd get no output?

So the lower voltage is only for the earlier stages? OK, not class H
then.

Hard to guess without the diagrams, etc. But two things came to mind here.
One was that the output stage has voltage gain. e.g. bipolars with
collectors to the output and emitters to the rails. The other was FET
devices with an aburdly high resistance or minimum source-drain
requirement. However I'm also wondering... does the above mean *only* +60
for the output and not +/-60?... Curious.


Audio amps are real buggers because of the global feedback. A dozen
different faults can lead to the single symptom of the output being
stuck at one of the rails. Luckily you haven't got that problem so you
have a fighting chance.

Yes. I can recall the bafflement of people I worked with who were used to
ac coupled stages when first confronted with dc throughout and overall
feedback. 8-] The trick, of course, is to look for where the voltages on a
component or stage are clearly inconsistent and so only explainable by a
specific fault at that point. Can be a poser if you don't know the circuit
well - particular if you have 'protection' providing other influences and
interfering! Glad not to have to fix such things. :-)

Slainte,

Jim

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