Fender Frontman 212R major issues- HELP!

[jaret]

Hi there. I have a Fender Frontman 212R that was humming when nothing was plugged into either input or the power amp in (FX return). Otherwise, it worked fine and the guitar signal was as it should be. Here's a schematic for reference:
http://docs.google.com/leaf?id=0B5eUEwC ... ist&num=50

While I was testing transistors in the 'Mute' section (Q6,7,8), I accidentally shorted the legs of either Q8 or Q10 (next to each other). It was only momentary, but now the amp hums all the time and the guitar sounds fizzy and distorted through the output. I tried replacing all of Q8-Q13 as a shotgun approach, but no luck. Since the hum is really loud through the 2x12" speakers (4 ohm total), I connected a 10ohm dummy load while testing voltages. However, the dummy load gets very hot, suggesting to me there's a LOT of current running through it. This same dummy load takes the output of a 100W tube amp without getting warm, so I'm thinking this is abnormal.

By shorting legs of Q8 or Q10, what components could have been fried? I don't have parts for Q14-21 so I'm really hoping it's not in there! The hum seems to be worse after replacing only Q13- the original tested bad for hFe. Perhaps the replacement is incorrect, although I'm using the equivalent, according to NTE (2SC2383>NTE31: http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/).

I'm up sh*t creek with my inexperience with solid state- hoping someone can throw me a paddle!

Thanks,
Jaret

[David Root]

Sorry guy, I think you're in the wrong forum for solid state. If it's beyond diodes/rectifiers/FETs, effects chips and maybe JFETs for switching, there are very few folks here that have any experience or interest in solid state.

[Cliff Schecht]

I have all kinds of experience with solid state amps. That's why I won't touch them now. You'll be hard pressed to get me inside of a PCB amp to repair it unless it's for a friend or is getting gutted (my preferred choice of course).

The last SS amp I fixed was a friends Ampeg Rocket bass amp combo. It was a nightmare to get apart and I remember the construction itself being quite goofy. One of the transistors sat inside of a heatsink and somehow shorted out. Since this was the "balancing" transistor that is in between the output PP pairs, it pretty much took out the power stage as well. Replaced this, all of the power transistors and the fuse and the amp has been working like a champ since.

[Cliff Schecht]

Also we use one of these at practice as a PA/extra guitar amp. Works alright but has that famous Fender issue of channel switching when you power down which is quite annoying sometimes. I remember that amp so well because it likes to shock you when you use a separate tube amp and an unbalanced mic to sing:D.