B-side C4S board issue at voltage check [resolved]

[Loquah]

Just in case the tube could influence things I switched the 7-pin tubes and it had no impact.

[Paul Birkeland]

Did the high voltage switch to the other large PC board?  If you rotated the middle board and the high voltage went from IA to IB on the small PC board, then you would swap the large PC boards to see if the offending voltage follows one of the high current C4S boards.

[Loquah]

Did the high voltage switch to the other large PC board?  If you rotated the middle board and the high voltage went from IA to IB on the small PC board, then you would swap the large PC boards to see if the offending voltage follows one of the high current C4S boards.

I've swapped the large PCBs already and the voltages remained the same and didn't change sides with the PCB swap (i.e. the large boards aren't the cause of it).

Am I understanding you correctly with this tes?

[Paul Birkeland]

OK, so the high OA voltage on the big PC board stayed put when you switched the big boards around.  Did this big board OA voltage also stay the same when you flipped the small board 180 degrees?

[Loquah]

OK, so the high OA voltage on the big PC board stayed put when you switched the big boards around.  Did this big board OA voltage also stay the same when you flipped the small board 180 degrees?

No. Switching big boards left the high voltage reading identical on small board. Rotating small board made high voltage switch from IB to IA so the issue stayed on the same side of the total circuit regardless of the board's orientation

[Paul Birkeland]

I would be interested in seeing the 7 pin socket on the offending side.  Does the tube glow in that socket?

-PB

[Loquah]

Here are some photos of the D socket. I've included one to show that the red wire attached to pin 5 isn't shorting with pin 6 because it looks like it from above.

https://photos.app.goo.gl/udjU7uq8tuPffGGp6

https://photos.app.goo.gl/TBWDe7YvHztAb4ws8

https://photos.app.goo.gl/jzS4HsLnNmGgvZGs5

[Paul Birkeland]

Does the 6AQ5 glow in that socket?

[Tom-s]

I've had issues with a C2A build where it was all related to the 6AQ5's not biassing properly on first power on. Switching them from side to side got the sleeper to fire up.

[Loquah]

Does the 6AQ5 glow in that socket?

Yes. Glow test passed on all tubes (and still does). Swapping tubes also had no impact

[Paul Birkeland]

Can you verify that the resistance of the little blue resistor between 5 and 6 on that 7 pin socket still measures appropriately? 

[Loquah]

Can you verify that the resistance of the little blue resistor between 5 and 6 on that 7 pin socket still measures appropriately?

236.2 ohms measured in-situ

[Paul Birkeland]

This should probably come visit my bench and I'll have a look over it.  As a last resort, if you had a broken wire between the high current C4S OA and pin 5 of the 6AQ5 socket, I suspect you'd end up with this issue as well.

[Loquah]

I'll rewire OA and D5 first and then it'll be time for a return trip 🙂

[Loquah]

For anyone following along at home, this kit ended up making the long trip from Australia to the USA over the Christmas break. I'm looking forward to an update from Paul when he has time to look at it (I believe he was quite swamped at the time so I appreciate the offer of assistance).