[sdiy] OT: Fixing RME Fireface 800 power supply

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Tue Jul 3 16:05:41 CEST 2012


Flyback switched-mode power supplies are notoriously difficult to fault 
find.  Start-up faults like this are difficult to diagnose because the 
controller IC is usually powered from a small winding on the flyback 
transformer tapping off some of the output power from the supply itself. 
You effectively have a chick and egg scenario whereby the supply needs 
to start up and provide some significant power in order to keep the 
controller IC powered and hence the supply to continue running.  A 
supply that fails to start up, or repeatedly starts and shuts down 
(tweeting or clicking noise) can often be due to a dozen or more 
possible faults.  (Sometimes not having enough load on one of the 
outputs is enough to make a functional PSU repeatedly shut down.)

It's unlikely you will be able to get a schematic for the supply as it 
is probably bought in from a third party Chinese or Taiwanese 
manufacturer.  In reality the schematic is probably quite close to those 
in UC3842 application notes, as all flyback supplies are pretty similar.

SMPSUs also pose a significant safety hazard during measurement, so 
please be careful!  I certainly wouldn't attempt to do any investigative 
work on it unless you have high-voltage probes, a variac and an 
isolation transformer.  If your power electronics skillz are sketchy 
sometimes the best option is to blindly replace all of the 
semiconductors and power resistors in one go and re-test the supply.  
(Also replace any bulging or leaking electrolytics!)  If this works it's 
a quick and cheap fix.  If it doesn't I'd just right it off.

I think your best options would be to contact the OEM and request a 
replacement supply board, or try to identify a suitable replacement 
SMPSU that will fit in the available space and provide the necessary 
voltage rails, meet the necessary safety standards, etc.

I hope this helps,

-Richie,



On 2012-07-03 14:28, Gil W. wrote:
> I got a dead FF800 which I'd like to repair. I used the PS unit from
> a working FF800 and the dead unit came alive, so obviously the PS is
> dead. Off circuit, no output voltages from the PS are seen. No burnt
> out parts, no shorts of transistors/diodes/diode bridge, looks clean.
> I noticed that in the dead PS unit, there's no "click" sound heard as
> you power the unit on... On the working unit, you will head a click
> sound a second after turning it on, but I don't see any relay in the
> PS, the click comes from what appears to be a transformer.
>
> I took parts from the working PS unit and places instead the one in
> the dead unit to see if I can find the defective part (RME doesn't 
> let
> the schematic become available...). This way, I replaced all large
> transistors, the photocoupler, the UC3842 ic, coil and transformer. I
> also ran over all solder joints just in case there's a crack. So far
> no change except when I replaced the UC3842 which made the 
> transformer
> make a high pitched noise when powering on and off (no click - and 
> the
> working PS doesn't make that noise).
>
> Before I'm going to look for a new one, any suggestions of what can
> be check further ? Thanks very much !
>
> Here's how it looks like:
>
> 
> http://www.fileden.com/files/2009/5/8/2434738/2012-07-03_11-12-35_4371111111.jpg
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