[sdiy] OT: Fixing RME Fireface 800 power supply

Gil W. gil_we at yahoo.com
Wed Jul 4 18:48:27 CEST 2012

Hi all, thanks very much for your replies. As I said I replaced all semcondictors, the transformer, coil and ICs - no help, so I decided to just get a new unit. Thanks for your help !

--- On Tue, 7/3/12, Steve Ridley <spr at spridley.freeserve.co.uk> wrote:

> From: Steve Ridley <spr at spridley.freeserve.co.uk>
> Subject: RE: [sdiy] OT: Fixing RME Fireface 800 power supply
> To: "Gil W." <gil_we at yahoo.com>
> Date: Tuesday, July 3, 2012, 11:54 PM
> Gil
> I've had reasonable success fixing supplies like this - some
> you win, some you lose, but there are some common faults.
> Warning - the large high voltage cap can hold it's voltage
> for a long time - I've had a nasty surprise off a SMPSU
> which was switched off the previous day! Check and discharge
> if necessary. I've done a lot of SMPSUs professionally (as
> part of larger, expensive equipment) - I rarely measure
> anything with the supply powered - I try to discharge
> everything thoroughly then measure components with the power
> off. Much safer. If I have to measure anything with the
> power on, I attach the test gear before I switch on.
> First thing - was the fuse blown? A blown fuse usually means
> power transistor failure and a load of blown and damaged
> parts, but if the fuse is still OK, you've got a chance. If
> the's smoke or a smell, I usually go no further.
> The UC3842 etc usually get power from a high value resistor
> from the rectified mains. Look at the data sheet, find the
> supply pin then trace and locate the supply resistor. They
> often go bad or high resistance. Also, the two small caps
> nearby sometimes dry out, so the local supply goes high
> impedance. Swap with similar.
> Does the output voltage give a brief kick on power on?
> There's an optocoupler in line with the transformer giving
> feedback. If this goes bad, the supply may go over voltage
> and shut itself down.
> I'm not sure what that white rectangular thing is. How many
> pins does it have? It could be a 110v/230v switchover relay,
> which would explain the click. If the coil drive circuit is
> bad or the contacts have welded closed, it might be stuck on
> the wrong voltage. Worth a look.
> Work safely...
> Steve

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