LEDs on Large Board not lighting. Voltages for OA and OB very low. [resolved]

[Colonl_Charisma]

I was just posting about taking measurements for diagnosing a channel imbalance. Well, prior to turning my amp over and plugging in my headphones, I thought that I would measure my voltages because my LEDs on my large board seemed to not be lit (I took some measurements with my amp on).

Besides the channel imbalance, amp has been working fine for months.

LEDs are not lighting on the large board.

OA and OB DC voltage = ~1.5 V
Ground = 0 V

 

[Paul Birkeland]

How are the voltages on the small board?

Did you happen to plug the 6080 in with the key in the base not aligned with the socket?

 

[Colonl_Charisma]

6080 is plugged in correctly. I checked that to make sure, and was keeping my fingers crossed hoping that was the issue.

LEDS on the 9 pin socket are lit, and so are the LEDs on the small board.

Small board DC voltages:
OA = 74.4
OB = 79.2
1A = 217
1B = 217
B-A/B = 0

 

[Paul Birkeland]

Is the 6080 glowing?

Having both OA and OB go out at the same time on the big board would have me inspecting the wires going to B+ and ground on that board.

 

[Colonl_Charisma]

I can confirm that the 6080 is glowing. I flipped it over and turned it on so that I could readily see the tubes start glowing.

I will inspect those wires and their connections.

 

[Colonl_Charisma]

Good call on the wires. Ground wire going from the large to small board looks fine. However, the wire going from B+ to 4U may have a cold solder joint. The connection on the large board looks fine, but maybe flipping my amp over a couple of times may have knocked loose a wire on 4U. The joint does not look 100% after getting a better look at it. I also did not crimp the wire around the joint.

I will fix this later today, and I will update the thread.

 

[Colonl_Charisma]

I do have an update, and I wish that my photos would come through. I have two LEDs lit on the large board. I could only tell when I cut the lights off. LEDs on 9 pin socket and small board are brightly lit even with lights on.

The two LEDs that are faintly lit on the large board are the ones closest to the middle of the board. The two LEDs that are closest to the edge of the board show no signs of being lit even with lights off in room.

 

[Paul Birkeland]

Illumination or lack thereof on the big PC board is somewhat meaningless.  Either the voltages are correct or they are not.  Since they were correct, then you moved the amp, and now they are not correct, you're most likely dealing with a loose or broken wire.

 

[Colonl_Charisma]

Right on! Here are my voltages:

OA: 1.4
OB: 1.7
B+: 219
G: 0

 

[Paul Birkeland]

You can post some build photos and we may be able to spot something.  To have a perfectly working Crack then to get to where you are, the possibilities that come to mind would be:

1.  6080 tube not indexed in the socket properly.  This can be checked by looking for the heater glow from the tube itself.
2.  Loose or broken B+ wire to the board.
3.  Loose or broken ground wire to the large board.

If there is any other possible information you have that maybe you've left out, there could potentially be other sources for the issue, but both halves of your large board are not working properly, and the only connections both halves of the board share are the B+ and ground connections. 

 

[Colonl_Charisma]

Thanks, PB! If I can get photos to go through, I will post them. In the meantime, I will replace one of the wires, check, and then replace the other one if the first test fails. I appreciate the help! 6080 is definitely glowing, so we can cross that one off.

 

[Paul Birkeland]

You could also check the voltages on each pin of the 6080 socket. B2 and B5 should be the same as whatever terminals 13, 2, and 4 are.  B1 and B4 should read the same as terminals 1 and 5.  B3 and B6 connect to the output terminals on the big PC board. 

It's very unlikely that both the output wires snapped off of the big PC board, but I suppose anything is possible.

 

[Colonl_Charisma]

Good news! I went ahead and replaced both wires, and now I have all my LEDs on. I also went ahead and took my voltage measurements.

DC Voltage:

OA: 110.3
OB: 106.6
G: 0
B+: 186.7

 

[Paul Birkeland]

There you go, working voltages.