fender frontman 212r distorted output

[pontiacpete]

Plugging the pre amp into another amp shows that the pre amp is working properly. Plugging into the power in jack the sound is distorted.
Tracing from the beginning of the power amp the signal looks good through U6a and b and until the base of the differential pair and signal looks good out of Q9 and 10 emitters. From there the signal on Q11 doesn't look good but there is voltage on all pins there E 30 C -29 B 30. Should there be a clean signal anywhere on this transistor?

Where do I go with tracing from here?
Thanks

[J M Fahey]

Once you get to the power amp itself (anything to the right of and including the differential pair Q9/Q10) itīs all a solid block.
Since SS amps have HUGE NFB, signals inside not necessariy are sinewaves by any means, rather "error correction signals" which may be wild looking on their own, but "just whatīs needede" so the output signal is a perfect sinewave.
Everything "inside" is part of a tightly coupled block.

So start by injecting a clean sinewave at power amp input, straight into "Power amp in" J4 since apparently U6b works fine, and scope speaker output driving a 4 ohm load.

Try to make it reach rails, show actual screen capture.

If one side is missing or heavily distorted, make the "good" half just reach clipping, so we may guess whatīs happening to the bad half.

Repeat without load, drive signal will be quite smaller (half or less than before) to see whether you can now reach rails.
I want to check whether itīs a "voltage" problem, output can swing only "that many" Volts (way before reaching rails) or a "current one" ; it can swing but not supply significant current to a load.

As a side note, I find this power amp design stupid: they complicated it so it "clips/behaves like a Tube amp" ... and then applied massive NFB (like on any conventional SS amp) abslutely ironing and smashing any special sauce flavour they might have accomplished earlier.

Compare it to building a combo cabinet out of nice wood, taking care of pattern, colour, etc ... and then Tolexing over it.

Oh well.

[pontiacpete]

Thanks Juan for the explanation
Here's photo of the wave. (this is the last wave this scope is making for a while, it just died on me, wow what timing)

[pontiacpete]

Huh, now it's not distorting. Maybe a case of a dirty J4?

[J M Fahey]

Maqybe, why not?

For the next time: I was asking for a sinewave *just* reaching clipping, your screen capture shows heavy clipping.

Also not flattops but peaky waveform makes me think you were driving a quite inductive load ... was it the speakers themselves?

And you show way too many cycles, so seeing , let alone analyzing waveform is next to impossible.

I would have adjusted sweep speed so as to show 1 or 2 full waveforms left to right, not much more than that.

In any case, glad it works.