[heathkit] AA-1640 Mod

["Jeff" <tractorguy14@worldnet.att.net>]

Twenty six years ago I built an AA-1640 Stereo Power amplifier.  I noticed
that the left channel of my AA-1640 was running too warm, and it did
eventually fail due to thermal runaway. This may be what happened to the
person that mentioned having a circuit board with a hole burned through it
as I did experience a similar occurrence. I called Heath Technical service
and was informed that this thermal runaway could in fact happen, and that
sometimes when they "factory assembled" an amplifier, it would try to. I was
told that it was being caused by the fact that the output transistors were
not "matched sets" based on their current gain. The mod was simple. Add a 22
Ohm resistor across D208, one of the output stage biasing diodes. I
performed the mod to both channels, ran my tests, and then put the amplifier
back into service. It has not failed since! The mod is quite easy to
accomplish as D208 is easy to get to. I used a half watt film type resistor,
installed it on the component side of the circuit board and soldered it to
the leads of the diode. I could probably be installed on the other side with
just a little more work. But like I said, it hasn't failed since.

Jeff