Bugera 1960 NON infinium

[Danglin' Fury]

Lol! Apparently I still had the preamp tubes still in it. Say it is a leaky cap... the schematic doesnt specify the voltage of said caps and the physically look awfully small. What do you fellas suggest? I have until 5:00 central time to get to the electronics store to procure one. I'm about to lift the leg and measure...

[Helmholtz]

Lol! Apparently I still had the preamp tubes still in it. Say it is a leaky cap... the schematic doesnt specify the voltage of said caps and the physically look awfully small. What do you fellas suggest? I have until 5:00 central time to get to the electronics store to procure one. I'm about to lift the leg and measure...

Coupling caps should be rated at least at PI supply voltage + bias voltage, so I'd say 500V should be safe.

[Danglin' Fury]

I just replaced them with 630 Volt caps. Same value. Getting similar readings to last time with the standby engaged. -31 on the right pair pin 5 and -1.43 on the left pair pin 5. I havent flipped it to play yet.

[Danglin' Fury]

Flipped it to play... very similar readings... +28vdc on pin 5, right pair and ~457 volts dc on the left pair pin 5. Similar readings at the coupling caps c26 and c29...

[Helmholtz]

You might have a conductive/carbonized board.

What do you mean with "standby engaged"? Either the amp is in standby (= high voltage supply disconnected) or not ( HV connected).

[Enzo]

Unless I missed it, did we check for open resistor? Bias supply comes from supply and BIAS test point through R21 and R22. One resistor for each side pair. One side seems to work as you get -42v on the grids, so trace that -42v on the other side from the bias test point through whichever resistor feeds the bad side and up to the tubes.

An open R21 or R22 will kill bias to half the tubes.

[Helmholtz]

Unless I missed it, did we check for open resistor? Bias supply comes from supply and BIAS test point through R21 and R22. One resistor for each side pair. One side seems to work as you get -42v on the grids, so trace that -42v on the other side from the bias test point through whichever resistor feeds the bad side and up to the tubes.

Resistors were checked and (unnessarily) replaced.
Where did you see -42V ?

[Enzo]

Oh, I guess I didn't, I saw that ~457 as -45.7 somehow. The tilde looked like a negative sign.

For what it is worth, I wasn't thinking open resistor so much as an open involving one, as in a cracked trace or some such.

Idle thought, with such odd voltages, any chance ribbon cable over at X20 could be on the pins wrong? (Upper left page 2 of drawing)

[Danglin' Fury]

X20 is the shitty connector Bugera likes to use on the power transformer, not so much a ribbon cable. This amp did work at one point. I have a post from 2018 (?) I believe. The output transformer was bad. I replaced it and it worked for a good while. I have checked the OT and it seems to be ok... those 220k ohm resistors... r23 reads pretty solid in circuit at 216k-ish ohms... but r15 is all over the place. And when I measure pin 5 of the right pair to pin 5 of the left pair, I should read 440k ohms, yes? But it fluctuates between 345k ohms and 283k-ish ohms.... this is pretty strange. The ceramic sockets dont look damaged. But I guess it could be underneath the sockets. Looks like I'm removing some sockets.

[Enzo]

Oh, I thought X20 was the connection between boards.

[Danglin' Fury]

Well... I took the power tube sockets off the board and there is no sign of burns that would explain why I have basically B+ voltage on pin 5 of my EL-34s... holy hell...

[Helmholtz]

Sure you didn't measure pin 6?

[Danglin' Fury]

I am double and triple sure. It is odd because the right pair (looking at the back of the amp) doesn't have said voltage on pin 5. It does have the high voltage on pin 4. But not 5. But the LEFT pair, has the HV on BOTH pin 4 and pin 5. The voltage on pin 4 is around 460 VDC. Then I measure pin 5 and it is 450-ish volts. This is very disturbing. Also, I measure the high voltage at the coupling cap off the anode of the phase inverter... so guessing it is either up stream, or it is part of the feesback/presence... there is no evidence of burning on the board...well, if there is it is extremely difficult to see as the boards themselves are black. No idea why they couldn't use green, but ok.

[Helmholtz]

Does any of the power tube sockets have a short between pin 5 and pin 4 or pin 6?

[Danglin' Fury]

I removed the left pair of sockets. They, off the board, have no short between pins 4 and 5 nor 4 and 6... the corresponding EL-34 solder pads on the board show completely open between 4 and 6, but 3.27K-OHMS between 4 and 5. The right pair of sockets show a resistance of 300K-ohms between 4 and 5 and between 4 and 6 it is open.