Arfa Daily wrote:
Anybody got any experience of an Audio Telex Communications of Sydney P.A.
amp model AMIS 250 ? I am particularly looking for a copy of the schematics,
but any help with it from anyone who knows the model, would be appreciated.
So far I have only been able to find a user manual.

I can't help you, I'm afraid. I've never serviced PA equipment.
You might have better odds asking in aus.electronics, which is a pretty
lively group. I've added them to the newsgroups line.

As it came to me, the main bridge reccy had one limb s/c, and the wires to
it looked as though they had got pretty distressed from heat in the past.
With the reccy replaced, it seemed to come up ok, but had no output. Signal
is getting up to the front pcb from the rear one, and the signal presence
detector for channels 1 and 2 is working, as the channels 3,4,5,6 mute relay
triggers when signal is applied to Ch1 or 2. You can also see the signal at
the detector opamp. But that's it. There doesn't seem to be any further
preamp activity at all. There is an 'insert' jack on the rear panel, and
this should carry preamp output, which it then links back to power amp
input. There is no preamp out signal at this jack. Nor anything at the 'tape
out' RCAs on the rear panel.

One further thing. Sometimes when you switch it on, the output stage very
quickly runs very hot, and the bridge gets very hot, which I guess is the
problem which led to the demise of the original, and made the wiring look as
though it has got hot, so when this problem occurs, there's some serious
current going through the output devices for some reason.

Supersonic oscillation maybe? Have you tried hanging a scope on the
preamp output?

When it's doing
this, there are no signs of any instability - in fact still no output of any
description. I would add that this is a crap lead-free product, and every
joint on the output board of course *looks* bad.

Ew.

I have not been able to
provoke the heating up problem when it is running cool, by any kind of
disturbance, but with lead-free, in my experience, that doesn't necessarily
mean that the problem is *not* a bad joint, and that is still my gut feeling
about it ...

Fair enough, too.

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