Volume imbalance and high voltage in one channel

[antjuneau]

I believe you still have a 31.6 Ohm resistor on the PC board feeding terminal 1. 

If the resistors are in their proper places, then you may have both red wires going to that board sitting in the "O" pads, though the LED's on the board shouldn't glow in that case.

[antjuneau]

I believe you still have a 31.6 Ohm resistor on the PC board feeding terminal 1. 

If the resistors are in their proper places, then you may have both red wires going to that board sitting in the "O" pads, though the LED's on the board shouldn't glow in that case.

I've triple checked resistances across all four R1 resistors - they're definitely in the right place.

All wires going to and from terminals 1-5 have been double checked and re-soldered. Same thing for the 9-pin socket.

Any other ideas? I'm about ready to just order 2 more small boards with parts and solder them in    I've been trying to figure this one out for 2 weeks now.

[antjuneau]

I went a re-soldered a bunch more connections and now I'm getting a different issue. Instead of high voltage on terminals 1 & 7, I'm getting low voltage on terminals 2, 4, 7, and 9.

2 - 95
4 - 95
7 - 71
9 - 83

And now only one LED isn't glowing - one of the LEDs closest to the edge of the big board, specifically the side of the board connected to 3U & 9L.

When I unplug the power tube and leave in the 12AU7 I get high voltages at 1,2, & 4:

1 - 182
2 - 205
4 - 205

Terminal 5 is correct at 79 volts.

Hope this combination of clues helps. I've checked and re-soldered just about everything south of the tranny.

[antjuneau]

Voltages now:

1 - 60
2 - 94
3 - 0
4 - 94
5 - 86
6 - 0
7 - 59
8 - 0
9 - 85
10 - 0
11 - 0
12 - 0
13 - 94
14 - 0
15 - 140
16 - 0
17 - 0
18 - 70
19 - 68
20 - 0
21 - 186
22 - 0

Low voltages everywhere. Can anybody help? Blackplate?

[antjuneau]

I think I found it. I'm getting 0 resistance between the outer legs of one of the MJE350 transistors. Going to order one and report back if it fixes the issue.

[Paul Birkeland]

Please pull the 6080 and recheck the voltages on 1-5.

To reiterate, you must get 60-90 volts on terminals 1 and 5 and roughly 200V on 2 and 4 before going back to run the 6080.  Until you can pass this check, please do not run the amp with the 6080.

-PB

[antjuneau]

Please pull the 6080 and recheck the voltages on 1-5.

To reiterate, you must get 60-90 volts on terminals 1 and 5 and roughly 200V on 2 and 4 before going back to run the 6080.  Until you can pass this check, please do not run the amp with the 6080.

-PB

Is this true even with the bad MJE350? I'm getting:

1 - 80
2 - 198
4 - 198
5 - 173

[Paul Birkeland]

So when we first did this test, terminal 5 gave you the voltage you were looking for and terminal 5 did not.  Now, terminal 1 is correct and terminal 5 is not.

Completely ignoring the 6080 and the big PC board, what else has changed in the amplifier that could have precipitated this change?