Voltage test problem + 2 LEDs dark [solved]

[George Napalm]

Hello everyone,

I finished building my Mainline a couple of days ago. Power supply test and resistance check were fine. But the voltage test brought the following problems:

+275vDC on the Power Supply Board: 278
+6.3vDC on the Power Supply Board: 6.2
IA on the A side C4S Board: 276
IA on the A side C4S Board: 274
Breg Regulator Board A side: 216
Breg Regulator Board B side: 142
-reg Regulator Board both sides: 0
Kreg Regulator Board A side: 11
Kreg Regulator Board B side: 58

In addition to that, 2 LEDs on B side of the right-side C4S board are dark. I've seen similar problems with other users, but their voltage readings were quite different. I've rechecked all connections on the Regulator Board and the right-side C4S board, as well as B and C sockets. I also resoldered some suspicious-looking connections on the rights-side C4S Board and Breg Regulator Board B side, but the problem persists.
I also need to mention, that I seem to have different 431 regulators: A side one has "WS TL 431A", B side has "FKB31 LM 431" written on them.

Does anyone has ideas what is the problem here? Thanks in advance.

Pictures:

 

[Paul Birkeland]

Pull the 12AU7, then measure the voltages at IA, OA, IB, and OB on the B side C4S board.

 

[George Napalm]

IA: 285
OA: 283
IB: 283
OB: 172

 

[George Napalm]

And I got similar results on the left-side C4S board, but OB on the left (A) side was 180

 

[Paul Birkeland]

Can you use the bias trim pots to trim out your OA voltages to more like 150V?

I'm just whittling down possibilities here.

-PB

 

[George Napalm]

A-ha! I reinserted  the 12AU7, then set the bias to 145 on both sides - and all my LEDs are now on! And those offending voltages are normal now: Kreg on both sides are 11V and Breg are both 217V.

I guess I'm all set! Thank you so much! I'll do listening tests tomorrow.

 

[George Napalm]

I did the listening test yesterday and had a bit of a scare. There was a big channel imbalance (on all volumes). After verifying that attenuators are built correctly I decided to double-check the tube bias. And indeed it drifted to 155 and 135 volt on left and right sides. I must have done the initial biasing before the amp warmed up properly. I waited for 10 minutes and set the bias voltage back to 145v on both channels. It totally fixed the problem.

The amp is up and running and produces beautiful sounds Thanks Bottlehead for terrific product and excellent support!