In article ,
Jim Lesurf wrote:
In article , Dave Plowman (News)
wrote:
I've been given this rather large and heavy AV amp dating from the
last century which is faulty. It will yield a pretty decent mains
transformer even if I scrap the rest.

Basically it started making funny noises from one of the rear channel
amps

What sort of 'funny noises'?

Didn't hear them myself but was told they happened with the rear amps not
being fed. Ie, Dolby off.

- and after a few weeks now powers down a couple of seconds after
being switched on. My guess is the speaker DC protection cutting in due
to a faulty output on that amp.

Is it worth attempting a repair?

Does a scope/meter show an excessive output dc voltage or one that
waggles about along with the noises?

I've not even had the cover off yet. But can see it's pretty densely
packed through the grills.

If you can put a current meter in the rail to measure the current (use a
2mic cap to bypass the meter) see if the quiescent jumps about.

Afraid I don't know a thing about the specific model. Hard to say from
the above if the fault is trivial or serious. Might be a loose
connection or fuse. But might be something more costly and
hard-to-diagnose.

FWIW One of my power amps a few years ago developed occasional
'rustling' noises which were accompanied by changes in the output dc
level of about 100mV. Freezer spray followed by replacing some
pre-driver transistors fixed this. Turned out to be an intermittent
connection inside the pack of one of the transistors. Replacement device
cost about 20p IIRC, but was a pest to find which one to replace. At
least your fault isn't intermittent so can be relied on to show up when
you are trying to nail it down. :-)

I also have encountered cases where the monitor circuits misbehave and
the actual amp is fine. In such cases you can sometimes just disable the
protection. But that obviously is only safe if you know this is the
problem when using it with speakers - as distinct from dummy loads you
don't mind frying. 8-]

TBH I never liked active protection for power amps. Just one more
bolt-on to fail or get in the way. Prefer fuses in the rails.

Thanks, Jim. I'll report back when I do some tests.

--
*Plagiarism saves time *

Dave Plowman London SW
To e-mail, change noise into sound.