17 Mar
2005
17 Mar
'05
11:23 a.m.
Ashley Carder wrote, in part:
Based on all the info that you and others have
provided,
it looks like an option is to remove the capacitors from the unit and hook
them up directly to the variac. Or is that not the right thing to do?
Maybe I've become confused along the way - it's definitely happened
before <g> - but if you're looking at reforming electrolytic capacitors,
I believe hooking them directly to the variac is definitely _not_ what
you want.
The idea is to feed the cap a trickle of DC current to get all the happy
little electrolytes to slowly rebond themselves to the capacitor's
plating, and as you get part of the cap restored you can then apply
greater voltage so more rebonding can take place until eventually the
capacitor is reformed to its full rating. There seem to be two standard
ways of doing this, which may perhaps be creating some confusion since
it's not always obvious which is being discussed; so for clarity I'll
arbitrarily call them the bench method and the variac method.
(Note to rest of list: if I basically screw up all the details on this,
please tell me. I'm still new at it myself. <g>)
The Bench Method: Remove the capacitor, hook it up to a variable-voltage
DC bench power supply, find a way to limit the amount of current (either a
current-limiting power supply, or a big fat resistor), and slowly (e.g.,
over the course of several long hours or days) turn the voltage output up
on the power supply:
Mains --> DC Power Supply --> Current-Limiting Resistor --> Cap
^-AC ^-low-voltage DC ^-low-current DC
The Variac Method: Since cap reformation is usually an issue with old
power supplies, and the power supplies are specifically converting AC
voltages to DC, you can use the variac to supply the entire unit with
ridiculously low AC voltage and let the power supply figure it out for
itself. The variac voltage is gradually turned up over hours or days as
before, and the capacitors in the old power supply all reform from the
same source. (Doing this means one should also disconnect the power supply
from the rest of the equipment, and put some sort of
dummy load in its
place, so you don't inadvertantly fry your classic
equipment.)
Mains --> Variac --> Old Power Supply --> Caps
^-AC ^-low-voltage AC ^-converts to DC internally
The variac method is a little easier because you don't have to dig all
the caps out of the machine, and you can skimp on the math; but it's less
precise, harder to monitor and has the capacity to destroy more equipment
much faster. I've done it once, and it worked, but as I've learned more I
don't think I want to try using that method again. :)
Excuse me if I'm asking dumb questions, it's
because I'm learning as I go.
Yeah, well, as my numerous disclaimers above may indicate, _I'm_ sure as
hell no expert. :) :) :)
-O.-