Finished build, 2 dark LED's on C4S [resolved]

[itsikhefez]

hi,
I just completed the build and something is not correct.
The continuity checks from pg.72 of the manual are all correct.

Voltage checks:
+275V: 283V
+6.3V: 6.29V
IA (A side) 283V
IA (B side) 283V
Regulator-A side (B, -, K): (220V, 0, 10.7V)
Regulator-B side (B, -, K): (193V, 0, 13.6V)

All 4 LED's light up on the A side C4S. IB=220, OB=144
On the C side C4S, the top A LED's light up, while the bottom B LED's stay dark. IB=193V, OB=146V

Any idea's where to look?
Thanks for the help!

[Paul Birkeland]

What do you get at OB on the faulty side?  (I'm guessing that's also about 190V)

[itsikhefez]

I just rechecked.
On the faulty side, IB is actually 220 for a few moments on power up, then drops to around 193.
OB is around 146-148 the whole time.

Thanks for the help!

[Paul Birkeland]

I just rechecked.
On the faulty side, IB is actually 220 for a few moments on power up, then drops to around 193.
OB is around 146-148 the whole time.

The 193V/13V tells us that the 12AU7 is not being permitted to really draw any current.  The 146V coming out of OB right when you turn it on is odd, as the 6C45 takes a bit of time to warm up and OB should be about what IB is until that happens.

Regardless, I would reheat all of the joints on the bottom side of that PC board before continuing.  Also look for the usual suspects like a PN2907 swapped with a TL431, or a jumper that isn't where it's supposed to be (review the applicable pages of the manual while resoldering the board).

[itsikhefez]

Just to confirm, should I resolder the joints of the faulty C4S board, or the regulator board?

[itsikhefez]

The 193V/13V tells us that the 12AU7 is not being permitted to really draw any current.  The 146V coming out of OB right when you turn it on is odd, as the 6C45 takes a bit of time to warm up and OB should be about what IB is until that happens.

Regardless, I would reheat all of the joints on the bottom side of that PC board before continuing.  Also look for the usual suspects like a PN2907 swapped with a TL431, or a jumper that isn't where it's supposed to be (review the applicable pages of the manual while resoldering the board).

You are correct that OB is similar to IB when directly after power on. Once the tube heats up, and IB starts dropping from 220V to 193V, then OB drops to around 146V.
I reviewed the pages of the C4S and Regulator board instructions and confirmed that I have the correct parts and jumpers on my boards. (PN2907 on the C4S and TL431 on the regulator board).

Since I will only have time to reheat the joints again properly late tonight, is there anything else I can check after that if reheating them doesn't help?
Thanks

[Paul Birkeland]

You'll want to reheat the MJE531A transistors on the high current C4S board.

[itsikhefez]

I did a few more checks, will add the info in case it can help.
I can bias both 20 and 30 to ~145V.
All 3 tubes are glowing, but C-side tube is slightly dimmer.
IA of the offending side is 278V, and the center pin of the MJE5731 reads 193V.

I reheated all the joints on that board, and in particular the MJE5731, without success.
Attaching a photo of the underside of the board.

EDIT: Looks like we were typing together

[Paul Birkeland]

Tube glow is controlled by pins 4 and 5 on the 6C45PI sockets and pins 4/5/9 on the 12AU7 socket.

Come to think of it, a loose connection at pin 4 or 5 of that 12AU7 socket could cause these issues.

[itsikhefez]

Tube glow is controlled by pins 4 and 5 on the 6C45PI sockets and pins 4/5/9 on the 12AU7 socket.

Come to think of it, a loose connection at pin 4 or 5 of that 12AU7 socket could cause these issues.

Thanks. I've tried reheating those pins as well, but the issue did not resolve.
I also swapped the 6C45PI and re-inserted 12AU7.

I'm not sure the issue is a cold joint. I don't want to sound over confident about the joints but I have a bit of an OCD when it comes to soldering, I examined every single one meticulously and redid them if necessary. I use a Hakko workstation and its heat is pretty consistent.

Could it be a bad transistor? Based on other threads, the issue was usually a human error and not a faulty part, so this seems unlikely too.

Either way, I measured the voltages of the MJE5731A pins.

On the "good" C4S:
(E-C-B voltages)
A side (278V, 220V, 277V)
B side (217V, 144V, 217V)

On the faulty C4S:
A side (278V, 190V, 277V)
B side (142.5V, 142.5V, 141.8V)

[Paul Birkeland]

What happens to Kreg on the faulty side if you remove the 6C45PI, leave the 12AU7 in, then power up the amp?

Faulty transistors tend to short out, so you will be able to read zero ohms between a pair of legs on a bad transistor.

[itsikhefez]

When the 6C45PI is removed, Kreg reads 3.5V on the faulty side.

None of the transistor pins are shorted, so I guess they are fine.
Also, the center pin is outputting 220V

[Paul Birkeland]

When the 6C45PI is removed, Kreg reads 3.5V on the faulty side.

Interesting, what do you see at IB under those conditions?

[itsikhefez]

Interesting, what do you see at IB under those conditions?

IB reads 220V.

Thanks again for the help!!

[itsikhefez]

IB reads 220V.

Thanks again for the help!!

I ~think~ the issue might be the PN2907A.
I removed the 6C45PI from both sides.
On the good C4S, E,B,C all read ~220V.
On the faulty C4S, E,B read 220V, but C reads 135V.

135V is also the value of OB.

Unless I traced it incorrectly, that seems to be the first location of 135V on the board.
(D1 and R1 both read 220V)

I tried reheating the pins of the PN2907A along with its surrounding components without success.
Thoughts?