Smoking resistor!

[yaryaryar]

Hello all,

Vague question here: I recently bought a project amp that's producing no sound whatsoever, and whenever I turn it on, one resistor starts smoking within 10-15 seconds. So then I shut it off and stare at the circuit board, wondering what to do next. I assume that the resistor is a symptom and not the problem - so just replacing that would be no help if it's just going to burn up again (I don't have that value resistor on hand anyway - I'd have to order one and wait). I'd like to turn it on and check voltages everywhere and see where the problem is coming from. But I certainly don't want to leave the amp on while there's smoke pouring out of it.

So my question is: Where do I begin here? Remove the resistor and test voltages without it? Wouldn't removing the resistor affect the voltages I'm trying to measure?

I know this is pretty vague, so if you need more info to answer the question, please let me know. If it helps, it's an early 70s Ampeg V2, and the resistor in question is R217. Schematic is attached. Otherwise, please feel free to tell me "just take it to a professional". That's kind of where I'm leaning right now anyway.

Thanks!

[Randall]

If C208 were shorted it could smoke R217. I'd start there.

[g1]

Other suspects would be C207, or the tube itself, V202.
And V202 is the reverb driver, so the amp should still function (without smoke or reverb) with V202 removed.

[Enzo]

Isolate the problem. UNplug the reverb pan. If that stops the resistor from burning, then C209 may be shorted. Then pull V202. if it still burns, then C208 is likely shot. It pulling the tube stops the burn, then the tube itself is suspect, OR maybe C206. SO with the tube out, measure pin 7 of V202 socket. You should see close to zero volts. If you have something like +50v there, then C206 is likely shorted. C207 is not really ne of my suspects, but no reason to ignore it. With the tube in place, in the time it takes the resistir to overheat, is the voltage on pin 8 of V202, or the top end of R216, whichever is easier to get at, zero or something much higher than 6v? You can of course also check to see if C207/R216 is shorted.

And of course, make sure there is no loose piece of hardware lodged under the board, shorting the circuit to ground.

[yaryaryar]

You guys are amazing. C208 is shorted. The others you all mentioned do not test for continuity, so I'll assume they're ok. In hindsight, of course I should have checked those... but this is my first troubleshooting experience where I've had to worry about starting a small fire in my basement...

So: I'll replace C208, and also R217. But that brings up questions. C208 is a weird looking cap. Will any .022uf cap above 400v (as the schematic notes) work? Or is this a specific type of capacitor? I ask because it is the only capacitor of this type in the entire amp. I've attached a photo - C208 is the yellow one, center right.

And also R217 - it's one of those hollow-type resistors. I remember reading something about that being a coil of some sort, right? So does that need to be an exact replacement, or will any 10k/5w resistor do the job?

Next question, I notice that one of the big electrolytic caps is bubbling out on one end, so I might as well replace that, while I'm ordering parts. That's the 20/20 silver cap in the other attached photo. Is this cap from fliptops.net a drop-in replacement? 20ufx20uf@450v Electrolytic Cap New! - Product Details . I'm assuming it is, but I want to be sure, since I know the multi-caps are sometimes tricky.

With all that out of the way, do you think that the shorted C208 would prevent the amp from making any sound at all? That being just the reverb circuit? I would still think I'd get some hum or some other awful noise. I'm assuming after I get this patched up, and I can fire up the amp and test voltages and such, that I'm going to find some other serious problems. I suppose I'll cross that bridge when I come to it.

Thanks, all. This forum is great.

[g1]

That would be a wirewound resistor, any 10K 5W will do there. The cap for C208 is nothing special, any of the same value & voltage will work.
The multi-can is being used with the 2 sides joined in parallel. So no need for a multi, any 47uF 450V will work and be much cheaper.
None of this should be responsible for no sound. R51 in power supply may have been stressed, check supply voltages.
Also wiggle all the tubes around, in case it's just a dirty socket causing signal loss.

[Enzo]

C208 was no doubt loading down your B+, but just remove it. Now does the rest of the amp work? Or not?

[yaryaryar]

Thanks, all. I have a few new parts on order, but in the meantime, I did have time to remove C208 today and switch it on today. No more smoking resistors, which is nice. But also, still no sound whatsoever. Not even a pop or static when I first flip the switch. So, there are still major problems to be uncovered in this amp. I didn't have time today to really get in there with a multimeter - hopefully I'll find time for that in the next few days and report back. But, now that this amp is officially "Not a Fire Hazard", I'll be comfortable taking those readings when I get time.

Thanks for all the help so far!

[Randall]

Just to state the obvious, is the speaker working? Can you measure it on an ohm meter? Does this amp have a speaker output jack? Can you plug in a replacement speaker to check?

[yaryaryar]

Hey! It works!

I'm still waiting on parts for all the stuff discussed above, but in the meantime, I did get the amp to make some sound. I had a slow day at work today, so I searched through the forum for every mention of an Ampeg V2. Somebody mentioned a while back that these Ampegs are especially susceptible to corrosion on the jacks/switches. Not sure if that's true or not... but it turned out to be true in my case. I had given the pots/jacks a shot of contact cleaner when I first got the amp, but today I really attacked the main speaker jack and also the impedance switch. That seems to have done the trick! It passes a good, clean guitar sound, along with a bit of hum. Not nearly as much as I expected, just a bit of buzziness. I assumed this guy would need new filter caps as well - not sure if I'm ready for that project myself, so I may take it to a pro. If it were a Fender-type, with drop-in replacements, no problem. But reconfiguring those multi-cap cans might be beyond me for now.

Anyway. It makes sound! I'm sure as I really dig in to this amp, I'll find more oddities that I'll need help with - and I'll probably start a new thread, as we're beyond the "smoking resistor" problem. Thanks so much for all the help so far. You guys are amazingly knowledgable and encouraging.

[Enzo]

Nothing special about V2s or even Ampegs. This happens to ANY amp with jacks.