Fender Princeton 112 Plus output power

[slideman82]

A friend of mine's got one of these, I had to swap output transistors for the right ones (had bipolars instead of Darlingtongs), Tip142 and Tip147, the fact is when a strong signal is applied, volume shuts down. I thought it was a channel switching problem like the one I had with a Roc Pro 1000, but I plug the guitar directly to the PA (using a stompbox to increase volume) and happened the same. Could be a output transistors problem?

Any suggestions will be grateful!

[J M Fahey]

*Maybe* the output transistors overheat and the thermal switch cuts it off. Check that. Play with the chassis out and every 5 or 10 minutes touch that aluminum bar to which TIPs are bolted. Do not touch them directly or you'll receive a mild shock. If your hand can't stand the heat and the amp shuts off, here's the culprit.

[slideman82]

Could be that, it doesn't do it till a couple of minutes, and this thing heats up quite much! I thought adding other pair of Tip, so they'd work less hard.

[J M Fahey]

OK.
That kind of Fender amp , the one driven by an op amp and 1, 2 or 3 pairs of TIPs , make me nervous, because I see them overbiased but worse than that, unadjustable.
They have to forward bias 4 junctions (two in each TIP , which is a Darlington), so *in theory* the 4 diodes in series they use, *should* be perfect, and *should* not need any trimming or adjustment.
In my experience, that is a difficult balancing act, because 4 "regular" diodes are a little too much. They rely on a specific brand and model there, and the popular 1N4002 which should work well, is too "hot", and, by definition, unadjustable.
I suggest that, to be sure, you turn your amp on, but with all controls on "0", and monitor that heatsink with your high-tech, NASA approved ultra-electronic detector called "finger".
If it overheats quickly with no signal applied ... now you know what's happening.
Just to cover all bases, measure that you have no DC voltage in that speaker out, both with and without speaker. (Up to 100mV is fine)

[slideman82]

GREAT TIP! In fact, I already know that, but never done it! Uhm, I remember I changed those ultra fast diodes for others, a bit less fast than the original... but I'm gonna find the orginal ones for better.

[phatt]

OK.
That kind of Fender amp , the one driven by an op amp and 1, 2 or 3 pairs of TIPs , make me nervous, because I see them overbiased but worse than that, unadjustable.
They have to forward bias 4 junctions (two in each TIP , which is a Darlington), so *in theory* the 4 diodes in series they use, *should* be perfect, and *should* not need any trimming or adjustment.
In my experience, that is a difficult balancing act, because 4 "regular" diodes are a little too much. They rely on a specific brand and model there, and the popular 1N4002 which should work well, is too "hot", and, by definition, unadjustable.
I suggest that, to be sure, you turn your amp on, but with all controls on "0", and monitor that heatsink with your high-tech, NASA approved ultra-electronic detector called "finger".
If it overheats quickly with no signal applied ... now you know what's happening.
Just to cover all bases, measure that you have no DC voltage in that speaker out, both with and without speaker. (Up to 100mV is fine)

Thanks JMF,, very well Explained :tu:
I somehow new there was something half cooked with those Fender Amps.
The Performer 1000 I fixed a while back is very similar, From my limited understanding the power stage is just a current boosted opamp.
thank goodness the poweramp was ok cause I would not be able to fix it.
At a guess I think every possible way to impliment feedback is somehow used on that circuit
Phil.