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

[Stratapastor]

I do... it will take me a minute to get the base leg loose... hopefully less than 10 min.

[Stratapastor]

Tested fine, not a fun place to solder

[Jazz P Bass]

So where is the +voltage coming from.

As far as I can see, that is the place.

[52 Bill]

So where is the +voltage coming from.

As far as I can see, that is the place.

Q1 and Q2 are part of the slow turn on mute circuit. Q2 starts out turned off and then as Q1 gets turned on, it turns on Q2.

[Dave H]

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

Measure the voltage between Q3 B and Q9 B. This is the bias voltage for the output stage. It should be less than 3V. From the above it looks like it could be 16V or more. This is bad as when the output transistors are replaced it will turn on both Q4 and Q10 shorting the +HV and -HV supplies together. I suspect Q5 is bad. Did you test it as Bill suggested in post #54?

With the speaker disconnected and drivers removed there is no path for the base current of Q6 so it will turn off, turning Q7 and Q8 on pulling Q9 B negative (I think).

[Stratapastor]

Measure the voltage between Q3 B and Q9 B. This is the bias voltage for the output stage. It should be less than 3V. From the above it looks like it could be 16V or more. This is bad as when the output transistors are replaced it will turn on both Q4 and Q10 shorting the +HV and -HV supplies together. I suspect Q5 is bad. Did you test it as Bill suggested in post #54?

With the speaker disconnected and drivers removed there is no path for the base current of Q6 so it will turn off, turning Q7 and Q8 on pulling Q9 B negative (I think).

Q5 is testing fine in circuit.

Q3 b-Q9 b starts at 15v and then climbs to over 20... I stopped there but it was climbing at a steady pace.

[g1]

Probably worth removing Q5 to test. Certain circuits can make an open junction appear 'good'.

[Stratapastor]

Probably worth removing Q5 to test. Certain circuits can make an open junction appear 'good'.

no change with q5 removed. However, probably from movement while i've been taking the board on and off, the outside legs of q5 were broken off at the board. once I know what else I need to replace i"ll be ordering that new too.

[52 Bill]

no change with q5 removed. However, probably from movement while i've been taking the board on and off, the outside legs of q5 were broken off at the board. once I know what else I need to replace i"ll be ordering that new too.

This is why i don't recommend removing parts or boards until you have a real reason to. There's just too many chances to do more damage than you fix.

I will have to look at the schematic again, but you may be able to just jumper across the bias transistor to see if that was the entire cause of your problem. The power amp may power up correctly with the two halves tied together. It won't sound right, but it may just power up okay. Please wait until I or somebody else has a chance to look at the schematic and tells you that this will work.

[Stratapastor]

it had been moved around quite a bit from my efforts to trouble shoot. it's really not anyones fault but mine. If I need it to trouble shoot further then I'll order a new one and let you know when it comes in.

[Jazz P Bass]

I still am not sure where that +35 volts is coming from on Q3 Base.

[52 Bill]

Is the amp currently back together, with the transistors (except q5), etc. back in place? If it is, jumper across Q5's Emitter and Collector and power up the amp with a light bulb limiter and report back what happens.

[Stratapastor]

It is not back together but it can be. At this point am I done with all the trouble shooting that can be done in it's current stat?

also.. I could temporarily solder on some jumpers to q5 to get it back in circuit in the short term if that is beneficial.

[52 Bill]

It is not back together but it can be. At this point am I done with all the trouble shooting that can be done in it's current stat?

also.. I could temporarily solder on some jumpers to q5 to get it back in circuit in the short term if that is beneficial.

Done? I don't know, you have removed transistors and tested them. If you are positive that they are good then they need to be put back in and other things need to be tested.

The usual reasons for an amp to blow fuses is shorted output transistors, shorted power supply rectifier, bad power transformer, shorted power supply capacitor and then assorted other problems. You have proven that the transformer and power supply parts are working with the correct voltage values. You have removed and tested the output and driver transistors. There are maybe three additional transistors in the power amp. Have you tested all of them?

As was stated here before, the bias transistor is what keeps the two halves of the power amp in check. If it goes open or becomes disconnected from the circuit, the two halves of the power amp both turn on at the same time and will cause the amp to blow up or at least blow fuses. If you are certain that the broken leads only occurred after the initial tests, then don't bother trying the earlier suggested test or jumpering it in place. If Q5 has been doing its' job, then the problem with the amp is still to be found.

We are all at a disadvantage here because we do not have the amp in front of us and we can only assume that you have tested things correctly and followed the directions that have been given correctly. So please understand that we are trying to help, but can only do so much from where we are.

[Dave H]

At this point am I done with all the trouble shooting that can be done in it's current stat?

You can't be sure where the drive voltage will settle with the output/driver transistors removed and feedback inoperative so +35V on Q3 base may not be a problem. At this stage I'd reinstall Q3,4,9,10 and jumper Q5 C to E then with the speaker disconnected power it up with the bulb limiter in circuit. Measure the HT rails and the output voltage to the speaker. The output voltage should be close to zero.

If the power rails and output voltage measure OK the next stage would be to check the values of R26,27,69 then install a new Q5 and repeat the above.