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 here:
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 ...
Thanks in advance,
Richard Rasker