R!f@@ is the right man for the job.

You definitely do not want to replace the power transistors without further tests.
With no input source, no speaker connected and Q5-Q9 all removed, turn on and measure the base, collector, emitter voltages at Q1 and Q2 with respect to GND.

On second review, ignore what I said.

Looking at that circuit diagram I cannot figure out what is your reference GND.
It is labelled as P which is different from your input reference A.

 

R!f@@ is the right man for the job..

Thanks Chips for the compliment.

You definitely do not want to replace the power transistors without further tests.
With no input source, no speaker connected and Q5-Q9 all removed, turn on and measure the base, collector, emitter voltages at Q1 and Q2 with respect to GND.

Not this baby I think.

On second review, ignore what I said.

Remember what I said about being similar to Peavey

Looking at that circuit diagram I cannot figure out what is your reference GND.
It is labelled as P which is different from your input reference A.

It uses no Solid GND. No center tap.
This one uses a Virtual GND and smoothing caps is a part of speaker output.

So all in all gives a headache when trying to measure DC referenced to ground.

But I have an Idea that is quite easy to figure out if there is any other issue.

 

All emitter resistors are great.

Q1a is good. However Q2a was only .026 across b to c and c to b while on the board. After taking out, it tests good.

 

If you can locate any point in the circuit shown as P in the circuit diagram, use this as your reference point.
Then measure the voltage at the base, collector and emitter of Q1 and Q2 with respect to P.

 

All emitter resistors are great.

Solder them back in if you want to.

Q1a is good. However Q2a was only .026 across b to c and c to b while on the board. After taking out, it tests good.

Q2a is showing a problem on board eh!

Referring to the diagram on that channel ch A
Then you need to check the components around the driver and bias
( remove them to test to make sure)
Check these components one by one.
R13
Z3
D1
D2
D12
R15
R16
D13
Z4
R12
TR1 and TR2 and TR3 are the SC pots and Bias, you need to be careful with these three pots. Remove them one by one. Never turn them before measuring the Readings from pin to pin. Note them down and then test there resistance by turning the pot to make sure they are not worn out, make sure the meter readings vary smoothly . After testing re-adjust them to there original value as these values are necessary for adjustment of the drivers. Do not confuse them or take them out all at once, take out one by one. Jungle up the pots and u asking for trouble. Do not turn the pots without noting down the initial values.

 

Let me ask you this:

With Q2a removed from the board, using DVM (set to diode check), I put one lead on the nearest end of R16a, and the other lead on the nearest end of R18a. DVM says 026. Keep in mind that the Q2a is removed, so the circuit should be completely open.

Here is a where I put the test leads. The three boxes are R16a, R18a, and Q2a.

question a.pdf

 

And that is why I told you to check the associated components. Specially the diodes.
Check them, you will find a bad one.

There are other components that come into play when you measure the tracks. You need to check the biasing components before moving on.

 

Measure the voltage across R17.
Measure the voltage across R18.

(with Q1 and Q2 in circuit)

 

Measure the voltage across R17.
Measure the voltage across R18.

(with Q1 and Q2 in circuit)

He is not measuring Voltage, Chips.
Besides, if he powered it up it might go haywire without output transistors.
Unless OP confirms the Drivers components I cannot go on.

 

Roger that R!f@@. Thanks for the help.

I'm going to have to buy me a solder removal tool, I believe. I will let you know the results of your list. Not sure how to test a zener, though. I'll have to do some research on that.

Thanks, again. Btw, just so you know, my wife got stranded in Kentucky for the last two days in a blizzard. 24" of snow. Major highways became parking lots around Louisville and Frankfort. So enjoy your heat!

 

Why would one output transistor short and leave the others undamaged?

I would be inclined to install one pair of good transistors (one NPN and one PNP) and turn on the power with no speaker connected (or install a 10Ω resistor load in place of the speaker).
With no input source, measure the voltage across the output (load resistor). You should get 0VDC output.

Just my thoughts.

 

Zener can be tested same as diode.

One transistor might blow occasionally and sometimes the whole string blows. I dealt with lot of times with multiple blown outputs and drivers with it.
OP is lucky to have just one output. I thought he would have multiple transistor shorts and the best part is his drivers are OK.
Just that low R measurement he is getting concerns me that a diode may be bad.

 

You can bet I am enjoying heat. I rarely go out in the heat. Always tries to stay in the workshop than go to sites.

thinking out loud here. ..how did OP removed the transistors if he needs a solder removal tool now ???

 

The answer to that is obvious....

The hard way.

 

Sorry it took so long.

Everything on the list is good.

However, d7a is showing no voltage drop in either direction.

 

D7a is open eh !.
Is it the only one showing a problem. It was not what I was hoping but since you confirmed there are no other components faulty I will take your word for it.
The Amp will work without D7a, it's just a protection but I advised you to find a Diode and replace it.
Put back all the components without the power transistors.
Find an appropriate fuse too.
Time to power up. Let me know when you are done soldering and I will tell you the testing procedure.

PS. you did say that Q2a tracks are showing low R..well it should not. This is possible if D13a is faulty. Please double check.

 

D13a is 492 drop with neg on cathode. OL with pos on cathode.

Did you say to NOT to put the power transistors back? Oops.

I gotta go find this d7a diode. Be back.

 

To clarify, d7a was CLOSED shorted. No voltage drop.

Power transistors removed. Everything else is installed.

 

Replace the shorted diode first.

I am leaving to go home now from workshop
I will update from home with in 1 hour.

 

Diode replaced.

Thanks again for your time.