Workingman's 15 blew a fuse when turned on. (help me troubleshoot?)

[Stratapastor]

At some point pulling the transistors (Q4 & Q10) off the board for testing may be beneficial. Also in cases where a component can't be accurately measured in circuit de-soldering one leg off the board will be all it takes to measure it correctly. Sorry for my long winded post last night, but hopefully a few bits of it helped. Just focus like Bill said on the problem at hand and get at the root of the problem. Also, to point out that there is 11.8vdc on the speaker output.

I pulled D10 and it tested correctly. Since Q4 and Q10 are the same transistor I assumed that they were both probably ok.

could a bad cap cause this? I believe I read that it can cause oscillation.

[52 Bill]

Please retest the two output transistors and post the E-C readings.

Have you tested the rectifier bridge?

[Dave H]

Since Q4 and Q10 are the same transistor I assumed that they were both probably ok.

Never assume, measure

I'd remove Q4, Q10 and the drivers Q3, Q9 and measure them out of circuit. B to E and B to C should measure like a diode, C to E should measure open circuit.

With all four transistors removed power up the amp and measure the +HT, -HT rails And +15V, -15V rails.

[Stratapastor]

Please retest the two output transistors and post the E-C readings.

Have you tested the rectifier bridge?

pulled and retested rectifier bridge, it's good. E-C readings were open 1 direction and .543 the other.

[Stratapastor]

At some point pulling the transistors (Q4 & Q10) off the board for testing may be beneficial. Also in cases where a component can't be accurately measured in circuit de-soldering one leg off the board will be all it takes to measure it correctly. Sorry for my long winded post last night, but hopefully a few bits of it helped. Just focus like Bill said on the problem at hand and get at the root of the problem. Also, to point out that there is 11.8vdc on the speaker output.

what are your thoughts on pulling and testing the caps?...

EDIT: never mind. I just discovered that my meter is about 6600 uf short of being able to measure my large caps....

[52 Bill]

what are your thoughts on pulling and testing the caps?...

EDIT: never mind. I just discovered that my meter is about 6600 uf short of being able to measure my large caps....

Test the two large filter caps with your ohm meter. All you need to know at this point is whether either one is shorted. You do not need to pull them or anything else until you find something that is suspect. Pulling things out of the board only increases the chances of damaging the board and the parts.

[Stratapastor]

Test the two large filter caps with your ohm meter. All you need to know at this point is whether either one is shorted. You do not need to pull them or anything else until you find something that is suspect. Pulling things out of the board only increases the chances of damaging the board and the parts.

Thanks, neither is shorted.

[Stratapastor]

Never assume, measure

I'd remove Q4, Q10 and the drivers Q3, Q9 and measure them out of circuit. B to E and B to C should measure like a diode, C to E should measure open circuit.

With all four transistors removed power up the amp and measure the +HT, -HT rails And +15V, -15V rails.

With the 4 above removed.

+/-HT = 62.8, =/-15 = +/-15

all 4 test ok out of circuit.

[52 Bill]

With the 4 above removed.

+/-HT = 62.8, =/-15 = +/-15

all 4 test ok out of circuit.

Have you tested Q5? If the voltages all come back with the outputs and drivers removed and all of those transistors test okay, then the bias circuit around Q5 is suspect.

[Stratapastor]

UPDATE: I've pulled out Q4,10,3, and 9. With all 4 removed there is of course no hum and the clip light is off. All of them test ok. What's the next step that I should take?

[Jazz P Bass]

So with the components removed, what voltage dc readings do you get at Q3 & Q9 Base, Collector & Emitter pads?

[Stratapastor]

Grounding the neg lead to the chassis and using the pos prob on the pads I get the following
Q3 B 34.4 C 63.7 E 0
Q9 B - started at 18 and then went negative to 14... i'm confused. C -63.7 E 0

I left the amp off for a while and found the same result but I didn't leave it on as long. Also, the R1 and R2 are quite warm.

[Jazz P Bass]

Q3 base being that high (+ Vdc) is bad.
A high on an NPN transistor turns it ON.
Which (in this case) drives the Emitter high.

Look at the output circuit.
That high Emitter (with the transistor installed) will put a high on the outputs Q4.
Which will put that Emitter high (Vdc on the speaker!)

Working backwards, I think Q2 may be at fault (at least).
Q2 is a PNP transistor.
A high on the Base turns the transistor off.
That's what the 2.7K resistor is doing.
Turning the Base slightly On.
The Collector then will be slightly On.
Which it is not.

R1 & R2 have nothing to do with the output circuit.
They are for the preamp.
5Watt resistors do get hot.

[Stratapastor]

q2 seems to be letting current through both ways between the emitter and base.

Also... somehow in all my testing this is the first time I've tested this cap... I'd have remembered because it was hard to get at and the only one like it on the board.


EDIT: I'm dumb... pretty sure it was measuring across the resistor that's between the B and E... What now?

[Jazz P Bass]

What now, what?

Test the transistor.

Do you have a 'diode check' function on your meter?