Fender Frontman 212R: DC output

[EugenioN]

Hi all!
I'm fiddling around a Frontman 212R, plauged with the infamous "pop/crackle" sound caused by mute circuit.
While powering for initial testing, some magic smoke escaped from R84 and output went to -30(ish)V. I then disconnected the muting circuitry snipping out R74 to simplify the troubleshooting.
This is the situation: R84 is blowing out because 85mA are flowing through. After some measures, this current path should be the one depicted in red:



Q10 base-emitter is reverse biased (-3ish V), even if U6-A is doing his best to keep the base positive.
Every single diode and transistor is looking fine when probed for base-emitter and base-collector diodes, and this makes me going nuts

Hints?

PS: sorry for these vague measurements, I need to quickly turn on mains supply, take a look at the bench multimeter, then turn off before R84 will burn with flames and fumes.

[Kim Christensen]

I'm a little confused by some of the values marked on your schematic. Like R87, R88, R89 & R90:
What does the "Fu" mean? 
ie: R87 is Fu470... Is that 470 ohms? Or something else?

The smokey current through R84 could be coming from 2 places. Q11 or Q10... You could check the voltage drop across R86 to estimate the current through Q10. It should be zero if Q10 is operating correctly since you mentioned that it's base-emitter is reverse biased. That being Ok would leave Q11 as the next suspect.

[Chris56000]

Hi!

"Fu" followed by a number indicates a "fusible" type resistor, indicated in ohms!

Apart from Q11 s/c base–to–collector being the obvious path for excess current to flow, part of the current  should be carried back to the –42V rail via R89 and diodes D25, D26 and D27.

This type of circuit is difficult to analyse component–by–component because of the direct coupling between all the transistors, and you may have one or more of the various fusible resistors damaged from the original short–circuit!

I would begin by checking all the "Fu".prefixed resistors for any damaged or open–circuit ones, followed by the 1N4448 diodes ('4148s will also do!), and make certain you've not got any other defective transistors, you'll to check them ALL between the diff amplifier stage Q9/Q10 all the way up to Q21, out of circuit!

Chris Williams

[EugenioN]

Q11 was the offending one.
Despite measuring ok with simple on circuit junction check, it was definitely not ok when biased at full voltage
After replacing with another bjt (similar specs, not the exact part) everything went ok.

It was populates quite high with respect to the pcb, "hanging in the breeze" with long leads; it was the only one mounted this way: all its siblings were close to the pcb surface.
Maybe some vibrational stress caused its premature degradation.

Thank you for your help!