Fender 112 Deluxe plus hot output transistors and farting

[shadygreentree]

Hi, I am hoping that one of you can help me. I have a Fender Deluxe 112 Plus. The output transistors are getting very hot even without any input and thus the thermostat shuts off the amp. I am suspecting high frequency oscillation but I may be wrong. Another symptom is that when you switch off the amp, it goes off with a "fart" noise. Can someone please point me in right direction?

[Enzo]

I just responded to this over at DIY, please read what I said there. I'd probably rather continue here myself, this forum is more about guitar amps than that one. But that is only my personal opinion.

[shadygreentree]

Thanks Enzo, I have got your reply on DIY and working on your suggestion. I will report back soon. You are right, I do not have a scope, but I do know how to rig up RF probe if needed. Ham radio for 20 years.

[shadygreentree]

OK here it is:
No DC voltage on Speaker terminals
All output transistors have 1V on base (between 1V and 1.1V)
Cant make out if one transistor is heating up more then others.
All four current sharing resistors in emmiter of output transistors have same voltage across
Same voltage across all biasing diodes 0.05V

Does 'fart' sound at switch off have any significance?

[Enzo]

Probly not.


So without the speaker, does it still get hot?

Those four diodes? You can short across one of them, that biases the amp colder. ANy change?

[shadygreentree]

Hi Enzo, Thanks for quick replies. Yes the outputs are cooler, though still warming up, but not too hot. Base voltage dropped to 0.7 and -0.9 . What now?

[Jazz P Bass]

The output section voltages appear 'normal'.
The transistors will get warm at idle.
That is what the heat sink is for.
Do not, ever, power the unit up with the output transistors Not connected to a heatsink.
What voltage did you measure across the emitter ballast resistors? (R74 & 75)
There are voltage test points on the schematic.
I would start at the start.
The power supply itself.
Is the +40 & -40 Vdc supply steady.
Measure the Vac ripple.
Under 500 mv's is good.
Now test the opamp supplies.
They must be steady Vdc.
As to Vac ripple, ideally you want to see none.

[shadygreentree]

Thanks everyone for help. It is fixed now. Enzo I have taken your advice and shorted one diode to pull bias volatage down. It is .7 on bas of PNP transisitors and .8 on NPN. I cant make out any distortion in audio output so I presume the amp is still in class B. I have also replaced the MC1436 with OPA551 and that fart sount at switch off also disappeared.