23 Feb
2009
23 Feb
'09
12:22 p.m.
*Rob, possibly you made the 110/240 switch wrong,
but also possible is
that the PSU just wasn't ready to be turned on like that.
Electrolytic
Capacitors (of which there are many in that PSU) tend towards
non-functionality the longer they sit unused. Without those caps
working right, the PSU will do pops, smokes, and other alarming things.
Then good luck fixing it.
Do you mean that it will be difficult to fix, even if I can find someone who
really knows what they are doing?
What I do with an old PSU like that is test each cap prior to it ever
being powered up.. both for capacitance and ESR. Usually, some or all
of the caps need reforming or even replacement. Only after the caps
are back to health, do I then give the PSU power... at first with the
smallest load I can get away with.
I only have a basic multimeter so I don't know if I would have had the
necessary equipment to do this, do you have any advice on the minimum
equipment needed? Now that there has been some damage is it sensible to replace the blown capacitors and any other ones that
don't measure well?
I would recomend against using a Variac on an SMPSU unless yoy really
know what tyou are doing. An SMPSU is a fairly good aproximation to a
contant _power_ load, in other word the input current increases as the
input voltage decreases. Some SMPSUs can actually be damaged by running
them at too low an input voltage, while plenty of other types do strange
things.
If it's a switcher PSU (as that one is), I'll bring it up quickly to
about 90V using a variac... then in 5V increments every 1/2 hr after
that to 130V, then back down to 120V. This in the USA.
I looked up variacs but there seem to be an awful lot of different ones,
again any recommendation as to the minimum I would need? By the way, I am aware that PSUs can be very dangerous
to meddle with when
you have limited knowledge. How long should I leave the PSU between any
Indeed. Mains, and the rectified mains you find in SMPSUs, is probably
the most danagerous voltage you're going to come across.
tests to allow the capacitors to discharge? The label
on the PSU says to
leave it 5 minutes, I suspect it should be longer.
It's impossible to say. A good electrolytic capacitor with nothing
connected to it will hold the charge for quite a time. Of course when the
supply is working, the chopper circuit tends to discharge the mains
smoothing capacitors, so the charge won't remain for very long. And many
supplies have bleeder resistors (resistors connected in parallel with the
mains smoothing capacitors) to discharge them. But of course those could
fail too.
What I do is open up the supply carefully and then measure the voltage
across the mains smoothing capacitors (sometiems, as in the case of the
HP9845 and DEC PDP11/44 supplies there are clear testpoints to do this).
If it's more htan a few volts, I carefully connect a suitable resistor (a
few 10's of kilohms) across the capacitors to discharge them.
-tony