Carvin V3 power circuit help

[marc05]

I bought a Carvin V3 head from a guy second hand. About 20 minutes of playing after I got it home, it quit. So I did some troubleshooting, got some tubes, figured out it was not the tubes. So I pulled it out and began troubleshooting the circuit boards etc.

First I found was resistor R56 was open. It's a 4.7 Kohm resistor, and was reading open and no power was passing through.
So I replaced the resistor and bang, the amp began to function again. This was short lived as about 5-10 minutes after powering it up, I began to smoke, and i quickly flipped it upside down to find the capacitor C82 was billowing smoke out the top like a chimney, so I quickly shut the power down. To find resistor R56 had gone bad again.

My concern is possibly the power transformer is outputting too high a voltage. I am reading about 370-375v to ground, and the prints say it should be 350. My input voltage is about 124 volts. Has anyone seen a transformer fail in this way?
With R56 failed open, should I be reading the voltages listed in the prints? Or will they be high because that part of the circuit is open?
Your help is appreciated. Thanks

[Enzo]

Yes, they would read high that way. The resistor burnt because the cap failed. resistors don't burn up on their own, you replaced the first resistor without finding out why it burnt Then the cap revealed itself and took out the second one.

Replace teh cap and resistor and see what happens. NO I don't think the transformer is at fault.

[marc05]

Awsome, thanks for the quick reply.
Strange thing is the cap, after removing it, reads 22uf, which is what its rating is. Is that normal for a cap to smoke but still read the correct capacitance?

[Chuck H]

The cap may read fine with the low voltage from your tester and not perform once the high voltage is applied. It's also possible that r56 is fine. You just couldn't read it in circuit because of voltage on the HV rail countering your testers voltage. That would explain being able to read any amplifier voltages downstream of that resistor. Or it may indeed be open because of high current through the failing cap and what you read was voltage standing on the other capacitors because they are unable to reach a "bleed" circuit (not shown in that portion of the schematic).

[marc05]

Well I am happy to hear it is likely not the transformer as they are a tad pricy, and being in Canada would have put it at well over a $200 part.

Oh and the high voltages I was refering to were upstream of R56. If I remember correctly, I was reading approx 480v DC on the line side of the resistor, and it would go to -.6v or something on the load side of the R56. That was before the first time I replaced it, as the second time I did less testing once I seen the cap turn into a smoke stack.