Bugera 333XL-212 power amp stage problem

[Carlito]

Hi, first time posting here. I need help to troubleshoot my Bugera 333XL-212 combo tube amp (NOT the infinium one).

I bought this amp used a few days. I got a good discount because when testing it, it didn't make any sound. It was supposedly working fine when it was last used several months ago. It is not clear if it had been moved since.
It turned on, all the LEDs (including power and standby) worked, buttons for channels selection, reverb etc worked, the channel selection pedal also worked, but the four power tubes (EL34) and the 12AX7 tube (which is on the same board as the EL34s) (tube 5 on the schematics) didn't glow. The three pre-amp 12AX7 tubes (which are on the main board), on the other hand, glow.
I noticed that the amplifier output impedance was set wrongly to 4ohms, whereas the two 8ohms speakers were connected in series, for a total of 16ohms (but that should not damage anything, right?).

When I got home, I started troubleshooting:
-the sound coming out of the effect send jack is fine! So it seems that the problem is located at the power amp stage.
-after opening the amp, no trace of damage at all.
-continuity test of the four fuses on the main board, and the main fuse at the back was successful.
-I poked the connectors with a wooden chopstick: all of them seem connected properly and most are secured with glue.
-continuity test for shorts of the five tubes which don't glow was successful: conduction between pins 2 and 7 only for the four EL34 tubes, and between pins 4, 5 and 9 only for the 12AX7 tube.
-I measured ACFIL_A and ACFIL_B on the power tubes' board with the amp ON, and it was 24V if I remember correcly (so some of the connectors are working).

I went through the ALL-TIME great information, but couldn't find something that matches exactly my problem.

Is there anything else I should check, before I remove the power amp stage board to check the surface for damage and trace out a circuit problem? Or should I start by tracing out the main board, even though the pre-amp stage works?
Any tips on how to discharge the capacitors which pins are hidden underneath the main board while it is still mounted? Or is it safe enough to unscrew the main board, flip it, and then discharge the capacitors?

Thanks in advance.

[scstill]

It sounds like there might be a filament break to the power board. Did you check that 6v is getting there?
#2 #3 on your schematic second page
What I cannot find is the source of the X16 connector in the schematic

[Carlito]

Thank you for your answer.
The X16 connector of page two is connected to the X25 connector of page 1 (the X25 pins are spread all over page 1), if I'm not mistaken.
I can check whether there is 6v or not. When you write #2 #3, do you mean X16 pins 2 and 3, and do you mean a difference of 6v between the two (not to ground)?

[Carlito]

I measured the voltage between X16 pins 2 and 3, it is 0v. So there is no heating to the five tubes, which would explain why they don't glow.
I started tracing back to the transformer and I found a burn mark on the connector to the transformer: the infamous Bugera 333 burned up transformer molex connector. How could I not notice during the initial inspection?

[scstill]

"infamous" Bugera 333 burned up transformer molex connector.

does this happen often in this amp?

[Carlito]

Yes, Bugera amps were rumored to catch on fire back then.
I should have suspected that was the problem.

I removed the glue, unplugged the connector, cleaned the pin, broke the lever part of the female terminal out of the melted plastic, mounted it inverted to increase the contact surface (clean on that side) with the pin, plugged back the connector, and tested the amp: everything works great. Damn this thing is loud.

I can see why this type of connector fails: the contact between the male and female connectors is just a point, not a surface. I will buy a proper molex connector with square female connectors, and a crimping tool, and mount that instead.

Thanks scstill for sending me in the right direction.

[scstill]

Haha They even have the problem right in their name
"Bug"era