Frontman 212R loud hum

[Randall]

Frontman 212R took a hit PT bell cover hard enough to push the thin aluminum chassis in full inch. It is silent with no channel mode lights until something is plugged into the input, then it passes no signal except a loud dangerous sounding hum. It also has +40v on the speaker outputs, though I'll be dipped if I can see how it's getting there. I don't know if the two are related. I haven't pulled the board yet, but poking around in circuit I don't find anything that looks like it is shorted.

Q20 and Q18 don't look shorted. Same with Q16 (it's actually a 2N4401). C55 looks good (actually a 47uF, another misprint). D38 looks good. Those are the only paths I am seeing for the + rail to get to speaker out. Except for R101 thru R107, what's up with that?

Another interesting thing is the DC at the output goes away if I pull the input jack, so it would seem the mute circuit is in play here somehow.

FM_212R_-_Frontman_212R_Schematic.pdf

[olddawg]

First thing to check on those amps is the main power supply filter cap connections to the pcb pads. Especially after rough handling they tend to disconnect themselves.

[Enzo]

You cannot assume an output transistor must be shorted to put DC on the output. Anything that turns them on puts DC on the output.

You mention the input jack has to be full. Same message, look at the mute circuit, lower left of PA. That mute controls the emitter side of the diffy pair, and that controls whether the rest of the power amp comes on. That is consistent.

Your problem looks to me to be to the left of Q14,15. I mean between them and the diffy pair, not like immediately to their left.

[Randall]

Turns out it was one of the cement resistors again. I forgot to look at the designation, but it was one of the pair on the left side of the board close to the input, R144 probably. I had been in this one a couple of years ago for this same issue. My repair held, a soldered in resistor leg splint, but this time it broke where the splint ended and the bare trace began. It must have taken quite a bad hit to do that, because I try to make sure the component is snug to the board so there can be minimal movement.

Anyway, if it was R144, then I also probably didn't have my +16v rail, which probably explains why it wouldn't channel switch. I still am trying to see where the 40v came from. Anyone care to venture a hypothesis?

[g1]

If you were missing one of the LV rails, an op amp may have had DC on it's output because of it's unbalanced supply.

Edit: U6-B specifically, putting DC on the diffy pair, which is carried on to the output. When nothing plugged in at the input jack, the diff. pair is turned off so the DC is gone.

[Jazz P Bass]

Best to keep in mind that most guitar power amps are 'dc coupled'.

Meaning there are not any capacitors in the signal path to 'block' Vdc.

[Randall]

"Edit: U6-B specifically, putting DC on the diffy pair, which is carried on to the output"

Could this be explained a little more in depth please? I see the 40v rail on the collectors of Q9 and 10, and I can imagine the unbalanced U6 putting DC on the base of Q9, turning it on, but where does it go from there?

[Enzo]

Or off, depends on polarity.

Q9,10 control Q18,19. If the IC loses one rail, say +16, then Q9 turns off, leaving the collector to pull up to rail voltage minus a little. That allows the base of Q18 to pull upclose to rail. And the output of the amp follows the bases of the output transistors. Voila - +40.