Having trouble with A side c4S board [resolved]

[quad]

I have put together a few bottlehead kits no big issues (great stuff!), anyway with my latest build I am having trouble with a Kaiju amplifier build at the c4S build section. Was getting close to finishing it :-)

The D/left side c4S board is working perfectly, IA is at ~ 525V and OA at ~ 296V, adjusting pot set 46 U to 175V. Sweet!

Now the A socket board: initially IA was ~ 525V but OA was at ~ 110V. Found a resistor miswired (R4 B side of board), fixed that, no change.
Checked again, found I had globbed some solder on transistors Q1/Q2 (and some pins were shorted) on A and B side of the board, icky, cleaned that up, reinserted board, now OA measures ~296V (good), but board won't adjust 32U to the right voltage. Currently that voltage only adjust slightly around ~ 95V. Tried swapping tubes between sides, didnt fix the problem. Took the A socket board out again, resoldered all the joints but no change.

Perhaps some of the transistors are damaged? Not sure.

Any suggestions on what to measure or how to proceed? Thanks for your time.

[Paul Birkeland]

Check the Kreg voltages and let us know what you see.  Specifically, the Kreg terminal with the trim pot should change in voltage as you move the trim pot, but might not if there's some underlying issue.  Knowing what that cathode voltage is and whether it changes with trim pot rotation will help the debugging process.

[quad]

Checked the A side here is the data in volts:
 IA: 519 OA: 295.5 OB (32U): 93 Kreg A: 5.35 Kreg B: 6.46
turned pot 5 turns clockwise:

                              OB (32U): 92.7 Kreg A: 4.86  Kreg B: 6.45

[Paul Birkeland]

OK, 6V of bias and 90V on the plate indicate that the tube is not drawing any current.  Can you measure the DC voltage across the 4.99K resistor on the PC board?

[quad]

Measured 2.486V across the 4.99K R3(A) resistor.

[Paul Birkeland]

OK, so that tells us that the TL431 is operating and receiving enough current to do so.  I'm leaning fairly strongly toward there being a jumper miswire on the board.  Feel free to post a few photos of both sides of the board and we may be able to spot what's out of place.

-PB 

[quad]

Hmm based on your fine input, I started to take pictures, and started to pull off the board again.

Found the 215 1/8 watt resistor from A3 to the center socket pin, was broken? off (odd) near the A3 side.

Ordered some from Mouser. Hopefully when I repair that, things will work :-)

[quad]

Finished amp. Works perfectly! Tx for the help!