Bugera 333xl 212 combo problem

[sclemmer]

Hey everyone, I recently picked up a non-working and out of warranty 333xl combo from a guy on Craigslist in hopes of it being a simple fix based on most of the issues these amps have.

The problem with the amp is that it blows the fuse where you plug the power cable in everytime you turn it on.

Following some of the threads here and other places, I started checking things inside the amp.

The first thing I did was check the other fuses inside which are all fine. After that I checked the connector going to the board for a burnt pin; the connector had a lot of glue all around it and inside one of the pins and looked like it had a small burn mark on it. I went ahead and snipped all the leads and soldered them directly to the pins on the board. After this I put it back together and it still blew the fuse.

I had read it may also be the power tubes so I replaced all of those with JJ 6l6's from eurotubes and made sure to put the bias switch to the 6l6 setting and I put the bias adjust knob dead center just to see what would happen and it still blew the fuse.

I'm at a loss now as to what to try next.

Any help or advice you guys can give would be greatly appreciated.

[Enzo]

SO all the fuses inside are OK, and it is blowing the main fuse outside?

Isolate the problem.

There is a powr switch and a standby switch. Both must be on for the amp to operate. But when you are doing this are both on or just the main powr switch. If it blows fuses with the standby off, that tells us something different than if it only blows once the standby turns on.

Pull the tubes. If it blows fuses with no tubes, there is no point in having tubes in it. Once we have it running without tubes, then we can install them and see how it goes.

So, pull the powr tubes, now does it blow fuses with just the power switch turned on? Does it blow fuses with the power switch off? If the power switch doesn;t even have to be on, you have a short in the mains wiring.

If it blows when the power switch turns on, and doesn;t wait for the standby switch, then ther eis either a power supply failure or a short in the power stage.

I would measure from pin 3 of each power tube socket to ground - are any shorted to ground? That would be a bad flyback diode probably or a bad output transformer.

And so on.

[sclemmer]

Thanks Enzo. I'll give this a try later today and report back.

[sclemmer]

Here's what I've done so far:

Fuse blows when the power switch is turned on.

Removed power tubes, fuse still blows once power switch is turned on.

I thought at first that just plugging the amp in was blowing the fuse but that is not the case.

I will check pin 3 of each power tube and post my findings.

[sclemmer]

Yay forum is back up.

Enzo, checking pin 3 of each power tube against ground there appears to be a short as each one gives a reading of different values (never zero).

[Enzo]

What does that mean? Are there low resistances? Or do you get a "charging up" sort of reading?

FInd the center tap of hte output transformer primary and disconnect it from the B+. If that stops the fuses blows, you have at least isolated it to the very output.

[sclemmer]

I don't know what you mean by the center tap of the output transformer?

[Enzo]

Somewhere hear the power tubes, is there a connector with three wires and empty spaces between the wires - the wires being blue, red, and brown? Probably says "X18" next to it on the board? Disconnect that and see if the fuses hold.

[sclemmer]

Fuse still blows with that cable disconnected.

[Enzo]

OK< then we just eliminated the power tube circuits and the output transformer.

Refresh my memory, the fuse blows even if the standby switch is still off, and only the power switch on?

There should be a five wire molex connector with two yellow, two orange, and a violet wire - X33. Pull it off the board. There also should be a couple of light gray wires going to somewhere near there - X29. Pull those off the board as well. Now turn the amp on. If it still blows fuses, your power transformer becomes the suspect.

IF THAT IS THE CASE, find th power transformer primary wires and disconnect them from the system. Now fire it up. if it no longer blows fuses with the transformer primary disconnected, then we have a bad transformer. If it still blows fuses without the transformer, then yoiu have either a wiring short, a miswired main power switch, an internally shoprted power swtich, some short in the mains wiring, or a shorted cap across the mains.

[sclemmer]

Correct, the fuse blows when I switch on the power only. I never touch standby.

With both of those connections removed from the board, when I switch on the power the fuse does NOT blow.

[Enzo]

OK, that is progress. We eliminated the OT and the powr tube stage. You just proved the power transformer was OK with its secondaries unloaded.

SO something on the board is the problem. It would either be associated with the two gray wires (the B+ circuit), or with the 5 wire Molex stuff. You COULD just connect one or hte other of those without the other and see which one blows the fuses. But we could be a bit more scientific.

Before connecting those secondaries back to the board, set your meter to ohms and measure between the two pins where the gray wires go - are they shorted together? Is either shorted to ground? Same checks on the 5-pin molex. Pin 4 is ground - the violet wire pin. Are any of the other four pins shorted to ground? Are any of them shorted to another pin? Pins 1 and 2 are the yellow wires and they carry the heater current. So you would be measuring the low resistance of the tubes in their sockets, so lift the 10A F4 fuse from its clip before checking those two pins.

If any of those shorts show up, then find out why.

JUst a thought, check if either end of the standby switch is shorted to chassis. A broken switch can short its internal contacts to the metal bushing.

If the connector tests seem OK, then also look at the nearby rectifier diodes: D10-13, D16-19. Check them for shorts. Check those large 350v e-caps for shorted too.

If those check out, then lift all 4 of those internal fuses. PLug the transformer secondary cables back onto the board and apply power. If it again blows fuses with those internal fuses lifted, then something basic right at the connections would seem to be bad.

If that holds, then push the 10A F4 back into its clip (power down first of course) and apply power again. If the fuse now blows again, we have isolated the problem to the heater circuit. If the heaters come on, then power off and push down the 1A F2 and F3 into their clips and power up again. Once again we want to see if their circuit is what is doing the damage. That leaves for last the high voltage fuse, F1. If restoring it to its clip blows the main fuse, then the high volatge supply has isssues.

[Stormrider]

Hey, I've taken over working on this amp, and I think I have it narrowed to a bad PT

I soldered all the leads to the board, and got rid of the problematic Molex connectors. I'm slightly stumped at this point however. Not having the schematics hinders me slightly.

Both of the orange leads are reading ground. The B+ and the heater wires are reading fine.

I cut the orange wires off of the board, to test the board, which I thought as the problem. Both of the connections on the board read fine, with no connection to ground or to each other. I checked the orange secondary leads from the PT, and both of them read to ground.

However, with the orange leads disconnected, it no longer blows fuses, BUT nothing else works either (i.e. no heater, no lights, no hum.)

I'm at a slight loss on what to do at this point, I'm fairly sure that it's the PT, but you seems to know this amp a lot better than I (Marshalls are my thing) so any advice you can throw my way would be very welcome.

Thanks

-Josh

PS I was using both the sleeve of a jack, and the screw right in front of the power jack PCB as a ground reference. FWIW both checked out against the center pin of the power jack.

[Enzo]

You already determined that the fuses do not blow when the PT secondaries are disconnected, so I tend to think the PT is OK. If it were shorted, it would still blow fuses.

Your orange wires are for the low voltage supplies, and the violet wire is their center tap. Since the center tap is grounded, the orange wires will show continuity to ground.

Follow the traces from where the orange wires connected. They should go through fuses F2,F3, 1A slow. Are those fuses there? And are they 1A fuses? After the fuses, follow the traces to diodes D16-19. ANy of them shorted? Those diodes rectify the AC from the ornge wires, and they are filtered by C56,C57. Either of those shorted? The + supply there, among other things goes to 3-pin X32 - the cable to the LED board. Try unpplugging that.

[Stormrider]

Yea I started thinking about it last night and though it might be center tapped. Ok, sounds like the next plan is to check the diodes. I'm going to put my current limiter together first, so I don't spend a small fortune on fuses. Thanks for the help!

-Josh