C4S LED and voltage issue

[Loquah]

I had a drop-out in the left channel today and discovered a broken wire caused by a recent episode of tinkering. Having fixed the broken wire and re-tested all voltages, I decided to check the biasing while the amp was upside down and powered.

The B side bias was no issue at all, but the A side bias only has a voltage range of up to about 6.5V

Any ideas what might be going on?

[Loquah]

I had a drop-out in the left channel today and discovered a broken wire caused by a recent episode of tinkering. Having fixed the broken wire and re-tested all voltages, I decided to check the biasing while the amp was upside down and powered.

The B side bias was no issue at all, but the A side bias only has a voltage range of up to about 6.5V

Any ideas what might be going on?

Had a thought to swap the tubes left/right and the issue switched too. Guess that's a spontaneous dead tube, yes?

[Loquah]

The story continues...

I installed some different tubes I had here and the B-side LEDs on the C4S for tube socket C stopped lighting up. Voltage checks show that the kreg on the B-side is only at 4.5V or so.

I'm going to back away from the amp for now and wait for some advice because I feel like each fix I find reveals another issue...

[Paul Birkeland]

I'm really very confused by your information.  You are giving a voltage of 4.5-6.5V, which voltage are you reporting? 

The manual says to measure terminals 7 and 20 to set bias.  If those are measuring around 5V DC, you have a serious problem, but I don't know if that's what you mean?

If you have 4-6V at Kreg on one side, that can just mean that one 6C45PI isn't conducting.

Which wire in the amp did you find was broken?  It could be that this caused a failure in the amp that we now need to diagnose.

[Loquah]

Hi Paul,

Sorry for any confusion. I'll step it out carefully so I can make some sense.
1) wire connecting 19L to +reg is what broke
2) after connecting new wire, the bias voltage (measuring terminals 7 & 30) was only around 5-6V
3) I swapped tubes and the C board LEDs didn't light up now (B-side of C board) so I ran voltage tests again
4) During voltage tests, kreg on B side only 6V

Have I gotten "lucky" with a dead tube in my spare set? Could the original 19L-to-+reg wire break kill the tube that was installed?

[Paul Birkeland]

Does the low voltage move with tube swapping? 

[Loquah]

I'll check when I get home.

Do you think the original wire break could have killed the tube I had installed at the time? (Different to the tubes I am now testing with)

[Paul Birkeland]

It's not impossible.

[Loquah]

Does the low voltage move with tube swapping?

No. After swapping tubes, the B-side LEDs above socket C do not light (or very dimly light) and the Kreg voltage on the B side of the regulator board is still low (around 5V)

[Loquah]

Just realised that it's possible the 19L to +reg cable broke when I was investigating the original issue of no sound from right channel. That would make sense based on the C-socket board not working right, wouldn't it?

Either way, the current problem is the Kreg voltage and non-lighting LEDs so I'll await your next instruction or diagnosis.

As always, thanks so much for the help!!

[Paul Birkeland]

Either way, the current problem is the Kreg voltage and non-lighting LEDs so I'll await your next instruction or diagnosis.

Isn't the problem that your bias voltage can't be set to 150V? 

What are IA/OA voltages on the big board on the side that isn't working?

[Doc B.]

It looks like we may be having some confusion due to terminology. Lachlan, can you give us all terminal voltages that are out of spec?

[Loquah]

+275 vDC = 271-272V
+6.3vDC = 6.29V
IA side A = 270V
IA side B = 269.2V
Breg = 220.5 (B side) and 219.2 (A side)
-reg = 0.5mV both sides
Kreg = 5.16V (B side) and 11.36V (A side)

Sorry for the confusion. The issue is that the original fault / out-of-spec measurement shifted while I was trouble shooting. I'm assuming the tube caused the original biasing issue and now we're dealing with something different.

[Paul Birkeland]

The B side C4S on the side with your low bias isn't supplying current to the 6C45, or there's something weird going on with that socket.

The easiest way to debug this is to swap the two big C4S boards to see if the bad voltage follows a board.  If it does, then you can set your meter to beep for continuity and test each pair of transistor legs for each transistor on the B side of the offending C4S board.  If any transistor has a short, just replace them both and consider replacing the LEDs as well.

If the low voltage does not follow a board, then I would be looking at the 6C45 socket wiring and double checking that the middle section of the middle PC board doesn't have any broken wires (around the area with the blue trim pots and 2N2222 transistors). 

-PB

[Loquah]

Brilliant. Thanks Paul. My money is on a connection around the socket failing during the tube switch, but I'll work through the steps as described.