I suspect that this is a common failure, so I am putting my fix out
there in the wild should anyone else want to use what I learned.
I purchased this amp in the early 1980s and have always liked it. It
was from Sony's high end specialty line at the time.
The problem was that the speaker protection circuit first started taking
a long time to kick in after power on and then eventually wouldn't kick
in at all. You can tell this by the fact that the LED in the volume
knob would stay red instead of turning green and the tell tale sound of
the speaker relays kicking in never happened.
I bought a scan of the schematics off the net. A bit of a rip off, but
I couldn't find free ones.
From the way the problem developed I figured that it was probably a
capacitor which had gotten old. Oddly enough, the circuit is split with
some components on the main board and some on the power supply board.
So, I replaced capacitors C404 (10 uF,16V), C405 (100 uF, 25V), C406 &
C421 (both 10 uf, 50 V NON-POLARIZED). Based on the circuit diagram I
suspect that C405 was the main trouble maker. It looks to me like R406
and C405 create the RC time delay which is an input trigger to the
HA12002 control IC. All of the caps I changed out are on the power
supply board. Be very carefull, those traces lift from the board very
easily. Even with care I buggered two of them up and had to do a bit of
add on wiring to make things right. For everything except the
non-polarized caps I was able to purchase replacement units with higher
voltage ratings. Maybe these will last 40 years
.
Finally, I also switched the rear panel speaker impedance selector to
the 4-8 ohm setting. I found out that all this does is to lower the
voltage rails to the output stages and thus limit the peak current
availability. I never push this amp hard, and given it's age I feel
that pulling back on the output drive voltage a little might buy me some
extra lifetime.
Happy Listening!
John