Bugera 333xl (non-Infinium) popping fuses

[Cytagasm]

Hi there,

I've had a faulty Bugera for a number of years that I'm finally now able to start diagnosing and attempting to fix. Any help would be greatly appreciated

When I turn the power switch on the light will come on for a second, there'll be a little hum from the speaker; then the whole thing will shut down, taking the fuse with it.

I removed the power tubes and it continued to blow. I then proceeded to disconnect the power transformer from the board by removing x29 (2 grey wires) and x33 (2 orange, 2 yellow, and 1 purple wire). It would power on. With just x33 attached it would power on. So using my awesome powers of deduction(!) it would seem something with x29 appears to be the culprit.

I re-attached both x29 and x33 and then removed all the fuses on the board. The amp continued to power on and I found I could have fuses F2-F4 on the board, but when I put F1 back in it would blow the mains fuse. I tested the fuse with my multimeter (continuity I think is the word) and my understand is it still works!

I managed to arrive at this stage using information gathered online (but mostly this thread http://music-electronics-forum.com/t13578/) but I haven't found anything to help get passed this stage yet. As you can probably gather my knowledge of circuits is and electronics is pretty poor but I am learning! (I know not to touch the capacitors!) I do have a multimeter so I can take rudimentary measurements

Any advice would be hugely appreciated.

Thanks in advance

[52 Bill]

Thank you for starting a new thread, you have given so much more information to go on now.

I do have a multimeter so I can take rudimentary measurements

You will need to test the high voltage power supply diodes as a first step. There are four of them and they connect to fuse F1.

The diodes that you need to test are D10, D11, D12 and D13. Does your meter have a diode test function? Do you know how to use it?

[Cytagasm]

Thanks a lot for taking the time to offer advice. I looked up how to test diodes online (my multimeter does have a diode test function) and from my understanding *each one is working correctly. All 4 give a reading between 743-748 in one direction, and between 532-537 in the other direction*

Edit: I guess I managed to completely miss the whole point of diodes haha. They're only supposed to allow voltage to go in one direction! So I've attached my black wire from my multimeter to the banded side of the diode and the red wire to the opposite side and that shows 532-537. Reversing the red and black gives 743-748

[mozwell]

Are those readings are with the diodes in circuit, desolder one end of each diode & repeat the measurements.
We are not getting 000 on any diode, so that is a good start.
Normally on diode scale on a multimeter we read voltage drop. The meter injects a small current into the diode & displays the voltage drop across the diode, so 532 would be 0.532
I would expect a diode to measure between 0.4 and 0.8 one way, and open, or overscale the other

[Cytagasm]

Are those readings are with the diodes in circuit, desolder one end of each diode & repeat the measurements.

I'm having trouble removing one end of the diodes- do I need to flip the board over?

[52 Bill]

I'm having trouble removing one end of the diodes- do I need to flip the board over?

Probably, how are you trying to remove the diode lead now?

Seeing that none of them read zero, I'd probably move on to test the caps and other components first. With the amp unplugged, measure the resistance from either side of the standby switch to ground. Try both positions of the switch, on and off. If you get a very low reading that doesn't change (rise or fall), you will need to try and find out why.

[g1]

There is a connector from the power supply to the output tube board. It's labelled X25/X16.
It looks like a good place to disconnect the circuit that is after C43/C44.

[Enzo]

Disconnect the output transformer from the power supply. Any difference? Or doing it a different way, measure resistance from pin 3 of each power tube to ground, any low resistance?

I am suspecting a shorted or leaky flyback diode.

[Cytagasm]

Probably, how are you trying to remove the diode lead now?

It took me a long time removing all the glue from the different connections but I've finally manage to flip the board over. (All cables disconnected except for that pesky 10 pin x25 x16 which I cannot figure out how to remove without breaking!) So I can go ahead and unsolder the diodes to measure them if need be.

Seeing that none of them read zero, I'd probably move on to test the caps and other components first.

I've been testing caps and they all seem to read ok (the resistance keeps increasing). Except however for C57 which gives me a value steady at 5.78 M Ohms.

With the amp unplugged, measure the resistance from either side of the standby switch to ground. Try both positions of the switch, on and off. If you get a very low reading that doesn't change (rise or fall), you will need to try and find out why.

Sorry I'm not 100% sure what you mean by 'either side of the standby switch'. There are physical clips on each side and if I measure them they jump all over the place!

The third pin of the output tubes measure .769k Ohms for the first two and .833k Ohms on the second two.

[52 Bill]

Now that you have removed the connectors from the power supply board, retest the diodes. any difference from the original readings?

If you disconnect the connector from one side of the standby switch and read the resistance from the connector to ground what happens?

Are the readings from pin 3 of the output tube sockets with or without the transformer connected? With or without the tubes? What happens when you reverse the meter leads?

[Cytagasm]

I was having difficulty removing one end of the diodes. I measured diodes 10-13 again using the diode test on my multimeter. There was no difference in reading with the power transformer disconnected versus conencted.
With red on the marked end of the diode:
0.747v on D10 and D11
0.743 on D12 and D13
With black on the marked end:
0.525 on D10 and D11
0.528 on D12 and D13

I measured the 2P X23 connection from the standby switch to ground and I got 100+ Ohms on both pins
I measured the 3p X32 connection from the standby switch to ground and I got 16.48, 0.9 and 0 Ohms to ground on the 3 different pins.

I measured the 3rd pin on the power tubes with the following results:

With Power Transformer disconnected and no power or preamp tubes there was an ever increasing resistance across all tubes.

With preamp tubes installed, but regardless of whether Power Transformer or power tubes connected there was 4.69 to 4.71 k Ohms across all tubes.

[52 Bill]

To me the only odd readings seem to be the reverse readings of the 4 main diodes.

Try reading the resistance across the four 220K bleeder resistors R85, R87, R101 and R102. Because these are across the filter caps, the readings should rise and or fall slowly as the cap charges up from the voltage from your meter.

[Cytagasm]

I have taken those measurements and they are as follows:

R85 rises and falls slowly
R87 rises and falls faster than the others
R101 rise and falls slowly
R102 rises and falls slowly

[52 Bill]

I have taken those measurements and they are as follows:

R85 rises and falls slowly
R87 rises and falls faster than the others
R101 rise and falls slowly
R102 rises and falls slowly

And what was the resistance readings across each of the resistors?

[mozwell]

If the resistance rise & fall as you say for each resistor, my guess is the power supply is OK

Have you done the checks Enzo suggested in post #8
I bow to his years of knowledge