switching power supply Q

[Curtis H. Wilbar Jr.]

Lee Davison wrote:
>
> > Anyone got any ideas as to the oily film ?
>
> Most likely it is from a capacitor.
>
> > If this is acting up, for testing purposes it would seem to be safe
> > to remove it to see if the voltage changes (jumps back to around 12V) ?
>
> No, that would remove all regulation from the 12V rail.
If wired per the reference in the PDF... for testing with no devices 
attached
(will prob need a dummy load for 5V I'd imagine) to see if the part is what
is causing the voltage to drop to 8.5V.  Not to actually use.

This is also a small part... wouldn't it be dissipating a large amount 
of current
clamping the rail from 12+V to 8.5V ?  How would it survive ?  There are no
heatsinks/etc ... I'd imagine this is to eliminate overvoltage situtations ?

>
> You can test this part out of circuit. Connect REF to K, pins 1 and 3 on
> the TO92 part, and these two to +5V through a 680 or 1K ohm resistor.
> Connect A, pin 2,to ground. With the 5V on you should read 2.495V +/-2%
> between K and A.
>
> This part can be replaced by a TL431, LM431 or equivalent.
Good to know
>
> > So, suggestions, ideas on the film, and best way to go about
> > troubleshooting the 12V rail are welcome :-)
>
> An ESR meter to test caps is pretty much essential. Some obviously
> fail but others can look fine but still be high R. Electrolytics
> are by far the biggest single cause of switch mode supply failure.
>

I have an ESR meter.

Have you ever seen a cap 'leak' oil ?  This looks like a film of light 
machine oil.
And the caps have none on them... the part with the most on it is the 
C1093...
but I've never seen a semiconductor device leak 'oil' ?  The only other
component with a lot of this 'oil' on it is the 12V adj pot.

Doubt this 'oil' got there by human intervention.... as it is on the 
inside end of the
power supply (you need to take apart the computer and almost take the power
supply out to really get to this end of the supply.

Guess I'll have to plan on pulling the caps to see if they could be the 
source of
the leak.... but if so... I'd imagine they should have a film on them 
too, no ?

And of course they have that white 'glue' between them (more like a hard 
caulk than
glue I guess ?).... what is that for anyway ?

Thanks for the reply.... I'm hopefully it won't be too hard a fix.  
Guess the toughest
will be if the 'oil' contaminated the 12V adj pot... in which case 
hopefully it won't be
too hard to get the same pot to replace it with.

-- Curt


> Lee.
>