UPDATED:Please HELP! My RM80 has Failed!

[JKMV12]

Well i picked up the amp from the tech last Friday after he installed a new Mercury Magnetics O50JM-MM OT (the original OT shorted internally). So i bring it home and plug it in. Rebiased to 32mA (EL34's). I start playing the Super Clean and it sounds OK. I switch to the 1087 and find that the gain is severely compromised...which i have come to expect from that module...i think that one of the tube sockets is bad. If i readjust a tube slightly then it usually is fixed.

Though the real trouble starts when i switch back to the Super Clean and push the gain a little higher. I had Master at 50%, Gain at 50%, and Module Level at ~40%. I start playing and within seconds i see two white spots of light reflecting off the back of my guitar case behind the amp. Before i have time to turn off the amp it shuts off completely. I check the fuses and the power tubes fuses as well as the mains fuse are blown. Upon further investigation inside the amp i found that there is a big black spot around where one of the terminals from the OT primary connect to the board. The plastic terminal head is also black with some splatter on the metal plate behind it. So ya the power tubes arced internally and caused some collateral damage. Nothing else on the board was fried so that is good. I didn't see the underside of the board but hopefully that is fine too. I am mainly concerned about the OT. I just spent $182 on it.

It is with the tech again and i should have more info tomorrow...especially about the OT. He believes that the old OT screwed up the power tubes. Now i am going to stick with either 6550's or 6L6's. I might even try some KT88's. I just hope that the OT is fine and that it won't be too expensive to fix. I am also just glad it isn't worse.

 

[kc2eeb]

It's not the tubes, it's the board itself.
Please look at these pictures I took of my amp while I was repairing it.
http://mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair
The problem was the board itself arched HV to ground.
It was an intermittent problem until it finally shorted enough to leave evidence of a short.
I had tried different power tubes, checked and replaced the screen resistors,
and finally the OT and power amp circuit board.

 

[JKMV12]

so the arching on the board caused the tubes to arc and the fuses to blow. is this a defect in the board? the tech said it possibly had a bad solder joint. The impedance's on the old OT were different so that is why he said that it shorted. Is this why the arcing didn't happen first? I want to figure out the chain of events. now you said yours happened at the plate voltage connectors. mine happened only on one of the OT primary terminals. Everything else on the board looks fine. Does this point towards a board defect or could the transformer be a bad match?

 

[kc2eeb]

In my case I think the board was arching first. I would hear sounds that I mistook for computer or cell phone hash coming through either the ac line or the high gain preamp. I replaced the output tube 3 or 4 times (JJ 6V6s) thinking it was a bad run of tubes. Then, during a rehearsal, I played the amp louder than usual, and it finally arched enough to do the damage it did.
I measured the nominal dc impedance on all the OT leads and they seemed OK. So, the first repair attempt was to the original PA board. I did not reinstall it. Instead I ordered a replacement PA board from Randall as well as a new OT. I really would have to pull the old OT apart to see what's up with it since it doesn't read open or shorted. When I get time to, I'll open up the OT and maybe rewind it. As far as the PA board, I suspect the coupling caps to the PI are leaking DC which would raise the bias over the top.
The problem with the Randall pc boards is they didn't allow enough electrical clearance between the HV traces and ground on the original boards. That's why you will see some of the pc board material ground away on some of the boards. The HV short from the power supply connection to the board is fairly common.

 

[JKMV12]

Well i got the amp back again a couple days ago. It has been playing great since i got it. I have some new Solvtek 6L6's in there. The MM OT is fine and after several more hours of playtime should sound even better.

Apparently the arcing on the top of the board was caused by my brief moment of forgetfulness; i forgot the plug in the speakers for a few seconds. That terminal is now fixed. The tech also noticed some arcing underneath the board which was caused by the ground being way too close to the HT pin on the power tube socket. He did a simple fix but if it starts to act up again he will have to do some more work on it. It has been running fine so far so hopefully it stays that way. He also fixed up my 1087; sometimes it would play fine but other times it would have a massive loss of volume. It turned out to be the tube sockets; some of the contacts were pushed too far apart because of all the tube changing. Now everything is playing great and i can finally spend hours playing guitar again

 

[kc2eeb]

Great to hear that! Enjoy!