I cleaned the power transistor that sparked, tested it, seems fine. Put it back in and now with the bulb limiter the amp won't turn on, not even the indicator light, though the bulb lights up and stays lit. When tried without the limited (yes, dumb I know) it blows its fuses. So it seems cleaning the corrosion off of the power transistor leads has exacerbated the short.

Quote from: galaxiex on September 15, 2016, 09:34:27 PM
The bulb should never "blow".
Unless the filament has reached end of life and simply burns out.
Then nothing works, bulb or amp.

Remember, all power to the amp passes thru the bulb filament.
At most a dead short in the amp causes the bulb to glow with it's "normal" brightness.

Think of it like this, the bulb is normally plugged directly into the mains.
By putting it in series with the amp you have simply added the amp into the circuit with the bulb.
So if the bulb can normally withstand full mains voltage, why would it blow with the amp in series with it, even if the amp has a fault?

I normally start with a 25W bulb for SS amps.

For the rest of your post I'll let the pro's speak. 

Not sure exactly what's wrong with the amp, so I wasn't sure what to expect from the bulb. The amp being a 120W and the bulb being 60W I thought blowing was a possibility.

Replaced Q14, Q15, made a light bulb limiter. Powered it back on and saw some smoke coming off, I think, the large inflammable resistors (R77, R78, R83) saw a spark, along with audible snap from one of the output transistors and measured 25 VDC at the CH1 speaker output. Bulb in the limiter is 60 W, it lit up, stayed lit up, but didn't blow. That blueish/white glow at the output transistor in the lower portion of the photo is the spark.

A bit hazy on transistor testing, but the closest transistors seem like they've bought the farm as well.

(Q14) 2SC1624
(Q15) 2SA814

The A814 shows OL on all combinations with the DMM and the C1624 reads a variety of low resistances (200ohms and less) across all leads back and forth. Neither of these results seem right for a transistor, could someone confirm that? From what I understand from this thread,

http://music-electronics-forum.com/t16431/

Those are the driver transistors. A lot of that thread is over my head, could someone with a bit more understanding of transistor specs suggest available replacements for these? Cheers.

Quote from: phatt on September 13, 2016, 08:55:20 AM
Meantime you might need a light bulb limiter.
http://www.ssguitar.com/index.php?topic=2093.0
You will need that to test the replaced parts are in fact working.
These are high current DC circuits and the light bulb (Current limiter) will stop instant destruction at turn on if something is not right.
Phil.

Yeah, I'll totally whip up one of these, I'd heard of them but never realized how simple it is until now.

Quote from: phatt on September 13, 2016, 03:19:12 AM
Now set DMM to DCV and note the *voltage at speaker output terminals* it should be very close to Zero,, If not the power stage is faulty. (100mV either side of Zero is normal)

Speaker output leads of the the bad channel, CH1, read 45 VDC. Outputs on the good channel, CH2, read 0.1 VDC.

Heyall,

Came across this old thread,

http://www.ssguitar.com/index.php?topic=3575.0

While trying to repair an '81 JC-120 I purchased as-is and seems folks in these forums can probably help.

The schematic is here,

http://music-electronics-forum.com/attachments/30030d1407958634-sm_jc-120_11016.pdf

CH1 speaker is dead, when I connect a good speaker to CH1 I get a tone but no signal, seems like around 100hz. CH2 works fine, so, everything works except for CH1 and thus Chorus because it requires CH1. Any ideas what I should be checking? I've got some experience with electronics, amps and pedals, built amps from kits and small ones from scratch. Have an older oscilloscope, multimeter and such. Thanks.

Update: Okay, found a couple of bad resistors (R72, R73, R74) and a bad diode (D1) on CH1 and swapped those out, now I get some crackling from that channel but no signal.

Update: Seems like R75 and R76 are bad too, which are fusing resistors -- I suppose I could replace those with non-fusing ones for now?

Update: Replaced R75 and R76, still just crackling from CH1, but softer.