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In Reply to: RE: FCUPS versions, other info posted by Thorsten on July 15, 2013 at 14:03:20
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.
Follow Ups:
Hi,
Do you want to use the negative voltage?
If so, do you have a lower voltage tap available?
Otherwise you need to de-solder two diodes, I need to look if I have the details somewhere.
E-Mail me directly, I should have the schematic for the older version.
Ciao T
Sometimes I'd like to be the water
sometimes shallow, sometimes wild.
Born high in the mountains,
even the seas would be mine.
(Translated from the song "Aus der ferne" by City)
If its just the negative bias supply that's buggered up I can make do. My transformer has a 50V tap.
The C- output appears to be derived from a neg. FW rectifier (2 diodes in series), then a zener/emitter-follower, with those 2 10uF 630V film caps between each stage. So, if I can just desolder those rectifier diodes I should be okay, correct?
Hopefully, I didn't fry any other bits. It did appear to be giving stable B+ supply.
Hi,
The Bias come from a voltage doubler, that was needed for the use of a single, non centre tapped winding.
So there are some other parts that need removing. Please e-mail me.
Ciao T
Sometimes I'd like to be the water
sometimes shallow, sometimes wild.
Born high in the mountains,
even the seas would be mine.
(Translated from the song "Aus der ferne" by City)
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