Failed Voltage Checks and some other weirdness [resolved]

[Paul Birkeland]

What are the OB voltages on the outer C4S boards under these conditions?

[petewebb]

I ordered some spare LM431, 2N2222A, PN2907 and MJE5731A that come in this afternoon. Should I try replacing any of those that would make sense in the path of the issue?

[petewebb]

215V for both.

What are the OB voltages on the outer C4S boards under these conditions?

[Paul Birkeland]

If you have 215V on an outer board OB and 1.5V on the center board OA/OB terminal feeding that side, that's a bit of a contradiction.   Those conditions would only really happen with about 100mA of plate current flowing, and that would drag other voltages down and burn up the 270 ohm 5W resistor in the power supply in the process. 

It may be best to just proceed with a repair on this one.  There's just too much that isn't making sense to know where to go from here.  Pulling out and replacing all the transistors is just a guess that will be a lot of work to complete and may cause damage to the PC boards.  A shorted transistor can be measured with the continuity beep on your meter and testing each pair of legs on each transistor.

[petewebb]

I think I may have measured wrong on that last step you mentioned. 

Buttttt.....I replaced the LM431s and 2N2222As on the center board, B-side first... no effect, same issue. However, when I did the A-side all C4S LEDs lit up! I was able to Bias the tubes at 145V all the other power checks succeeded with the Kreg's on both sides reading 11.2V.  All the switches work and they should, and I have music playing through the XLR and 1/4" now. Sounds Wonderful.

Thanks for your help PB!