3 Jun
2003
3 Jun
'03
7:02 a.m.
Anybody want to offer advice on reforming filter
caps? I'm planning to power up some stuff that
hasn't seen voltage in a long time. My plan is
to put several low-wattage light-bulbs in series
with the thing, to limit the amount of current it
will get, and hopefully allow those old caps
to reform instead of, say, exploding in my face.
Light bulbs will limit the maximum mains current in the event of a
catestrophic failure on the mains side of the PSU (this is particularly
valuable when repairing SMPSUs), but I doubt they'll have much effect
here. You really need to remoce the capacitors and connect them in series
with a fairly high value resistor across an adjustable DC power supply.
And then increase the voltage while keeping the current fairly low until
the capacitor will stand the rated working voltage.
It would be better to put the light bulbs (or
whatever kind of resistor) in series with the
caps themselves on the secondary side of the
Yes
Also, what kinds of things could be damaged by
getting less voltage than they were designed for?
Switch-mode PSUs for one. These things appear as a constant _power_ load,
and draw more current as the input voltage decreases. If you're unlucky,
this will kill the chopper transistor, etc.
I could imagine hard disks spinning too little to
lift the heads from the surface, for instance.
(This is moot, because there are no hard disks
in any of the equipment I'm looking at.) How
about CRT's? Could too little deflection make
the beam hit something it shouldn't? Anything
CRts are an interesting case. A low supply voltage will almost certainly
result in a low EHT, which will mean a less stiff electron beam. So the
deflection angle might actually increase!. However, I think the main risk
would be damage to the cathode by trying to draw too high a beam current
from it while it's too cold (the heater voltage
would be low too).
What are the devices in question? My experience suggests that most stuff
from the late 1970s and 1980s is fine -- the capacitors
are still good
(OK, they'll be the odd failure, but..). This particularly
applies to
computers and test gear which used good quality components in the first
place. Older, valved, stuff, needs to have the capacitors removed,
checked and re-formed. More modern stuff, particularly consumer-grade
stuff, may well have capacitor problems too.
Incidentally, all my HP desktop calculators, my Tekky 555 'scope, and
most of my minicomputers still all the original capacitors!
-tony