Fender Frontman 212 power amp distortion

[jonbo335]

seams that nobody can fix this common problem.
All components seams to be ok
there are a few mV on PWR input
I ran out of ideas

[The Dude]

Certainly not saying you have the same issue, but I just chased one of these around for quite some time and finally found Q12 & Q13 leaky/intermittent.

[jonbo335]

Q12 Q13 and also C51 C52 are new, but the problem remain
Tanks !

[Enzo]

What common problem? Looks to me like the amp functions but you are missing the upper half of the output waveform. Your scope is on AC coupling, I would imagine if put on DC coupling the flat tops would move to the zero line?

It looks like you have a load connected with the brown wires? Disconnect the load, is the top of the wave still missing?

Look at the bases of the output transistors, do both have a full sine wave signal there? Between them are Q18,19, your drivers, look on their bases. Full waveform or missing half?

Do you have +42v more or less at the collector of the V+ output?

[jonbo335]

Hi Enzo, thankyou for your patience
Scope is ok !
+- 42V is perfect on the collectors
Whith no load the distortion disappears, same distortion on 4 or 16 ohm load
testing with a guitar there is a lot of distortion on clean cannel and also in the power input

[Enzo]

I am sure your scope is working, but what I was asking was this: is the missing upper part opf the wave from zero voltage and up, or is it as it appears, clipped part way up the positive side? On AC coupling, it will center a half waveform on the screen.

If the output is a complete waveform with no load, but is missing half under load, that means the output cannot supply current to the load on the upoper half. That means Q20 is not doing its job. Q20 itself could be bad, but an open 0.22 ohm R109 could cause the same thing.

[jonbo335]

Ok Enzo, you are right, my scope was not setted to zero on dc or ground in the shot, bat I can confirm that the sinewave is in the center of the screen (no DC in the output).
I have disconect Q20 and the junction seams ok on the multimeter test, I try to find one new... R109 is ok

[J M Fahey]

bat I can confirm that the sinewave is in the center of the screen

WHICH sinewave? I see none there.
I see nothing centered either.
Please:
Confirm input is set to DC
Short input terminals and center horizontal trace, then remeasure, with and without load.
Repeat on power transistor bases touching no scope settings, as Enzo said.
Repeat on driver transistor bases.
Since you checked emitter resistor, you might have open power or driver transistors or rail voltage might not reach them or you might have open connections, including cracked tracks or solder.
—problems are not just "bad parts"—


"

[jonbo335]

Here is input, out no load, out load

[jonbo335]

Any Suggestion ?

[Enzo]

How did a new Q20 work?

[jonbo335]

Same whith new Q20, R109-112

[Randall]

I had three of those amps in the last year with distortion. All three had bad or broken connections on one or more cement resistors.