Otrona Attache power supply woes...

ard at p850ug1.demon.co.uk ard at p850ug1.demon.co.uk
Tue Sep 16 04:18:53 CDT 2008

derschjo at msu.edu wrote:
> After looking over the schematics pointed out to me 
> (http://oldcomputers.net/Attache_Schematics.pdf -- see pg. 8) I metered 
> out the voltages at the op-amp and I'm getting 8V (which, if I'm reading 
> the schematics correctly, is not unexpected).  I'm not sure where else 
> on the board to check (I'm still learning this stuff, but I'm getting 
> better, honest!).
> I get nothing at all at the actual power supply outputs.

This is a relatively unconventional and repairer-friendly SMPSU! The chopper
control circuitry is all on the output side of the isoaltion barrier and
the startup supply comes from a little mains transformer and linear regulator.

Start by checking for the +8V supply from U1 on the PSU mainboard. This, I 
suspect is present, it's the 8V you've seen on the op-amp.

Now check for the 5V reference voltage from pin 14 of U4 (the TL494).

Now, I think that TL494 should be oscillating, at least at startup. See if
you get anything on pin 8. A TL494 datasheet would be a useful thing to have!

What voltage do you see on pin 3 (Comp) of this chip? That's the output of the 
error amplifier IIRC. It appears there's an overcurrent shutdown circuit 
connected there, maybe that's shutting it down.

Main voltage regulation is the comparison of -ER (half the reference voltage)
and a divided down version of the 5V output on +ER. 

What about pin 9? If that's high (8V), it'll shut the supply down. There's a 
chopper overcurrent circuit and a +5V overvoltage circuit connected here I 

Whatever you do, don't disable any of the overcurrent or overvoltage circuits
If there really is a problem, doing that can prove spectacular and 


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