Hey all, hoping someone can push me in the right direction. This amp is broken up into 4 boards, Power Supply, Amp, and 2 preamp boards. I was able to get the schematic from Crate so bonus there!!
Symptom: Blows main Power Supply fuse when switched on.
I disconnected the power supply put a fresh fuse in and I get the expected -45/+45Volts DC out, so the PS is good at least with no load it's good. Next I connected the Amp but not the preamp and that blows the fuse when turned on, so I'm thinking something in the Amp is most likely. Nothing visual and I've gone through and check resistance values to make sure nothing is shorted to gnd etc. That is pretty much where I'm stuck, without being able to put voltage to the board does anyone have suggestions? I've attached the schematic

 

Check Q8 to Q13 for short circuits.

 

Did you do the same test with only the preAmp connected?

 

Check Q8 to Q13 for short circuits.

Emmiter on Q8 to Base of Q11,13 is 6 ohms but that's a straight shot, I don't see the R34 in the circuit

At least I'm assuming base, the Q11,Q13 can only has 2 leads so I'm a bit confused with those

 

At least I'm assuming base, the Q11,Q13 can only has 2 leads so I'm a bit confused with those

There are two wires sticking out which are the base and emitter. The collector connection is the big metal can. For each transistor check between the three leads of the transistor - emitter to collector, emitter to base, and base to collector.

 

E-C - 5 ohms
E-B - 122
C-B - 122

Q10,12 has the same readings

 

You need to learn that transistors look like 2 diodes, collector to base and emitter to base, so each diode has to be measured in each polarity, as in, reverse the ohm meter leads and use a diode test range if your meter has one.

Six transistors is a butt load of measurements (24), but these guys are steering you right. The most likely failure is a punch-through in the power transistors. It isn't fun and it isn't easy to remove 6 transistors, but that's what must be done.

 

You need to learn that transistors look like 2 diodes, collector to base and emitter to base, so each diode has to be measured in each polarity, as in, reverse the ohm meter leads and use a diode test range if your meter has one.

Six transistors is a butt load of measurements (24), but these guys are steering you right. The most likely failure is a punch-through in the power transistors. It isn't fun and it isn't easy to remove 6 transistors, but that's what must be done.

I can't believe how much basic info I've forgotten over the years, thank you for that memory jogger!! With that in Mind
Q11,13
C-B .148 / .135
B-E .145/.135
E-C Short both ways
All 4 Cans had essentially the same readings, all readings in circuit with diode mode
Q8
C-B .563/1.44
B-E .215/.215
C-E .571/1.48
Q9 was almost identical, again in circuit in diode mode

Do I need to pull these to get an accurate reading? I'm going to go through all the transistors and see if anything strange shows up, Thanks all for the help so far!!

 

I pulled Q11 out of circuit and it's basically the same readings

 

Came back to me as I was staring at the circuit that the E-C should be open, so I pulled Q13 and out of circuit it is, here is the readings for Q13 out of circuit which makes much more sense to me
C-B Open/.5
B-E Open/.5
C-E Open/Open

I'm going to pull 10 and 12 to verify but I may have found my issue in Q11

 

Ok pulled all 4 and Q11 and Q10 have shorts from C-E

 

Yes, that would do it - short-circuit across the power supply.

 

Hopefully that is all that is wrong!! Thanks for the help!!

 

Got the transistors in and replaced all 4, amp powers up but no audio just 60Hz hum, all the grounding looks good any suggestions on where to look now?

 

Do you get the hum even with no inputs connected?
Do the tone controls change the hum?
Does the volume control change the hum?

 

Steady hum indépendant of input or control changes, after a few seconds it crackles and fades in volume (like somethings shorting), turn it on again and sounds like it's sparking and lower volume like something else just went out, ugh.

 

Check the bridge rectifier. It may have been damaged by the shorted transistors.

 

Steady hum indépendant of input or control changes, after a few seconds it crackles and fades in volume (like somethings shorting), turn it on again and sounds like it's sparking and lower volume like something else just went out, ugh.

Sounds like the bridge rectifier has gone causing mains hum on the DC supply, i would change the smoothing capacitors too.

 

while I'm waiting for the Caps to drain, a ignorant question; shouldn't the main power caps both have voltage sitting on them? when I checked for voltage on the caps one had 39v and the other is 0, I'm thinking if one has voltage on it they both should since they are in series. My assumption would be regardless of the rectifier if one is getting voltage they both should be charging. I am seeing voltage from the positive lead of C7 to the neg lead of C6 but no charge on C6. Am I right or wrong?

 

pulled the rectifier diodes and smoothing caps, diodes all check good and the caps are pretty close to .1uf as they are suppose to be.