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Said output drivers, Q15 and Q16, basically both had their collectors
shorted to the chassis heatsink right through their own baseplates and
what was left of the thermal pads.
That sent the +60 and -60 volt rails straight
to chassis ground, which then looped back through the above resistor
to signal [e.g. power supply] ground, and, well, fzzzzzzt.
I learned more of the unit's history later; it was an ebay find and had
sustained some shipping damage. Its new owner and/or a buddy had
had to repair the thermal pads once already, and used what was on hand --
possibly the wrong stuff for the purpose, and then maybe a bit too much screw
torque, which over some number of thermal cycles eventually let the metal
transistor surfaces grind their way through to the heatsink aluminum.
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