🚀 go-pugleaf

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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