[sdiy] OT: Fixing RME Fireface 800 power supply

[Barry Klein]

I test power adapters at work.  I see bad ones too once in a while.  When
one part goes it often takes out several others.
Was the main switching transistor shorted?  If so, there is usually a
resistor in series with it and the primary that fails too.  Also check the
optoisolator.
If the cause was high voltage transient that may be all.  But if old age
there may be faulty caps somewhere.
I couldn't see the photo site - wanted me to register.  What is the supply
supposed to output as far as voltages?  Maybe just buy a wallwart and kludge
it.
At least one would provide some possible replacement parts.

Barry


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Harry Bissell
Sent: Tuesday, July 03, 2012 07:39
To: rburnett at richieburnett.co.uk
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] OT: Fixing RME Fireface 800 power supply

Amen.  I would not try to fix a SMPS unless I was an astronaut on Apollo 13
and not likely to ever see home again unless I was successful. In fact I'd
give serious consideration to just cracking the hatch open for a breath of
fresh vacuum...

If you can surmise the voltages needed, a replacement supply might be a
better choice.

In most cases, SMPS power supply failures involve multiple componant
faults... they work well when working... but failures usually run through
many components. A former colleague of mine used to call them "like jogging
with a loaded .45 automatic pointed at your head"  (fine, unless you stumble
:^)

Try and look at websites of such suppliers as "MeanWell" who make SMPS
clones of just about every OEM supply. I'd wager they have one that is very
similar to your photo...

H^) harry


----- Original Message -----
From: rburnett at richieburnett.co.uk
To: synth-diy at dropmix.xs4all.nl
Sent: Tue, 03 Jul 2012 10:05:41 -0400 (EDT)
Subject: Re: [sdiy] OT: Fixing RME Fireface 800 power supply

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_4371
> 111111.jpg _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

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Harry Bissell & Nora Abdullah 4eva
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