Fender Princeton 65 DSP short with signal

[pontiacpete]

This amp turns on fine but when applying a signal to the input it appears to have a short and voltage appears on all legs of the main output transistors, about 35vdc and there's a loud hum.
The output transistors don't appear to be shorted when tested in circuit with the unit off. And the voltages look normal without signal when turned on.
Into the power amp in jack the short like behavior is there. Plugging into the pre out jack there is nothing coming out to another amp.

So far I've been re-working the board, looking for something disconnected like a solder joint or a broken component.
Does anyone have any idea which way to go from here?

[Enzo]

Look at the schematic, the input and power amp jacks are connected together to control the MUTE line. Look bottom center of drawing, lower left corner of power amp, see where MUTE comes in to control Q21? Until something is plugged into those jacks, the amp is muted. Plug in and it turns on the power amp.

The loud hum is DC on your speaker, so TURN IT OFF. Use a volt meter instead of a speaker until that DC is gone.

[pontiacpete]

Thanks Enzo,
Speaker is disconnected. I put a dummy plug in the input jack and there is 35v across the output.

Do I need to leave it on the limiter to take voltages even though it's not blowing fuses or smoking?

EDIT: anyway with the limiter:
Q6
c 35vdc
b 15vdc
e 15vdc

Q7
c 37v
b 15v
e 15v

Q8
c 15v
b -28v
e -29v

[Jazz P Bass]

I would be looking at the output devices for shorts before the LTP. (the feedback path will throw off the LTP voltages)

[Enzo]

Except shorted output transistors would put rail on the output regardless of the jack mute. But do check them.

35v or -35v on the output?

[pontiacpete]

35v on the output.
In circuit the output transistors don't appear to be shorted. Q15, Q17, Q18 and Q19 seem to be good.

[pontiacpete]

Here are the rest of the voltages in the output. I wonder if there is positive voltage where there should be negative or vice versa.
Q9
E 38v
C -37v
B 37v
Q11
E -38v
B -37v
C 36v
Q10
E 38v
C -37v
B 37v
Q12
E 34v
B 34v
C 35v
Q13
E 35v
B 34v
C 35v
Q14
E 36v
C 36v
B 36v
Q15
B 36v
C 38v
E 35v
Q16
E 36v
C 35v
B 35
Q17
B 34v
C -38v
E 35v
Q18
B 35v
C 38v
E 35v
Q19
B 35v
C 38v
E 35v

[Jazz P Bass]

The most obvious transistor voltage that appears off is Q16.

Q14 & Q16 are the limiting transistors.

Remove either the transistors or the diodes to see if voltages fall into place.

Also check the resistor string on Q17 from base to collector.

Q17 base should be a negative voltage.

[pontiacpete]

I was thinking the same about Q16.
Pulled the anode of D34 at Q16's collector and my limiter went bright.
All these resistors R89,90,98,100,101 and 102 all measure up to their value.

[Jazz P Bass]

"All these resistors R89,90,98,100,101 and 102 all measure up to their value. "

O/k, they 'measure up'.
But do they connect to Q17's base?

Have you checked the transistors with a meter (diode check)?
Power off.

Q17's base having a plus voltage on it is all wrong.

[Enzo]

Q12,13 are the limiters. Lifting D27, D32 turns them off.

Q14,15 are your bias circuits.

Q15-19 (less 16) are the output and that is a current amp. Look at Q18, it is an emitter follower, so whatever is on its base will be on its emitter. Same thing at Q19. Note those bases connected together through a 47 ohm. A positive voltage at B of Q18 turns it on, if the voltage falls below the emitter, the transistor base is reverse biases and is turned off. That is what we expect in a push pull amp. Bias in such an amp has the job of maintaining a voltage space between V+ and V- side output transistor bases.

I figure with two base drops from the emitters, the bases of Q15, 17 ought to be about 2v apart at all times. And looking at your chart, I see B Q15 is 36v, while B Q17 is 34v. What do you know: 2v. Right now it is stuck up at 35v, but normally they rest centered on zero, and the music signal rides them both up and down together, but always that 2v apart. So with 35v on those bases, of course we have 35v on the output. Note 35v is the mid point between the 34 and 36v bases.


Q10,11 drive all that. See between Q14,16 their emitters join and are connected back up to collector Q10,11. Q10 grabs that mid point and shakes it up and down to the music. And wher they go, so does the output... Normally.

Your chart says C Q11 is at 35v. That makes sense, because so is everything to the right of it, including the output. So Q10, I can't quite accept you have -37 on any part of it. I'd believe all three legs were at about +35.

So my thought is that Q11 is open, or not getting enough base current, though the 1v across its B-E looks like it ought to be on.

I even stock those MPSW42/92 types specifically for Fender power amps.

[Jazz P Bass]

Thanks for clarifying that Enzo.

Yes, Q12 &Q13 are the limiters.

I really did not look at it that close.

Usually the limiters are drawn where the bias is drawn.

[pontiacpete]

JP Q19 collector is wrong because I wrote it in wrong. There is -38v on the collector. I can't even read my own writing some times.

Enzo, Thanks for the circuit description. That's really helpful. You are also right about Q10 there is all positive voltage for the same reason as above^^
E 37v
B 37v
C 36v
I apologize for making this even more difficult.

I have some MPSW42s so I put one in and still no change. Q11 is not open. I've been going through all the connections in the area around Q10 and Q11, so far everything is connected.
Should Q9 have negative voltage on it's collector? not sure what happens through the string of diodes.

[Jazz P Bass]

Something is being missed.

If Q9 Collector is at a negative voltage, why is Q17 Base at a positive?
From the negative supply, the base should read: 4.7K + 1.5K + 10 + 47 ohms.

Check the fuseable resistors again.
Q11: R82 & R83
Q12: R102.

[pontiacpete]

From the supply to the base of Q17 I measure about 5K ohms.

At D23 the anode is -37v and on the cathode there's 34v