Clip on Heat sink/heat load on the transistor before failure [resolved]

[Alonzo]

Well I got home today and my daughter tells me the stereo isn't sounding right.  I find that the left channel isn't playing at all, no sound from the speaker.  Tilt back the Stereomour II and all the leds are on but there's a heat sink on the table top underneath.  I shut it down and take it to the basement for work tomorrow.  She doesn't know how long the channels been out, turned it on around 9 and I turned it off at 5 or so.
So, how long can the transistors go without the heatsink?  I plan on checking the voltages on the shunt board in the morning but should I just replace it anyway because of the potential heat damage?
This is a Stereomour II with Shunt Regulator and DC filaments.

[Doc B.]

So, how long can the transistors go without the heatsink?

Precisely up to the moment they fail. You will either have to measure the transistor to see if it is shorted, or put it back together and see if it still works.

[Alonzo]

Measured voltages to see, on the shunt board
IA   391V
OA  115V
KregA  0.1V
IB   391V
OB  307V
KregB   2.8V

2 LEDs on the small board are also out.  I don't remember the precise volts but I got about half the value on the "out" LED's vs the "on" LED's, something like ~150 on the out and ~300 on the on ones.
 I'll look around for a new transistor. Check for other problems.

[Alonzo]

Replacing shunt board transistor didn't solve anything.  Voltage at 14 low at 110, 2 LEDS out on small board IB side.  Volts at 9 and 1B are 307, 12 and 1A are 114.  Swapped tubes and no change, don't see any loose connections or cold solders.  All the other standard voltage checks are fine (but the shunt board is still low at Kreg A and OA.)
Still troubleshooting.

[Alonzo]

Got those backwards, 9 and 1A are 307, 12 and 1B are 114.  Gonna check that transistor next, make sure I installed the heat sink correct and touch up the solder.

[Alonzo]

One last thing, initially all LED's light up and the ones at 1B on the small board then fade out.  Any ideas on what to chase?

[Paul Birkeland]

OA  115V
KregA  0.1V

Yeah, that's indicative of potentially no current flow. I'd replace the transistors and voltage regulator that serve that channel.

-PB

[Alonzo]

Yep, that did it, new TL431 and it's back working.  All the voltages check out.