Kreg voltage issues and two LED's dark [resolved]

[msmith]

Hello everyone, I finished assembling my mainline yesterday and I have a couple of issues on the voltage check I was hoping to get some help with.  All of the resistance checks were good and here are the voltage measurements I got:

+275vDC on PSB:  284vDC
+6.3vDC on PSB:  6.31vDC
IA on A side C4S:  284vDC
IA on B side C4S:  284vDC
Breg A side:  217vDC
Breg B side:  220vDC
-reg A side:  0vDC
-reg B side:  0vDC
Kreg A side:  4.54vDC
Kreg B side:  9.81vDC

As you can see, the voltage on Kreg A side is a little low (below the 8-12 range), and I have two LED's on the B side of the C4S board over the A socket that won't light up.  All other 6 LED's light up as they should.  I tried swapping the C4S boards to see if the problem followed the board, but it does not.

Also, I tried regulating the voltage at terminal 20 and 30 to 145vDC.  Terminal 30 will regulate when I turn the screw, but the voltage at terminal 20 does not change the when I turn the adjuster screw.

I've reflowed solder joints on the regulator board and double checked that I followed the instructions on page 28 to make sure the lead on the potentiometer under the screw is bent over and soldered to the center lead.

I've been going over solder joints looking for bad solders but so far have not been able to find anything.  Any ideas of something to check next?  I've been scratching my head on this one for a while...…

Thanks!

[Paul Birkeland]

I would recommend posting photos and we can give you an idea of what's going on.

The high voltage at the Kreg terminal suggests possibly swapped R1 resistors on the C4S board, a misplaced jumper, or cold solders.

[msmith]

Here are some photos.  I can't get the hole bottom of the regulator board without desoldering some wires, but I was able to get part of the bottom side of the regulator board.

I checked the resistors on the C4S boards.  R1 on the A side on both boards meassure 37 ohms, and R1 on the B side on both boards measure 50 ohms.

Paul, you mentioned the voltage being high on Kreg, but Kreg on the B side is within range, while Kreg on the A side is a little low.  Am I misreading something?

Thanks

[msmith]

Additional pictures of the sockets.

[Paul Birkeland]

Sorry, not enough coffee.

220V on the plate of the 12AU7 (breg) and 4.5V on the cathode (Kreg) means that the 12AU7 on the offending side is consuming all of the available current.  Though this sounds like an issue with the 12AU7, it's actually that the 6C45P isn't drawing its share of current.

What voltage do you have at terminal 20? (I would guess somewhere around 215V)

Do both 6C45 tubes glow?

[msmith]

Wow, you're exactly right.  215v at terminal 20.  And yes, both 6c45 tubes glow.  It looks like the tube on the A socket glows a little brighter.  Don't know if that means anything.

I've also tried swapping positions of the 6c45 tubes, but it didn't change anything.  I also tried swapping the 12au7 with the one from my Crack, but no change there either.

[Paul Birkeland]

Yeah, then no current is being drawn by the 6C45PI.  You can resolder pins 4 and 5 on the 9 pin socket to ensure that the heater is well connected.  The other super common issue that will cause this condition is underheated solder joints on the MJE5731A.

[msmith]

Resoldered the MJE5731A's and held for 10 seconds to make sure the joints were well heated.  Also resoldered pins 4 and 5.  No change...

[Paul Birkeland]

The center PC board can be removed and rotated 180 degrees and reconnected to change which half of the board feeds each half of the amp.  I find it likely that doing this will send the problematic voltage to the C side. 

[msmith]

Thanks PB, I'll give that a try as soon as I get some more solder wick!  Ran out yesterday!

Thanks again for your help!

[msmith]

PB, I rotated the center board and the problem did move to the C side.  That helps significantly.  At least now I know that I have a problem with the A side of the center board.  I'll concentrate on reworking the solder joints, again.  Any checks on the individual components you would recommend?

Thanks again for your help!

[Paul Birkeland]

There's not a whole lot going on in that biasing circuit, I would just reheat all the joints and ensure that no pins on the 2N2222A read shorted to each other.

-PB

[msmith]

Okay, so I removed the center board and resoldered all the joints.  I also verified that I had no shorts on 2N2222A.  Still no change, low voltage on A side Kreg and two LED's dark.....

[msmith]

Finally!!! I think one of the trim potentiometers is bad.  After my last post, I resoldered the entire board one more time just to make sure I didn't have a bad solder joint.  Still no change.  I then tried regulating the voltage at terminal 20 and 30 again.  Terminal 30 would regulate to 145v, but it appeared terminal 20 was stuck at 215v.  I kept turning the adjustment screw for the potentiometer associated with terminal 20 when suddenly the voltage dropped to 98v.  Once it dropped, I check the voltage at Kreg on side A and it was 10.2v and the dark LED's light up.  If I try to regulate the voltage at terminal 20 to 145v, the voltage slowly increases from 98v to about 123v and then it shoots back up to 215v again.  Once that happens, Kreg drops back to 4.5v and the two LED's go dark.

So, unless I'm missing something, I think I just need to replace potentiometer for the A side.

[Paul Birkeland]

You can measure the trim pot's resistance  with your multimeter.  On those little 10-turn pots, you can turn them infinitely in either direction, but there is only about 10 turns of adjustment.