Otrona Attache power supply woes...

Tony Duell ard at p850ug1.demon.co.uk
Sun Sep 21 12:48:54 CDT 2008


> > Sure.
> >
> > U4:
> > 1 - 0V
> > 2 - 2.40V
> > 3 - 0V
> > 4 - 0V
> > 5 - oscillating
> > 8 - 7.91V
> 
> > 9 - 4.43V
> 
> Hi Josh

At least the refernce voltage is more reasonable now :-). That old TL494 
was almost certainly defective.

>  Getting closer. Pin 9 should be almost zero. The two transistors ( Q17, =
> Q18 )
> form a latch. Normally this latch is not tripped and Q17 is on while Q18 =
> is off.
> Two sources of signals can trip the latch. One is an over voltage on the =
> 5 volt line
> and the other is an over current in the switcher transistor. The over cur=
> rent is detected
> by the optical coupler ( D1 in the switcher board ).
>  All the levels on U5 look as expected under the current conditions.
>  There are a number of possible reasons that the Q17,Q18 latch has fired.=
>  Not
> all are normal operation of this circuit.
> 1. D18 may be shorted and any voltage on the output trips it
> 2. Replacing the bad U4 has caused the voltage adjustment to be too high,
> causing the circuit to trip.
> 3. Q18 may be shorted.
> 4. Q17 may be open.
> 5. Something else

How about a genuine overcurrent-in-the-chopper problem? You seem to have 
neglected that inout to the latch. I suspect a shorted rectifier diode on 
the output side of the PSU could do that, or a shroted smoothing 
capacitor. 

Check for dead shorts between each of the outputs and ground. If you find 
one that tests shorted, desolder the assoicalted diode and capacitor and 
check those for shorts.

Do we know that this model of supply works correctly with no load? Some 
SMPSUs will go into overvoltage shutdown if they have no load. Equally a 
high ESR capacitor on the 5V output would put high-ish voltage spikes 
there (around 10V typically) which would trip that latch and shut things 
down. 


>  We know that U4 was bad so it may have been the only bad part. This
> makes #2 a likely possibility. Try adjusting the pot R39 such that the wi=
> per
> is closest to R38 ( maximum resistance ). Cycle the power switch.
>  Normally I wouldn't suggest adjusting any pots in a non-working circuit
> but this one should be harmless. Don't worry about remembering the

My rule is that I'll only tweak a preset if : 

a) Misadjustment can't cause further damage.

b) I know the procedure for settign it correctly and have the equipment 
to do so.

Here. even if the PSU overvoltages it won't do any damage because you 
don't have the rest of the computer connected to it. And the prcedurte 
for settign it is to adjust if for 5V on the 5V output connection, 
something you only need a voltmeter for.

> location on this pot as it would have needed some adjustment anyway,
> with the replacement of U4.
> Do not fiddle with any of the other adjustments!

Whateverr you do, don't disable the overcurrent trip circuit. Doing that 
is a sure way to gets bits of (expensive) chopper transistor distributed 
round the room and the magic smoke escaping from all sorts of components. 
Please don't ask how I found that out (OK, I was young and foolish!).

-tony



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