Fender princeton 65 blown output?

[pontiacpete]

This amp has, on the limiter, 38vdc, no speaker attached. I lifted several components to check for shorts and have found none. They are Q18, Q15, D35 & 36 and C53. These look good. But there is the 38 volts on the output.
I've tried the in circuit diode test on the, I think all of the other transistors and none appear to be shorted.
So the question I have, could it be that the output transistors shorted when voltage is applied?

[Enzo]

Sure. But you are wedging all your expectations on a shorted part. An open part or broken connection can cause exactly the same issues. The output of the amp is normally zero volts DC. it does that by centering between the two power rails, in this case more or less +/-42v. You get +38 there. You assume it takes a short on the Q15,18 side, but what if the Q19 side is open and cannot tug the output towards the negative?

You need to verify both power rails are present at the transistors. For that matter both 15v rails need to be present at the IC in that power amp. You need to not just look for shorted transistors, you need to do diode tests on all the junctions looking for opens as well as shorts. R102,104,105 open?

[pontiacpete]

Thanks for answering that question.
The voltages were taken with the limiter. Without it there's +/- 40 on C55 and C58.
R102,104 ,105 are not open
Its curious that the IC only has -16vdc on pin 4 and nothing else to speak of.

[Enzo]

That is not curious at all, that is why I mentioned it.

The amplifier wants to center between its supply rails. The center between +/-40 is zero. The input end of the power amp is an op amp. Op amps are the same, they want to center between their power rails as well. So +16 and -16 power rails, the center is zero.

So you have -16 on pin 4, but "nothing" on pin 8? Where will that op amp output want to center to? Not zero, zero is now its + power voltage. The poor thing will now swing negative. That this might cause a swing the other way at the output is irrelevant here. Get that +16 back to that chip or nothing else we do will have any meaning.


Here is a visualization. Put a mark on a rubber band. Make it a good sturdy one like the post office uses. Now hold that band in front of your face with the mark centered. Pull on the ends to stretch the band. Notice the mark stays in the center as you stretch the band? Your grip on each end is like the power supplies. No let go on one end. Where is the mark now? No longer centered, it is now over by the remaining grip.

[pontiacpete]

Big mistake on the IC reading, it has the +/-16. sorry, I'm trying to do too much at the same time. Turns out that there's a delay before all the voltages appear and I was rushing
Continuing around board I found D38 has -16 on one end and on the other end, TP24 there's 0vdc.
D38 is not opened.
Not sure how to test the jfet in this application. What purpose does it serve? Is it something to do with silencing the turn on pop that many solid state amps have?

[g1]

The mute Fet Q5 mutes the power amp during turn on and also whenever there is nothing plugged into the input jack.
You should get the -15V at TP24 if you plug a cord into the input.

(Edit: I thought this was the amp that needed a dummy plug at the input to be able to use the power amp in jack, but it is not. I know I had that problem with an amp but can't remember which make & model it was. This amp addresses the problem with the switch contact at the PA in jack that routes back to the main input jack.)

[pontiacpete]

Thanks g-one.
Got the -15v with the dummy plug.
Oh well, again, I thought I was on to something.

[g1]

How about some DC voltages for E,B, & C of Q15,17,18, & 19.

[pontiacpete]

here are some of the voltages
Q18
B - 38vdc
C - 40
E - 37

Q19
B - 37vdc
C - (-40vdc)
E - 37vdc

Q15
B - 38v
C - 40
E - 38

Q17
B - 37v
C - (-40)
E - 37v

[g1]

Check all the resistors labelled on the schematic as Fu (fusible). If you find one open, check it's associated transistor.
R80,81,82,83,94,102,104

[pontiacpete]

R80,81,82,83,94,102,104

All of these are looking good in circuit

[g1]

Sounds like you got a tough crowd there .
DC for ebc of Q8,9,10,11 (no hyphens please, just + or -).

[Jazz P Bass]

You are overlooking something.

The base of Q17 should Not be a positive voltage.

[g1]

You are overlooking something.

The base of Q17 should Not be a positive voltage.

No it shouldn't, and that is causing the positive voltage on Q19. But where is the + coming from? I'm hoping some of those readings from Q8 to 11 will give some answers, I guess Q12 and 13 voltages should be checked as well.

[Jazz P Bass]

Just to add: Q15 base being that high is not good.
I would expect +1 volt max.

And Q17 -0.8 volts.