Bugera 6262 crackling/power amp board issue...

[psycle_1]

Got a Bugera 6262 in. Main fuse was blown, replaced it, powered it up. Starting to get crackling from the speakers, so I shut it off and opened it up. Found R3 and R1 burnt, so I replaced both as well as put in new JJ 6L6's. Turned the amp back on, fine for a minute, starts crackling again. Started checking the other resistors on the power amp board, R9 reads at 15meg or something stupid like that and keeps rising. Pulled one leg, reads 220k. Checked R10 as a reference, reads 220k without pulling a leg out of the circuit. What gives?

http://bmamps.com/Schematics/Bugera/..._Schematic.pdf

[The Dude]

Caps charging or discharging will cause that. I wouldn't worry much about it as long as they measure good out of circuit. I would remove the output tubes and check voltages on all of the sockets. Make sure you still have proper voltages on the tubes. You could have a leaky/arcing coupling cap, which would show up as a positive voltage instead of negative on the grids.

[psycle_1]

Yeah, I thought about the cap charging/discharging effect, but since the circuit is symmetrical I would figure R9 should read the same as R10 in the circuit. I'll check the voltages as you suggested and report back. Thanks Dude!

[psycle_1]

Here are the voltages starting with pin 1:
V1: 0, 23.8, 472, 461, 461, 0, 23.6, 0
V2: 0, 24, 476, 470, 466, 0, 23.9, 0
V3: 0, 24.1, 479, 473, -39, 0, 24.1, 0
V4: 0, 24.2, 481, 475, -39.1, 0, 24.2, 0

So based on this, I need to look at the V1/V2 side, even though the blown resistors were on V3 and V4. Correct?

[psycle_1]

Tried swapping C1 and C2 to see if the positive voltage on pin 5 would follow, but it didn't.

[Mick Bailey]

I would check the tube sockets for arcing, plus check there's no leakage or shorts on the board. The positive voltage on pin 5 looks to me like it's originating on the tube-side of R9, otherwise the reading for the bias for the tubes on the other side would be worse. Usually I'd be looking for a leaky PI cap (C56 with this amp), but the voltage is high. Is X14 a connector? If so I'd pull it and check MAIN_B for DC

[psycle_1]

Checked all over the power tube board for arcing and shorts, didn't find anything. Voltage on bias switch side of R9 is negative, just as it should be. Checked the tube sockets and didn't see anything, but will check again.

[psycle_1]

X14 used to be a connector, but has since been hardwired in. Got MAIN_B disconnected, is around 308V. The power amp board side is -35V.

[Mick Bailey]

Looks like C56 is leaking to me, but the voltages don't add up to 460 or so volts measured on pin 5 of the power tubes. Either way, with MAIN_B disconnected there shouldn't be any DC on the PI side, unless the cap is bad, or there's a low resistance or solder bridge.

[The Dude]

I'm wondering about R9. Earlier you pulled one side to measure it out of circuit and it was ok BUT, maybe it is cracked or has a bad lead or something of the sort and is intermittent. If you have your neg bias on one side of it and not the other, I'd try changing it.

[psycle_1]

Yeah, I went ahead and changed it out just to be safe. No change.

[The Dude]

So what are the voltages on both sides of it?

[psycle_1]

-24 on the PI side, 421 on the tube side.

[The Dude]

There shouldn't be 421 there. Is this with output tubes removed? You likely have a shorted tube, or a socket, or trace problem.

If you don't have the tubes out, measure again with them removed. If you have the tubes removed, lift R1 and R3 one at a time and see if one of those lifted removes the 421V on R9. That will tell you which socket it's coming from.

[psycle_1]

Right, that is what has me stumped. Tubes are removed. Checked the sockets and there is no visible damage or arcing. Checking the traces is a bitch since they put black paint over the board covering the traces. but I did follow all the traces that I could and I didn't see any damage. Will lift the resistors and let you know.