QSC USA 370 problems

[dcline]

I'm working on a QSC USA 370 stereo power amplifier. When I got it, only channel 2 was working.
I found 3 bad output transistors, an open 22 ohm (R 18a) resistor and an open .22 ohm power resistor (R27a).
I replaced all them parts and still nothing out of channel 1.
D 11a and D11b are fine at -15 volts.
D 10a is only at 2 volts when it should be 15 and when I lift the hot side I get nothing.
D 10b is fine at 15 volts.
I don't know much about op amps but could a bad op amp cause this problem? Would I damage anything if I swapped the two to see what happens?
I've checked just about every other component in the amp. All diodes and resistors and transistors seem to be fine. I'm stumped. I normally work on tube amps but due to the economy have had no choice but to start learning solid state repair.
Any advise on what to check next would be greatly appreciated. I can provide a schematic but not sure how to do it on here.
Thanks in advance

 

[Nigel Goodwin]

Check Z1 (15V zener) you're probably find it's S/C - so you've got no +15V rail.

 

[leal]

Qsc usa 370

hellow guys. check this out.

 

[dcline]

Z1 appears to be just fine. I even pulled one end and checked it. It checks out good.
Any other suggestions?
Thanks Leal for making the schematic available

 

[Nigel Goodwin]

Z1 appears to be just fine. I even pulled one end and checked it. It checks out good.
Any other suggestions?
Thanks Leal for making the schematic available

If the 15V rail is missing then there's probably a S/C on it somewhere (the zener is the usual cause). You might also check the resistor that feeds it, often if there's a short it overheats the resistor and that fails as well.

 

[dcline]

I'm assuming you are referring to R40 which checked out ok. Am I wrong?

 

[dcline]

I found an open resistor. R21.
It slipped by me because is was getting a reading. But when checking it again I noticed it was in the mega ohms and it should be 2k. I pulled one side and sure enough it was open. Unfortunately I don't have any in stock so I'll have to order some. Boy I sure hope this is the final issue.
Thanks you again everyone!

 

[dcline]

I found some 1k resistors in my pile, wired them in series and that did the trick. The amp is working fine now.
I feel a little stupid that it was something so simple but I wanted to thank everyone very much for your time and help.

 

[Nigel Goodwin]

I'm assuming you are referring to R40 which checked out ok. Am I wrong?

Yes, you're in completely the wrong part of the circuit - it's R20 which feeds the positive 15 volt supply.

R21, which you say has cured the problem, feeds the -15V supply, with Z2 as the zener.