[Jack[_11_]]

Last night I pulled heat sink for the last stage output transistors to
get a good look at the back of the power amp board. This involved
moderate flexing of wires coming to and from the board. All the solder
joints looked sound. Apart from taking a soldering iron to all of them,
I could see no obvious need for attention so I put everything back
together. After that the left channel, which was cutting out
consistently, either at startup or after ten minutes or so of play after
bringing it back by turning up the volume, was now playing without
cutting out. This morning both channels started up and both have been
playing for an hour or so.

Last night, after an hour or so of consistent play, I took more
measurements of the transistors of the second to last stages, playing a
100Hz tone, and found that the AC voltages of the emitters and bases of
the left channel were half or less than those of the collectors and
bases of the right channel. These transistors are 2SB528 and 2SD358


2SB528
L Channel -- E 4.0 -- B 3.4
R Channel -- E 9.2 -- B 8.6

2SD358
L Channel -- E 5.2 ---- B 5.8
R Channel -- E 10.5 -- B 11.1



This morning, playing a local jazz station in mono mode, I noticed that
the left channel is lower in volume and the high is missing, cymbals
disappear. I switched speakers to make sure they weren't the problem and
they weren't. I switched the preamp out and power in jacks on the back
to determine if this was a preamp or main amp problem. The problem
stayed in the left channel. My guess is that these problems are related.

NTE has replacements for 2SB528 and 2SD358, being NTE292 and NTE 291
respectively. Fortunately for me, I live close to Al Lasher's
Electronics and they have both in stock. Being that the transistors in
the next lower stage all measured the same when the cutting out problem
was not manifesting itself and the these particular transistors have
widely divergent readings, the problem would appear to be the
transistors themselves or some associated diodes, resistors or capacitors.