Mainline voltage issues [solved]

[dpat1]

Well I finally got through my build to the voltage checks and the problems started. I had a couple of miss wires that I corrected initially but they may have had an effect on the current issue. The first was I incorrectly wired the filament for the A tube. Had them on 3/4 instead of 4/5, now fixed. The next one was a wrong connection on the regulator board. I connected the white wire from the B side (to the C tube) C4S board to the wrong pad on the regulator board, also fixed. That said this is where I am at now with the voltage readings.

All resistance readings were as expected.

PS board= +230V, 6.3V
IA on the A side = 229V
IB on the B side = 222V
Breg = 219V both sides
-reg = 0V
Kreg = 13.4V for the C tube; 3.2V for the A tube

On power up all of the LED's on the C tube light and stay lit. On the A tube the LED's farthest from the output transformers come on and then gradually go out. The LED's closest to the transformer never come on. I have looked at this for a long time trying to figure out what I did wrong and can't see anything obvious so it's time to dial 911 Bottlehead. Please let me know what else to look at and I will get the needed information.

Thanks,

David

[Paul Birkeland]

Hello David,

Go ahead and attempt to bias the 6C45's to the correct voltage, then let us know how the voltages look.

-PB

[dpat1]

Thanks for the quick response. I'll give it a try in the next day or so when I can get some time after work.

Dqavid

[dpat1]

I had a chance to dial in the bias with mixed results. The A side adjusted fine to 145V on terminal 30. The B side would only go down to 191V on terminal 20 before running out of adjustment. Kreg on the C tube remained at 13V. The Kreg on the A tube went to over 14V. Same issue with the LED's for the A tube. Let me know what I should look at next.

Thanks,

David

[Paul Birkeland]

Swap tubes to rule that out.

On the B side that would only go to 191V, can you give me the OA voltage on the big C4S board at that adjustment?

Can you also give the OA/OB voltages on the center PC board?

-PB

[dpat1]

PB,

I switched tubes and there was no change. The OA voltage for the C4S board is 172V for the A tube and 171V for the C tube. On the regulator board the OA was 1.7V and the OB was 0.7V

Thanks,

David

[Paul Birkeland]

If your OA voltages are in the 170's and your Kreg voltages are above 3, then I would be double checking the R1 values on the outer pair of C4S boards.

-PB

[dpat1]

PB,

Checked both sides and the resistances are close to the 50 ohm values that should be there. I re-soldered all four and there was no change.

Thanks,

David

[Paul Birkeland]

Hello David,

The "A" side should read 37 and the "B" side should read 50. 

Can you triple check this?

[dpat1]

PB,

The C4s for the A tube: A side 38.6 ohms, B side 50.2 ohms.

The C4s for the C tube: A side 37.4 ohms, B side 49.7 ohms.

All readings taken in circuit with a Fluke 87 III meter.

Thanks,

David

[Paul Birkeland]

It would probably be a good idea to post some photos.  Tops and bottoms of each PC board, as well as the wiring under them may allow us to pick something out that has gotten missed so far.

-PB

[dubiousmike]

Hey dpat - couple of thoughts:

1) Shouldn't your main psu board be outputting 280V -> IA, rather than only 230V?  Or maybe the US version of the amp that I have is different in this regard if you are overseas?  Did everything check out when you did the initial psu test on page 38 of the manual?

2) Another thing you might want to look at: the 220k (R4) and 2.49k (R3) resistors on the problem channel on your regulator board.  These control the output of the TL431 variable regulator, which is what you are measuring @ kreg.  I had a bad joint on one of these resistors myself - although my bad kreg voltage was even lower than yours (10.47 in the good channel and only 1.82 in the bad).  Here's my thread in case it sparks any other ideas: http://bottlehead.com/smf/index.php/topic,5126.0.html

Best of luck with your debug!

[dpat1]

dubiousmike, Thanks for the input. Yes the PS checked out fine at that stage of the build. I went through your thread and then went back to mine and did a bunch of soldering touchup  to no avail.

PB,

The photos are below so please let me know if you or anyone sees anything suspect.
 Photo 2463 is the suspect C4S board for the A tube.
 Photo 2465 is the good C4S board for the C tube.
 Photo 2468 is the regulator board.
 Photo 2469 is the underside of the suspect C4S.
 Photo 2470 is the underside of the regulator board.
 Photo 2471 is the B tube socket.
 Photo 2473 is the A tube socket.


Thanks to everyone for the help.

Regards,

David

[dubiousmike]

Hi David,

Sorry to hear my comment above didn't lead anywhere helpful.  I had a further thought, borrowing from PB's suggestions in my thread.  Have you tried firing up the amp without any 12au7 in the b-socket?

I was slow to take up this suggestion (simply forgot about it) when debugging my own unit, and it turned out to be pivotal since I was focusing on the C4S board when my issue was really on the regulator board.

As I understand it (and I may well be oversimplifying), pulling the 12au7 basically takes the regulator board out of the picture and helps isolate your issue.  If your C4S boards still measure differently, then they clearly become the focus.  Alternatively, if both C4S boards suddenly perform the same (as occurred in my case), then you can focus on the regulator board.

[dpat1]

dubiousmke,

Yes I did try removing the 12AU7 and the same problem remained.

Thanks,

David