B52 At-212 with intermittent problem

[Mr Johnny Birchwood]

Hey all, I just had a B-52 AT-212 show up in need of some help. Owner reports that kick-drum resonance in rehearsal space causes the amp to make 'horrible loud noises.' Also, amp was non-functional in either the Tube A or Tube AB settings. Seemed to work OK in solid-state rect. I haven't been able to re-create the reported problem on the bench but here's what I've found so far:

Rectifier tube Chinese 5AR4 burned to a crisp.

Power tubes worn-out and very rattly with obvious heat damage(likely from extended playing in tube A setting)

PS caps C83 C84 bulging at the top like they are about to explode

Obvious heat damage to connectors adjacent to R95 the cathode resistor(who's brilliant idea was it to put this resistor on the ps board 1/2 inch away from all those ultra-cheap capacitors? DUH!!!)

Amp was obviously dropped at some point. Back particle board panels are broken. No obvious signs of mechanical damage inside the chassis.

I've read some of the previous threads about the problems with these things, and the best plan of action I can come up with is to replace all power tubes and rectifier tube, replace all power supply caps, move R95 off the board by soldering in some leads and mounting it away from any sensitive components. I would also have to replace the bias caps, and all other electrolytics that are near to R95 and the rectifier tube. Lead wires from standby switch are burned to a crisp from proximity to R95. In general, there are lots of loose, cheap plastic connectors all over this thing. Considering that it will need new tubes, new caps, and a pretty complete and painstaking inspection, most likely soldering quite a few leads directly to the board in place of the cheap connectors, my question for you more experienced techs is, should I bother? Total repair bill for parts and labor would easily be $300 or more and I like to be able to guarantee my work. I'm starting to think that this amp may be a lost cause, that it may be more ethical to recommend that the owner cut his losses and buy a decent used amp that has a better track record of reliability.

Any thoughts, opinions, rants are welcome.

Also, I found a non-factory schematic on the interwebs, but I'm wondering if anybody has a factory service document they would be willing to share. My schematic doesn't have the jumper connections on it which makes untangling this rat's nest pretty time-consuming.

Thanx

[Enzo]

If the rectifier tube is bad, replace it, but if the amp has a solid state setting, go ahead and just use that for now. If the power tubes are shaky, replace them too. But it might serve you to find the problem before you do. Kick drum drives it nuts? SOunds vibrational to me. (Oh unless they have a PA system micing the kick drum, and the drum shots are sagging the mains and affecting the amp - I doubt it) Amp running, ball up your fist and whack the top of the amp hard. Does that cause a reaction? If so, you have discovered a loose connection. Loose conections can be inside tubes, so it is possible new power tubes would cure it, but diagnose it first. Loose transformer bolts can cause noises too. If fisting it beneath you, you can get official and whack it with a rubber mallet.

Bulging caps: is the cap itself really bulging? Or do you see the black plastic end cover bulging, and assume the cap under it is too. If the cap can is distended then it is surely bad, but MANY MANY caps will warp their end cover disk when heated. Take a blade to the plastic sleeve to free the end disc and expose the metal can. Bulging or not?

[Mr Johnny Birchwood]

You are right, the metal ends aren't bulging under the caps. The connectors in the vicinity of R95 show obvious signs of heat damage so that led me to suspect all the other components in the general vicinity. I still haven't been able to get the amp to go haywire by beating on it. Tried various methods, hitting, kicking, in various locations. The rattling in the worst of the power tubes is resonant at a D note played on a guitar but I still can't recreate the reported problem. I guess the next step is to power it up and wiggle all the connections until I hear some noise. Assuming it's a loose connector that I can track down(of all the loose connectors) I would still be hesitant to charge for the repair, a set of tubes and a re-bias and then send it out the door knowing that the PS and bias caps were previously exposed to such excessive heat. It does have a B+ fuse on the power tubes so if the neg bias failed and the tubes ran away it would hopefully blow the fuse before causing much collateral damage. Transformer bolts are all nice and snug, btw.

[gtrmann]

I would think the rattle of the output tubes might be the problem with the "horrible noises". $300.00 is not out of line for a extensive repair charge. If some one is paying you for your work, you should do a thorough job and be able to warranty it. The customer brought you the amp for repair, the price should determine whether the customer finds value in repairing it. That amp might give him something specific that he wants.