"Mike" wrote in message
...
well 0.47 caps, 1uf caps, and 2.2 uf caps read about 5. at 50khz, new and
old. so im assuming thats how it reads.


all 107 power supplys I have here, all 3 100uf caps are reading near 1
ohm, some a little over.


my new 100uf reads right at 0. (analog meter).


I'm not really following that. What relevance does the readings of those
other values have, in respect of the value you were asking about ? Your
reading of " 0. " on an " analogue meter " is also confusing me. Zero is a
short circuit. What sort of analogue meter are we talking here - ESR or ohms
? I have just checked a brand new out-of-the-drawer bog-standard 100uF @
25v, and on my Bob Parker meter, and it goes 0.3 ohms, which is in agreement
with the front panel legend.

As I said before, 1 ohm plus for this value, is a little high. If I were
repairing a SMPS, and found a cap of that value reading thus, I would feel
inclined to replace it, as being " on the way out ", not because I would be
particularly confident of it causing a problem. Although electrolytic caps
are probably the single biggest source of trouble in switchers, there is a
designed-in tolerance in most reputable designs, such that caps have to be
significantly out of spec, before they start causing real problems. ( 5 or 6
ohms possibly for that value cap )

As far as your other belief ( further down the thread ) that " worn out "
semiconductors - MOSFETs - are to blame, I think that the chances of that
lie somewhere between slim and zero. Whilst I have had bipolar transistors
that have gone noisy, and some that have gone low gain, and more than a few
that have gone leaky over the years, by far the most common failure mode for
semiconductors, and particularly power MOSFETs, is catastrophic junction
failure e.g. short circuit. I cannot think of a failure mode that might be
considered as " worn out ".

Depending on the topology employed in these supplies - self oscillating
primary, IC oscillator and control primary etc - if caps are not to blame
for the sluggish startup, then I would be inclined to start looking for
resistors that have gone high. Possibly a startup resistor on the primary
side, or a current sense resistor on the secondary side. Good luck with
them, and do be careful to observe the highly necessary safety precautions
for working on line powered switchers.

Arfa