fender 212r output stage issue

[angelothewolf]

Hi! looks like a bias problem right? but i cant find the problem.Voltages are ok, Output devices are fine, drivers pre drivers, etc. with no load is working fine.
i upload a video with the input vs output on the oscilloscope with the amp loaded, to see if someone can help me a little bit

any ideas?

FM212R_schematic.pdf

[mozwell]

It looks like you are missing one of the power supply rails for the power amp, or one of the rails for the op amps
Lets measure the power supply rails & make sure these are ok first
This is a solid state amp, so disconnect the speaker until we get everything working well.

Please measure:
TP1 to P4, should get 32VAC
TP2 to P4, should get 32VAC
TP3 to P4, should get +41VDC and very small VAC
TP4 to P4, should get -41VDC and very small VAC
TP5 to P4, should get +16.8VDC and very small VAC
TP6 to P4, should get -16.8VDC and very small VAC

Post results here

[J M Fahey]

Post ^^^^^^ results, but if the amplifier works well without load, rather than missing a rail we have a current drive problem here.

[angelothewolf]

well, while trying to isolate the problem, i discover that temperature affects the distortion, so with a bit of hot air and cold spray a found Q12 screwing up my positive cycle!

[Jazz P Bass]

Good call.

And replacing Q12 repaired the amp?