Re: When should a power supply be declared Perma-Dead?

["Luke Forat" <iceman@nospamdigidude.homeip.net>]

    It seems you have a "hard short" condition possibly on the input stage
(HV- primary switching transformer side). This may be due to:
    - faulty rectifier bridge (one diode shorted)
    - very leaky filter cap (you may notice a bulge on the caps)
    - switching power transistor failure

    Without tossing away further fuses, you can diagnose a hard short
condition with a cheap tester in Ohms. Set it to the 1K scale range and
place it across the line input of the apple PSU (apple PSU disconnected from
the mains naturally!) on the VDE connector with the switch on the ON
position. In normal conditions, you should see the Ohm range start from far
right then slowly decrease (increase in resistance) as the filter caps
charge and settle at certain Ohm value - also in reverse. If you have a hard
short condition, you will not notice this variation as the resistance may be
less than a few 100 Ohms (even near 0 if a rectifier short is present). In
any case, you will need to open the PSU up and check mroe in depth if you
feel up to it - check the above components. Always be careful to check that
the filter caps *have been discharged* before touching them or you will not
like what you feel - mark my words.

    Luke

"General_Failure" <tristan.mumford@gmail.com> wrote in message
1148878241.849912.142640@i39g2000cwa.googlegroups.com">news:1148878241.849912.142640@i39g2000cwa.googlegroups.com...
> I would scrap at least 4 computer power supplies a year. Mostly from
> the instant-fuse-blow disorder. Just recently my apple //e's power
> supply has been added to that list.
>
> Initially I suspected the Z80 card, which I still believe may have
> started the problem. But now I don't know.
> Even with the power supply on its own, not plugged into anything, It
> has an instant fuse disintegration. Thankfully not as powerful as the
> lightshow a lot of x86 power supplies give.
>
> I can't see any problems with the PCB or components in the Dyna Comp
> power supply. Nothing is even remotely suspicious. It actually looks
> freshly built.
>
> Should I try tracing the fault, or just give up, steal it's low voltage
> cabling and give the apple a PC power supply?
>
> Also, after the first fuse blow, I decided to fit a panel mount fuse
> holder. Very convenient.
>
> With AT/ATX power supplies I never bother with trying to repair them.
> The apple supply is a bit of a rarity though.
>