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Original Message

RE: FCUPS versions, other info

Posted by zarniwoop on July 15, 2013 at 19:10:19:
Thanks Thorsten,

"The 2008 versions had a "bug" in the negative supply. I cannot remember which particular connection triggered it, but I seem to remember it was with centre tapped transformers."

I think I discovered this "bug", and its quite a serious one! I am indeed using a center tapped transformer, wiring the center tap to the FCUPS CT terminal. I slowly brought up voltage on a variac (15K load on B+ output) and didn't immediately notice problems until i got up to 300VDC output, at which point I heard arcing sounds and shut it off. I, luckily, probed the film caps and found one w/ 850VDC across it. Probe slipped and I got a nice bang as the cap discharged. I'm glad I was cautious and didn't pick up the board with my hand.

Obviously, these should not be at this voltage! Since no schematic is available, I tried tracing some of the circuit board components and fitting it to the general layout as shown in the manual... Seems like it was one of the negative bias filter caps. I've since read your comment, and noticed there is the addition of a bias voltage (virtual)center tap connection on the newer boards. So I'm guessing this bug was addressed in these changes. I'm a little pissed off that nobody thought to mention anywhere that this "bug" exists on the version 1 boards. Lots of people use center tapped transformers. Is there a FIX for this that would be possible while keeping the bugged board? My plan right now is to just remove the connections to the negative bias rectifier if I can manage that with the pcb, as my transformer has a bias tap anyway.

Anyone with these older version boards, please take note and be cautious.