Heathkit AR-1250 FM stereo receiver problem

[John Farrington]

Has anyone had experience working on the solid-state AR-1250 receiver's
amplifier section? This has AB push-pull outputs with static bias circuitry,
30W RMS each channel at 8 ohms. Made around 1982, It's worked for years,
but I never played it at a higher volume until now, whereupon a bias
instability appeared that has me pulling my hair, and there ain't that much
left.

It works fine at normal volume levels, running all day and hardly getting
lukewarm, but with only somewhat louder music (peaks up to just 3 or 4W on
the output LED's) it gradually warms up until it reaches a point where
suddenly something that looks like thermal runaway occurs:   the bias to
the output transistors shoots up and drives them into heavy conduction,
giving them and the heat sink a large abrupt temperature spike and then
usually blowing one or both of the fuses in a channel (in series with
Vc +/- for the 2 AB output transistors). Once it fried one of the output
PNP/NPN pair. After cooling down it again works OK at normal volume.

It's not a speaker problem, as it does the same thing while they're
disconnected and using earphones in the panel jack - when the spike
occurred the DC current surge burned out a set of phones.

I made the following checks while it was running cool:

- Checked all the wiring to the board, and touched up all solder joints.
   No solder bridges, and the foil patterns are good.
- Reset the bias pots according to Heath's instructions, repeating the
   procedure after it warms up, as specified. No change in operation.
- Traced audio throughout with a scope, and all stages work.
- I checked all the typical in/out voltages stage by stage as listed on
   the schematic, and they were within tolerance.
- Next I removed and tested each semiconductor, and compared them to
   new components. They looked good, but just in case I replaced the old
   ones with NTE equivalents, with careful attention to thermal mounting
   of the outputs.
- Ran it again with the same results - worked OK at normal volume, but
   with higher volume thermal runaway eventually occurred as before.
- Next I removed every small resistor and capacitor in the channel that
   over-heats the fastest and checked them. No bad ones, but just in case
   I replaced them all.
- Ran it again with the same results - bias to the output transistors
   gradually increases as it warms up even at normal volume, then at a
   variable time after increasing the volume somewhat it suddenly spikes
   causing a lot of sudden heat and blown fuses.

As there's nothing left to replace I'm beginning to suspect that it's a
design fault rather than a component malfunction. Has anyone heard of
this problem, or have any ideas, and did Heath issue a tech bulletin
on it?

Thanks.

John   KE5ZB

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