Ampeg svt 3 pro Power Amp

[JMFahey]

OK, let's go step by step.
Measuring is the way to go.
I said earlier that even without the servo correction (the 1/4 TL074) which is the icing on the cake, the output rail should be close to 0V all by itself.
Why? because if we have symmetrical rails, and symmetrical components, they should meet halfway, i.e. 0V or very close.
From top to bottom we have (as main voltage dropping elements):
R14 > Q3 > R10 > Q2 > Q4 > R19.
You'll notice that string is symmetrical, the "bottom side" is exactly the same as the "top side", only it's its mirror image (and so NPN instead of PNP).
We'll measure voltages across that string, expecting symmetry; if not, that points to the culprit.
1000 times better than "mesuring parts" at random which is slow, tedious, can introduce new errors and is not the shortest path.
And it may miss the real problem !!!
Please measure and post voltages at:
R14 top and bottom
R10 top and bottom
Q2 emitter
R19 top and bottom

When I say top and bottom, this refers to "as drawn", check what pin connects to other components , following the schematic.
In the actual PCB parts can be oriented any way, depending on PCB designer convenience.

I know this looks an endless task, but truth is: I do not know what the problem *origin* is, no Tech knows beforehand, and the only way to know is measuring.
If I had the amp on my bench, I'd "guess" what's wrong, and take some measurements to confirm or discard.
If not successful, guess another possibility (based on experience and what I find here) and test it ... and so on and on (until getting close enough).
I may have a lucky day and confirm the first or second guesses ; or it might take 10 or 15 tests in a real bad day.
Reality is that all those possibilities may be tested in, say, an hour.
But since we are communicating by Mail ... it may take many days.
So, please, be patient, I'm sure we'll repair this amp.
Good luck.

PS: your measurements tell me the main (officially +/-65V rails are symmetrical, (+/- 32V) , good, so now we proceed to the next step.

[served]

I cant thank you enough for doing this!

[served]

So here is what I found.
I have to say that the voltage change during time, so they are not 0.1 accurate.
If I stoppend the measurement and checked some points again, then they were a bit off, but it should not make a huge difference I guess.

So What I noticed is that blue box number. It started to rise if I connected my DMM + probe to it. And it rised very fast (seconds) as I could not detect a start point. (my DMM is too slow)

Bias is set to low. Trim resistance max.
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[llwhtt]

The key is the -24.4 on the collector of Q3, why is that negative? Have you checked your source resistors on the positive side, R32,34,36, and 38? If those are open that might explain the imbalance.

Craig

[JMFahey]

If you had showed this voltage chart from the beginning, we would have saved a lot of time
Pulling parts and measuring then "outside" is slow, dangerous, and it's easy to miss a mistake.
While making it work, shows what's not normal" at once.

Anomalies:
1) Q3 : shows 55.7V on emitter, 54.1V on base ; so Vbe -1.6V forward biased (the base on a PNP needs to be more negative than emitter) -> Impossible value.
The base in a forward conducting PNP will be around -0.7V from emitter (It's a diode after all) so we have 3 basic possibilities:
a) Q3 is open. Replace with a good one. BIG suspect.
b) Q3 is miswired. Some pins went into the wrong holes.
c) because of messing with it, some pad or track cracked, same end effect.
d) to further confirm the above possibilities, Q3 stands 55.7+24.4V = 80V , yet passes no current, further indication it's open.

2) Q4 looks fine: 55-54.2= 0.8Vbe, forward biased, normal value.

3) Q5: Base and Emitter voltages look reasonable, but don't believe the 0V on its collector.
Please recheck.

4) Q6 C and E voltages look normal.
It has Vbe -25.1-28.2V=3.1V (positive) which in a PNP means reverse biased.
The value is possible and shows the circuit is trying (unsuccessfully) to correct those weird voltages.
Which it can't because Q3 is open, one way or another.

So, in a nutshell: check Q3, it may be open, miswired or tracks leading to it may be cracked.
Typically a hairline crack where it meets the pad, because too much soldering and unsoldering makes the pad contract and expand (relative to the cold track).
Check that and post results.
Worst case, we might have to momentarily pull protection transistors Q5 and Q6 (or lift one end of D28/29 , but first I'll wait for your results.

[served]

Changed Q3.
Nothing changed.

Q5 reading re checed and I still got 0VDC.

Diode is blocking - and cap is blocking DC.

[served]

So I found that R40 R41 R42 R43 are gone.

[JMFahey]

All four? Ugh !!
Well, it looks like replacing time

[JMFahey]

So I found that R40 R41 R42 R43 are gone.

To be more precise, what does "gone" mean?
How did you find out?

EDIT:

Changed Q3. Nothing changed.

But we *still* have a problem there.
It is impossible that you have 1.6Vbe there
Measure again, on the transistor legs themselves.
If still anything above 0.7V, pull that transistor (with amp off) and measure the BE diode in the multimeter diode scale.
Red to emitter and black to base.
Just for confirmation, read it the opposite way.
Are you using a real 2N5415 or some substitute?

[Zero Cool]

Originally Posted by served

So I found that R40 R41 R42 R43 are gone.

yup you had oscillation! that is a tell tale sign of what happened!