Failed Voltage Check on New Mainline [resolved]

[ScottAstroNut]

I have completed the build of my new Mainline. All the resistances checked out, but one voltage does not.

+275vDC on the Power Supply Board..... 275.3V
+6.3vDC on the Power Supply Board..... 57.6V
-6.3vDC on the Power Supply Board..... 51.5V
IA on the A side C4S Board..... 275.4V
IA on the B side C4S Board..... 275.3
Breg Regulator Board..... 217.8V, 217.0V
-reg Regulator Board..... 0V, 0V
Kreg Regulator Board..... 3.0V on A side, 10.1V on B side

Also, the LEDs on the B side of the A socket C4S board are not lighting. All the other LEDs light up.

I can adjust the bias on the A trim to 145V, but the B trim stays at 213V regardless.

I have tried reflowing joints around the B tube socket, as well as the connections to the A side C4S board and the regulator board, and I have changed 12AU7 tubes to see if that was a problem. All to no avail.

Thanks in advance for the help.

 

[Paul Birkeland]

One 6C45PI isn't drawing current.  This is kind of a good thing because it narrows things way, way down for the purposes of debugging. 

You could narrow things down even more by swapping the big high current C4S boards between channels.  That unchanging OB voltage will either move to the other side (which put the issue on the high current C4S board), or it will stay put (then you'd need to look at the wiring of that 6C45PI socket and be sure the wiring around the 2N2222 and blue trim pot on the center PC board is correct).

 

[ScottAstroNut]

I swapped the C4S boards, and the problem did NOT follow the boards. The same LEDs on the B side of the A socket C4S board failed to light. I guess that means that the C4S boards are not the culprit? The measurements for Kreg remained the same at 2.9V and 10.0V for the A and B sides respectively. I checked the wiring of the A side C4S board and the regulator board and don't find any incorrect connections between the boards and the chassis. I double-checked the wiring of the A socket and B socket and can find no bad joints or shorts. I checked the regulator board for any poor or shorted joints on the board itself and could find nothing amiss.

 

[Paul Birkeland]

What are the DC voltages on OA/OB on the center PC board? 

 

[ScottAstroNut]

OA = 1.36V
OB = 3.94V

Just for fun, terminal 20 reads 215V and cannot be changed using the B side trim pot. Terminal 30 can be set to the nominal 145V using the A side trim pot.

I just triple checked the regulator board and can't find a short or other wiring problem. Side A and Side B seem to be wired the same.

 

[Paul Birkeland]

So the 3.94V would be consistent with the 6C45P not drawing any current, or at least very little. 

If you rotate the B side trim pot, I would imagine the voltage doesn't change at OB on the center board? 

 

[ScottAstroNut]

No, adjusting the B side trim pot doesn't really do much, but the voltage does fluxuate some. I got it as low as 3.2V, but it seemed erratic.

 

[ScottAstroNut]

I think that the B side trim pot is set up correctly. It's the leads directly under the adjusting screw that are shorted, right?

 

[ScottAstroNut]

I also swapped the 6C45P tubes and was careful to fully seat them. No change.

 

[ScottAstroNut]

For good measure I reflowed the joints at the B side trim pot. No change.

 

[Paul Birkeland]

If the voltage at OB on the small board doesn't move when you rotate the pot, something is amiss with how that part of the board is populated, wired, or soldered.  If all else fails, you can post some photos of the top and bottom side of the board.  If you forgot to fold the pot legs over on that side (it's a red instruction in the manual on page 31), then you'd get no response to adjusting the trim pot and your bias voltage would always be high.

 

[ScottAstroNut]

I'll post some photos in just a bit. The leads on the B side pot under the adjusting screw are folded over on each other. I just reflowed that joint.

 

[ScottAstroNut]

Do you want me to remove the board from the chassis to get a better view of the bottom of the board?

 

[ScottAstroNut]

Here are some photos of the regulator board.

 

[Toobuzz]

I had the exact same issue. Check your PM for more details. I had to rotate one of the trim pots over twenty full rotations before the 200V+ bias even started to decline. Once I got it down to 145V the Kreg pulled into place nicely as well.

 

[Doc B]

It could be valuable to the community if a measurement of the resistance from either end pin of the trimpot to the center pin was measured before and after that kind of exercise of the trim pot. The result would imply that there was some change to the contact inside the pot, or it might imply that the conductivity of the trimpot's solder connections was improved mechanically by working on the pot.

 

[Toobuzz]

That is definitely a better suggestion than “turn it twenty times and see what happens”. My goal is to eventually be able to help someone on this forum. I hope I’m not creating too much noise in the meantime.

 

[Paul Birkeland]

That is a good reminder. Those little blue pots are multiturn pots, and you can turn them endlessly in one direction (they just cam out when you reach the end of the rotation, and you'll hear a tiny click when that happens).

You also have to turn them in the opposite direction that you would intuitively imagine that you'd need to.

 

[ScottAstroNut]

I just tested the trim pot. It has no trouble going from 0 ohms to 2K ohms.

 

[ScottAstroNut]

I am working here with a friend who is a retired EE who worked in the audio industry. He thinks he has narrowed it down to the 2N2222 transistor on the B side of the board. We pulled the transistor from the board to test it, and it measured open circuit on the base-emitter junction. Bad transistor?