Showtec Multi-Exchanger Demux Fault

[Biskit]

An odd one which I'm hoping someone may have come across before (though I suspect not unfortunately!).

I have one of those Showtec 72 channel demux units (also sold as Botex, and other brands including Strand at one point - as far a I can tell they're all identical except the paint scheme).  Trying to use it with some old Strand Tempus dimmers, so need -10V output.  So... there seems to be an offset of about 3V - ie. when a channel is at zero on the desk, the demux outputs +3V on that channel with respect to the ground pin.  At 100% the output is about -7V.  The dimmers don't like it at all, so are completely unresponsive.  This unit is a (admittedly second hand) straight swop for an identical one which had worked fine for about 15 years, but one day just went pop, so I do know that the pinout of my connectors is correct and in principle the setup should work ok.  The dimmers are definitely fine (tested with an ancient tempus desk just to be sure).  I've had others over the years, and always found them pretty robust and reliable.  Has anyone else encountered this particular failure and found a solution, before I just bin this one?!  A quick check with multimeter shows a similar fault with it set to other negative control voltages, however on positive voltages it seems to be absolutely fine.

Any suggestions?

[DrV]

Sounds to me as though the internal +/- 12 or 15 volt supply is not balanced around 0V. Perhaps the 0V connection from the PSU has come loose? Can you check the internal supplies with a meter?

[Biskit]

28 minutes ago, DrV said:

Sounds to me as though the internal +/- 12 or 15 volt supply is not balanced around 0V. Perhaps the 0V connection from the PSU has come loose? Can you check the internal supplies with a meter?

The PSU is part of the main board unfortunately, it just has AC connection to it correct from the mains transformer, and they appear to be ok. I'm currently seeing to taking the board out of the case to get to the underside to check the soldering on the power capacitors etc.

[Biskit]

Well I can't find any obvious problem with the internal power supply, and as this thing is all surface mount components I think it's destined for scrap unfortunately.  So if anyone has a working one they want rid of, drop me a message!  Otherwise, thanks for looking all.

Edited June 14 by Biskit

[Kazeja]

Are any of the Power capacitors bulging? (1000uf 35v from a very fuzzy memory).

 

May be worth replacing as a precaution.

 

K