Fender Powe Chorus output stage oscillation

[Mechapoch]

Hi all, my name is Eric. I'm having trouble with a Fender power chorus output stage. This unit has 2 identical power stages. When I power the unit one of the output stages starts oscillating. The strage thing is that it won't oscillate with a resistive load, only when the speaker is connected. I've changed all the capacitors in the output stage (a couple of them where leaking) but it still oscillates. Someone told me to connect a 4,7ohm resistor + a 470nf cap at the output to tame oscillations, but it won't go away. The problem must be somewhere else, since the other ouput stage works just fine. I don't know what else to try. does anyone has any ideas? thanks!

here is the schematic of the output stage:

[MarkusBass]

Can you post a photo of the power amp board? First I would look at the board with magnifying glass. Is R218 soldered correctly?
PS: I never replace "blindly" components unless I'm sure that the component needs to be replaced. Otherwise, you may cause additional problems and this may make finding the root cause of the problem much more difficult.

Mark

[Mechapoch]

Here´s a pic of the power stage



R218 seems to be ok, I´ve checked it whith a multimeter.

[MarkusBass]

Looks good. What was this brown spot around C107/C108? You could use some tricks like soldering a 47pF capactitor between base and collector of the output transistors but since the amp worked before, I would try to isolate the problem. Did you try what happens when you disconnect the input signal? Did you try to remove the power amp board from the amp and test it with external power supply?
I assume that you checked all the diodes and they are OK, right?

Mark

[Mechapoch]

C107/8 capacitors were leaky, I´ve replaced them with new ones. The amp has a single pcb for the preamp, power supply and output stage, so I cant isolate it easily. But the DC testpoints measure ok. I havent disconnected the input from the sistem yet, since the amp works fine with a resistive load and oscilates with speakers I guessed that the problem is somewhere else. I´ve also tried with another speaker.

I´ve checked all the diodes with a multimeter and they are all fine. Using the resistive load I´ve also measured biasing voltages around resistors r212-215 and they´re similar to the working output stage.

I dont know abot the zener diodes, the show ok with the multimeter, but I dont know if they start conducting at 3,9v or not

[Jazz P Bass]

You could try replacing U15.

[MarkusBass]

When posting the schematic you omitted big part of it, in my opinion the most important. You omitted the power amp inputs and the Return jack. With the Return jack you can perform several tests: you can check whether with a jack inserted (in this case the preamp is disconnected) the problem stays the same. Then, there is 56pF capacitor on the input. Just for test you can solder there 1nF capacitor and see whether it stops the oscillations. You can take a look at this capacitor. You can also connect an oscilloscope to the input and see whether you get oscillations there, or it is present only on the output. You could also try to replace the IC but I don't see what type it is (and I don't know if it is still available).
EDIT: I forgot to mention that it would be nice to clean the residual (?) - what was left from C107/108. Otherwise, the PC board may be conducting there and cause additional problems - for example oscillations :-).

Mark

[Mechapoch]

I´ll try soldering the 1nF capacitor and see what happens.
I can´t change the IC for the moment, I can´t find any replacements in my area (there are some from TI).

I thought that the problem was in the power stage itself, since it oscilates whith an inductive load and not with a resistive one.

[Mechapoch]

Found it! A diode failed open at the input of the opamp. I didn't check them the first time because they were back to back and I couldn't test them whitout unsoldering them. Then I completely forgot about it... -.-"

Thanks everyone for the help!