Champ 600 RI blew up, voltage questions.

[stoneattic]

I'm trying to help a friend fix his Champ 600 RI that went up in a puff of smoke. It was obvious from it's charred appearance that R11 was burnt and it measures 4M instead of 1K now, so I put a new 1K 3W in it and it started to smoke as well. I pulled the tubes and measured test points TP12, 14 & 16 (see attached schematic) and got:

TP16 should be 120Vac, measured 122 Vac (just checking the power coming in)
TP14 should be 280Vdc, measured 299Vdc (seems higher than it should be just because of the slightly high AC)
TP12 should be 366Vdc, measured 422 (seems way high)

My thoughts are that the 6V6 went and took out R11. Unfortunately we have to order one just to check since we don't have one laying around to try.

My questions are:

This may be dumb, but is there a way to test any aspect of the 6V6 with a DMM to see if it is shorted, etc?

Are the high voltages overly high and a concern? If so, there's not much between the wall AC at TP12 and R11 besides the transformer to TP14 and rectifier diodes and filter caps to TP16. Any idea what could be causing the high voltage? I'm used to resistors drifting causing higher voltages, not caps or diode, but I'm still a relative beginner.

TIA!



champion600_reissue_fullmanual.pdf

[Chuck H]

I think the 6V6 went for sure. You might detect an internal short on a bad tube with a DMM. I wouldn't bother. I think you have your TP #'s different from the schematic so it's hard to see just what your talking about without speculating. Basically, the power tube shorted and started drawing excessive screen current. Check all screen components. I expect the cap is good since, if I'm guessing right, you have high voltage at that node once R11 blows. But check it anyway. So, check C5 and R26, replace as needed. Replace R11 again and then plug in the new tube. Best to use a current limiter for the initial test to avoid more problems in case this doesn't catch it.

[stoneattic]

Thanks for the response and the catch on the test points. I corrected the test point numbers. The 6V6 doesn't have a ton of hours on it, so I'm kind of surprised and it's a JJ, not the stock Chinese one. I'll check R26 and C5, but they look okay. I know that doesn't mean anything, but R11 make a LOT of smoke.

[Bruce / Mission Amps]

...
Are the high voltages overly high and a concern? If so, there's not much between the wall AC at TP12 and R11 besides the transformer to TP14 and rectifier diodes and filter caps to TP16. Any idea what could be causing the high voltage? I'm used to resistors drifting causing higher voltages, not caps or diode, but I'm still a relative beginner.

TIA!

[ATTACH]17906[/ATTACH]

The high voltages are because, with no tubes, you have no load on the power supply.
I also think you should just replace the power tube, R11 and start over.

[g1]

You are wondering about high readings at the test points. By pulling the tubes, you have removed most of the load on the power supply. Power supply voltages will be higher when the supply is unloaded. The voltages shown are under actual operating conditions, with tubes installed.
Something you can try is measuring the voltage across R11 with the 6V6 removed.
There should be very little voltage across it. If so, as Chuck stated, the 6V6 is toast.
However, if you measure more than a couple of volts across R11 with the 6V6 removed, then the tube is not the problem, more likely C5 is shorted.

[stoneattic]

I just wanted to follow up with a thanks to everyone. We replaced the 6V6 and the all the suggested resistors (which tested okay, but were slightly brown) and cap. All is well now.