Assistance with signal path please?

[Enzo]

Would it be OK to run your car engine with no oil or water in it, "just for a moment"?

Never power up a power amp without its heat sink.

[Jazz P Bass]

Yup.

Never!

[Dave H]

Yes R8 is supposed to be 180 Ohms, so I'll need to replace that. I pulled the board and found Q13 open between C and E, but I don't understand what, if anything it has to do with R8, which once I pulled it seems to have been some smoke scraped off the board under it.

All transistors measure open between C and E. They should measure as diodes between B and E and between B and C. Measure the C3 you've removed. The only way I can see for R8 to be smoked is for C3 to be shorted. The current through R3 (33k) is not enough. It can only be a few mA.

[Dave H]

...the collector of Q13 is very difficult to access. I now read 43 vdc at the junction of it and R33. Also, I now see that R8 has been replaced with a 56K, which would seem suspect. It doesn't get hot, though. The metal can Q13 is a few degrees above ambient.

Is 43 vdc normal? I'd expect the collector of Q13 to be either 0V or close to the rail voltage (75 vdc). It's not pulsing is it? Check it with a scope. Q13 and R8 won't get hot with 56k in there. As someone thought it OK to replace 180R with 56k I'd check the other resistor values.

[J M Fahey]

The main problem there today is that R8 is 56k, some 300 times what is expected, meaning the amp is muted all the time or at least quite "stupid".
Don´t you have a 180 ohm resistor or thereabouts (220 would be fine) to continue troubleshooting today?

You can´t lose a week or more every time you find something suspicious, personally if some value is unavailable I test anyway with something close, at least confirms or dispels some theory.
Sadly I guess you still have other problems there.

Signal testing comes later, first get the DC voltages right, what is DC voltage to ground at C3 collector, and at J5, the speaker out rail?

Bias volage across Q4?

[Dave H]

You can´t lose a week or more every time you find something suspicious, personally if some value is unavailable I test anyway with something close, at least confirms or dispels some theory.

I agree. Why lose a week? Replace the 56k in R8 with a 220ohm and short Q14 C to E to defeat the mute circuit.

[Randall]

I replaced R8 with 180 ohm, Q13, and C3 and C 17. The amp now passes signal. I only left it on for a brief moment to measure a few voltages. I am still seeing 45v on collector of Q14. Is this a problem?

"what is DC voltage to ground at C3 collector" JM Fahey, I don't understand the question? Did you mean Q14?

The output rail has 0.33vdc on it, and bias across Q4 C-E is 1.8vdc. And I now see 0v at the collector of Q13.

[Jazz P Bass]

I am still seeing 45v on collector of Q14. Is this a problem?

+45 Vdc sounds about right.

And I now see 0v at the collector of Q13.

If Q13 truly has 'zero' volts dc on the collector, then Q14 would be biased 'full on'.
That would put Q14's Collector at something like +70 some Vdc.
Q14 is a PNP transistor.
The lower the base voltage , the more it is 'On'.

Something is still off..

Either R33 is 'open' or you mistook the Emitter of Q13 (which has to be 'zero volts' as it is at ground potential) as the Collector.

[Randall]

Having triple checked, the junction of R33 and the collector of Q13 measures 0.02 vdc. R33 measures 17.9K ohms in circuit. The junction of R33 and the base of Q14 measures 42 vdc. The amp is passing signal.

So, now I am even more confused.

[Jazz P Bass]

I may have been mistaken on Q13 Collector.

If Q14 Base is at 42Vdc, then all is well.

[Randall]

Is there anything more to check before closing up on this?

[The Dude]

If I'm reading correctly, you still have .33VDC offset at the output? IMO, that's a bit high and there may still be a problem somewhere.

[Randall]

yes, that is with an 8 ohm load connected. It starts a little higher and drops to about 0.3 vdc after maybe 30 secs. Any advice on what else to check?

[The Dude]

What is the offset with no load/no signal?

[Randall]

It starts off at about 1.5 vdc and settles in at about 0.3 vdc after about 30 secs.