fender frontman212r

[J M Fahey]

It's interesting what you say about the amp working properly for a second or two after you turn it off.
Just on a hunch, *if* you happen to have a Variac, hooh it to the amplifier and start lowering the voltage.
*Maybe* it works properly when fed from, say, 80VAC instead of the regular 120VAC.
If so, we might find something misbehaving.
Trouble with that particular amp is that Fender took a "normal" power amp and modded it with some funky add-ons to make it sound "more tubey", this complicates service somewhat.
You'll see in other posts about FM212 we have been chasing our tails because of that.
The "other" FM212 we helped repair had also a similar offset problem.
If you have a scope and generator, please drive it with 400 or 1K Hz and take some pictures of the waveform, both with and without load.
We want to see whether it can put out a clean *voltage* signal (no load), and then a *power* signal, with load.
Good luck.
EDIT: I forgot, that distorted signal with a spike on its shoulder is interesting, it might mean the short protection triggering, maybe because of a real problem or by itself working improperly.
When you get that waveform, disconnect the load and tell what happens .
Still distorted?.

[andyjazz]

I took a couple of shots of the scope with my phone but...my card reader is on a Vista machine and...I don't have the time to figure out all the in's and out's of how to upload a goddamned photo with Vista. I'll see if I can go another way around, take a photo with an old digital camera then use my XP machine to do the upload. What we have here folks is the diminishing returns of technology. Every time Bill Gates releases another version of his Dirty Operating System...well I've not going to bore you all with my venting. Suffice it to say that between Bill Fucking Gates and Leo Fucking Fender I'm not having a good day. Happy New Year

[J M Fahey]

Hi Andy.
Fully agree about Bill Gates of Hell, but poor old Leo has had absolutely no relationship with Fender Corporate Monster Inc. since around 1967.
He must be turning in his grave with all this.
Any digital camera (which only does that) is *much* better than any cell phone which tries to be jack of all trades but in a credit card size package, so go for it.

[andyjazz]

Even without posting a screen shot of the scope trace I can tell you that the square wave gets smaller as the input signal is lowered. when I first started tracing this signal through the amp I started at the speaker outs and then followed back through each component pair on the power amp rail. I progressed down from q20, q21 and q18,q19. When I got to the pair of q12, q13....q12 had no square wave so i wnet ahead and replaced thinking "Aha I've got er dicked now." Yeah well think again.

The more i think about this thing, I think that km6xz is right when he says: "The result is you will see something that looks like the distorted output everywhere in the signal path of th power amp section." I don't know wwhere to go from here. I'm ready to give up.