fender frontman 212 distorted output

[pontiacpete]

Hi all, this FM212 has a distorted output. I think I narrowed it down to the power amp.
Plugging the pre out to another amp the problem is not there. I gave the whole board a once over with some solder.
Chop stick hitting of the board isn't showing a change in the sound.
There's 42 +/- vdc on q 18 - 21. The wave is not looking so good. Clipped on one ended.
I've been going thru the power amp components testing what I can in circuit.
Anyone have any ideas? Any suggestions whatsoever would be appreciated, thanks

[Jazz P Bass]

Which side is chopped off. Top or Bottom?

Try the amp with & without a load & see if that changes the clipping.

And make sure the 0.22 ballast resistors are good.

[pontiacpete]

Here's a picture of a 300mv wave into the power amp in jack.

Both ballast resistors R109 and R112 measured in circuit with my fluke are .3 ohms.
I think these a good.

[Ted]

You might want to try it into some different speakers. I've seen three of these in the last few months, all of them either sounding terrible or completely silent from blown speakers.

[pontiacpete]

I'm using my test speaker. The problem is there without the speakers connected
Thanks for the input, though.

[Ted]

OK. Just a thought. I notice that Fender classify these amps as "Non-serviceable". Does that just mean that its cheaper to supply a new amp than repair warranty returns?.

[pontiacpete]

I guess that's what it means.

[Jazz P Bass]

If it's not a warranty repair, that does not mean that the amp cannot be repaired.

For a start, I would go with the admittedly low Fender test voltages.
5 mv input for a 2 volt output.
See where that leads.

Also, I would recommend measuring the output transistor bases at idle. (Vdc)

And don't forget the opamp power supplies. They should be checked.

[Enzo]

Non-serviceable ONLY refers to warranty repairs, it has nothing at all to do with electronics.


yes, it is cheaper OVERALL to replace these amps than to pay service center warranty rates.

[pontiacpete]

5mv is really low but I fed a 5mV signal into the input and raised the volume until I had 2v on the output. Is what I'm suppose to do, raise the volume for the 2v?
The picture of the wave is below.

The bases of the Q18-21 have 2vac.
ICs all have 16+/- vdc except for the U1 which has 15+/- vdc

[g1]

Your picture in post #3 is hard to see, but looks like the waveform is clipping equally on top and bottom. What is it supposed to show?
There is no pic attached to post #10.

Bottom right area lists test conditions and control settings to get the 2VAC at the output. But you could go into the power amp in jack and set your signal level so that you have 33mV at TP18 (U6 pin7). That should give you 2VAC at the output with 4 ohm dummy load.
I think this may be the silly circuit where power amp in jack is muted unless you put a dummy plug in the input jack.

[pontiacpete]

Here's the picture for post 10.

gone- you're right the first picture in post 3 is clipping top and bottom.
I'll do the tests tomorrow and report back.
Thanks all

[pontiacpete]

This is what it looks like thru the power amp in jack with a 33mv signal.

[pontiacpete]

U6 pin 7 there's 129mV
pin 1 has 16mv

Does this means something is wrong with u6?
Or should I raise the signal to 33mV at pin 7 ? probably not since it's higher at pin 7, maybe lower?

pin 5 has 12mv.

[Jazz P Bass]

The PA IN should read 91mv (the same as U5 pin/ pin 1 TP15).

The opamp outputs of U6 (pin 1 & 7) should read 33mv's.