80's Peavey Butcher actin' wild

[GremlinMachine]

R49 (the blue 400-ohm 10-Watt resistor) started glowing hot orange inside and nearly smoked a few seconds after taking the amp off standby! Looking at the schematic can you identify a common probable cause??

All fuses are still good and none of the Molexes have brown or burn marks etc and seem intact although I may hardwire the higher voltage ones after this. I just now replaced the rectifier diodes, upped screen resistors to fresh 470-ohm 5W, swapped all 220k resistors (x6) on the power board, check all solder joints on the main power board. I went ahead and put a fresh R49 in there even though the original one measured OK, but have not risked powering it back up without getting all your advice first. Also my access to variac is in another location altogether today so I'm being extra cautious.

History: This amp worked perfectly in the shop after a full electrolytic recap and bias mod and was biased safely ~36mA prior to selling and shipping it off. I had used it a number of times with zero problems. I am suspecting something about getting jarred in shipping may have brought new gremlins out, or the buyer could have done something weird but I really don't think they did.

The buyer played the amp a few minutes and it blew the 1A internal fuse. Had him send it back so I could diagnose any further issues and give it a once-over for testing reliability before sending it back.

I have done general restoration of several 80's Butchers, 4 or 5 VTM's, Rock Master head, 5150, JCM800, etc and never had this current type of problem arise so I am looking for help before powering it back up.

I honestly consider myself an intermediate level electronics tech with lots of various repair and restoration experience but not at engineering level education fyi. Any help and advice is greatly appreciated. I do have access to Variac, Signal Gen, and Oscope with a basic knowledge of using them if needed.

I can upload pics and follow any recommended steps and will gladly reply with updates. Thank you all!

[Helmholtz]

From the schematic the 400R/10W resistor should be R49?

[GremlinMachine]

Correct R49...blond moment, my bad.

[Enzo]

can you identify a common probable cause??

Piece of advice: it doesn't matter if it is common or not, we just need to find the cause. Another nit pick: It is the low voltage, high current pins on the molex connectors that burn up, not generally high voltage. As in the 6v for heaters.

The big resistor serves the screens node of the power supply, and especially considering you shipped the thing, by far the most likely reason for the resistor to burn is a shorted power tube. Sure it could just coincidently be something else, but you want common, there it is.

[GremlinMachine]

Enzo the PV master..I have read hundreds of your forum posts over the years to great personal educational benefit, so thank you first of all.

Those heater wires were actually the first ones I was going to do, thanks for the words on that aspect...high current indeed. I probably just would not be doing the indicator LED ones i.e.

Now...why would that resistor still do that with no power tubes installed?

Also I took the tubes to a shop with a tube tester and they tested good with no shorts before I reinstalled them. They were brand new matched quad of JJ's with less than 2-3 hours play time.

[Enzo]

OK, if the resistor burns with no tubes installed, look for something shorting the screens node to ground, or an extremely leaky filter cap. And rare, but I have seen, internal short to frame in the standby switch.

[g1]

Also check for any signs of arcing at the power tube sockets.

[Enzo]

Yep, and one trick there is to turn off the room lights, arc spark is way easier to see in the dark.

[GremlinMachine]

Thanks will physically inspect for shorts including underneath the power tube board etc. The flyback diodes tested OK on the MM anyway. It's likely got to be a short from B+ if I am looking at this correctly...will report back.

[glebert]

Now...why would that resistor still do that with no power tubes installed?

Are there any burn marks on the tube board or the PS board? I've had a very similar amp that had gotten a conductive spot due to overheating/arcing. If you haven't already I would try disconnecting the tube board from the power board to see if that changes anything.

[Enzo]

The flyback diodes are before that resistor and should not affect it.

[GremlinMachine]

SOLVED...sure enough I found a goofy short off the B+ under the power tube board. Sheesh!!!