Richard Rasker wrote:
Hi all,

I'm trying to fix an old Heath amp, the AA-1600. One day, it blew up one
end stage, and as the original PNP transistors 2N6609 are no longer
available here, I replaced the transistors with MJ15022/MJ15023 pairs.
Further investigation revealed that all four(!) 4K7/2W resistors
(R728/R729) were overcooked, and had resistances between 80K and
infinite. All other components seemed fine, so I thought I had the
problem nailed.

For testing, I put a 150W halogen lamp in series with the mains supply,
and everything still went fine. The end stage bias current seemed
completely stable, the amp operated on nearly its full mains voltage,
and both end stages behaved identical, both on low and high frequencies.
Of course, the supply voltage dropped when I applied a load on the output,
but even then, I could find no difference between both end stages. They
delivered 40W each without a problem before the mains voltage dropped too
much for the amp to function properly.

Then I hooked up the amp to the 'hard' mains voltage (without a series
lamp), and still all seemed well. The end stage bias current was fine and
stable, and both end stages delivered the same signal without a load
connected.

But alas, with an 8 ohm dummy load connected, the very same end stage
fried instantly as the output power got past a few watts. I was actually
monitoring the oscilloscope and volt meters when it happened, and there
was no warning sign whatsoever. No sudden oscillation, no DC voltage
drift, nothing at all. One second the amp worked fine, an instant later an
end stage transistor shorted out, blowing the fuses. :-(

So before I replace the end stage transistors once again, I'd like to see
if anyone here has a clue ... I considered replacing both driver
transistors as well (Q711/Q712), but based on the original type numbers
(F-317-B respectively F-417-B), I can't find any suitable replacements.

A somewhat low-quality schematic diagram can be found he

http://web.archive.org/web/200106071...th/aa-1600.gif

Does anyone have an idea what might be the problem? Or how I could trace
down the problem without blowing yet another set of end stage transistors?
I'm particularly puzzled by the fact that the wretched thing only blows up
when loaded and at the same time connected to the 'hard' mains voltage
(230V AC). Even with a low-ohm load in series, the amp behaves just fine ...

Hi Richard...

Happens that I still have the whole manual for the aa-1640, but no
idea at all how much it differs from the 1600. I'm also on the
other side of the puddle (Canada), though I doubt that would make
much difference other than to the power transformer.

If you think it might be similar enough to help, I'll be happy to
scan a few pages for you.

Take care.

Ken