C4S LED and voltage issue

[Loquah]

Swapped C4S boards and the error remained at Socket C. Going to check socket and centre board next

[Loquah]

I've checked everything now to the best of my ability and cants solve the issue. What I've done:
- removed and reconnected the power regulator board
- reflowed any suspect joints on reg board
- tested for continuity across the regulator board transistor pins (no continuity between any of three pins on both)
- swapped C4S boards (i.e. they are currently opposite from their original arrangement)
- tested and reflowed joints on C socket

After full reassembly I've got the same issue of the two LEDs that light up later on the C socket C4S failing to light.

I'm guessing I've got a faulty component somewhere, but don't know how to test for it. Any tips would be hugely welcome.

[Paul Birkeland]

The center board can be rotated 180 degrees, that is how I would check the center board.

[Loquah]

Oh, I tried that, but didn't include it in my list because I wasn't 100% sure that was meant to work. The result was the LED issue switched sides. The second pair of LEDs above Socket C also didn't light, but I realised after that the tube was still out of the socket.

I'll retest this with the tube in to be certain, but working on the assumption that the problem changed side (and the C socket LEDs just needed a tube installed), what's my next step for diagnosis.

Sorry this is a challenging one!

[Paul Birkeland]

If rotating the center board also rotated the problem, then there's a trimmer pot, a 2N2222 transistor, and a resistor that are being swapped in that process, as well as the high voltage regulator components.  Before swapping the center board again, what voltages do you see at OA and OB on that center board?

I have to be absolutely insistent that you not say anything about lit or dim LEDs moving forward.  It's absolutely 100% necessary to have voltage measurements.  Dark LEDs don't really tell me anything, while voltage measurements tell me a lot. 

[Loquah]

No problems, Paul. Only using the LEDs as an indicator. Happy to focus on voltages now you've told me what to measure. I'll test as soon as I can and report back

[Loquah]

Original orientation
OA: 3.32V
OB: 2.28V

Centre board turned 180 degrees
OB: 2.22V
OA: 3.2V

Other measurements with centre board rotated 180 degrees:
+275: 286.6
+6.3: 6.29
IA: 289 (A) and 289 (C)
Breg: 220.3 (B side, now closest to A socket) and 157 (A side, now closest to C socket)
-reg: 0.4mV both sides
Kreg: 4.45 (B side, close to A socket) and 1.56 (A side, close to socket C)

[Paul Birkeland]

Breg:  157 (A side, now closest to C socket)

Kreg: 1.56 (A side, close to socket C)

These have changed.  Can you pull the 12AU7 and recheck?

[Loquah]

You might be onto something...

Kreg B 1.16 and A 1.04
Breg B 286 and A 286

[Paul Birkeland]

With no 12AU7, are you still seeing that super low voltage at OB on the offending channel? 

[Loquah]

With no 12AU7, are you still seeing that super low voltage at OB on the offending channel?

189V at OB on socket C
For reference, it's 262V for socket A

[Paul Birkeland]

OB on the two outboard C4S boards shouldn't be that high.  You need to run the amp with no 12AU7 but with the 6C45PI tubes. 

OB on the outboard C4S boards is of interest under these conditions.

[Loquah]

That was with 6C45Pi installed on both sides.

Confirming measurements WITH 6C45s and WITHOUT 12AU7. All references to sides are based on the A and C sockets on the amp to avoid confusion from the rotated regulator board. Apologies for any redundant info. Just trying to provide a complete picture

+275 = 288
+6.3 = 6.3
IA = 287 (A) 288 (C)
Breg = 285 (A side)  285 (C side)
-reg = 0.3mV (both)
Kreg = 1.16 (A side) 1.02 (B side)

OB: 259 (A) 187 (C)
IB: 285 (both)
IA: 286 (A) 287 (C)
OA: 284 (both)

[Loquah]

That was with 6C45Pi installed on both sides.

Confirming measurements WITH 6C45s and WITHOUT 12AU7. All references to sides are based on the A and C sockets on the amp to avoid confusion from the rotated regulator board. Apologies for any redundant info. Just trying to provide a complete picture

+275 = 288
+6.3 = 6.3
IA = 287 (A) 288 (C)
Breg = 285 (A side)  285 (C side)
-reg = 0.3mV (both)
Kreg = 1.16 (A side) 1.02 (B side)

OB: 259 (A) 187 (C)
IB: 285 (both)
IA: 286 (A) 287 (C)
OA: 284 (both)

Just rechecked all my wiring because of your concerns about OB voltages. OB on C4S boards remained as per quoted post here.

OB on regulator board reads 2.24V (connected to C socket) and OA is 3.37V (connected to A socket)

[Paul Birkeland]

OK, so the OB voltage that really wanted to spend the rest of its life at 6V has popped back up without the regulator in the way.

I believe your issue will be resolved by replacing the 431 regulator on the offending channel, with the slightest possibility that you'll also need a 12AU7. 

-PB