On Jan 14, 15:52, Rob Kapteyn wrote:
> I have not yet tried this yet --
> In theory, electrolytics can be re-formed.
> There is a very thin aluminum oxide layer that disappears after a long
time.
> Running current through the cap will rebuild the insulating layer until
current will no longer go through it.
>
> I want to reform the original capacitors in my Altair "in place" -- i.e.,
without unsoldering them.
>
> I have the following setup in mind:
>
> 1. Insulate the computer from the world -- unplug it and put it on a
rubber table.
> 2. Use ultra mini test clips to connect to both leads of a capacitor.
> 3. Connect the test leads to one of those lab power supplies that have
adjustable DC voltage and a milli-ammeter.
> 4. Connect + on the lab power supply to + on the capacitor (very
important).
> 5. Start at zero voltage and increase slowly while looking at the
ammeter.
> 6. Stop increasing the voltage when the ammeter reads anything more than
a few milliamps.
> (If it draws too much current, the capacitor will heat up and may
blow up)
> 7. Wait for the current on the ammeter to drop to zero.
> 8. Continue to increase the voltage, and wait for the ammeter to drop
until you reach the rated voltage limit of the capacitor.
> 9. Repeat for every electrolytic cap in the computer.
>
> Has anyone done this ?
> Will it work ?
I've not done that in-circuit, but it might work. There are some gotchas.
If you're talking about power smoothing caps, any voltage you put on the
cap will also be powering the rest of the circuit. Disconnect any
transformer. You don't want to feed them DC. Also, the current drawn by
the rest of the circuit will make it impossible to gauge the leakage
current in the capacitor.
If you're talking about any other capacitor, putting a voltage on it may
provide power to some signal line connected to an unpowered device, which
the device may not like. In particular, TTL doesn't like power on some
signal inputs when there is a ground connection but no Vcc.
In either case, the voltage rating on the capacitor is likely higher than
the maximum for the logic ICs, and TTL in particular does not like
excessive voltage (NMOS is more tolerant). Don't exceed the lowest
maximum-voltage rating of any device on the board.
Of course, you're assuming that the capacitors need reformed. They might
not (but it's good practice to assume they might). If you can't remove
them without risking damage to the PCB, I would try feeding power into the
board through a current-limited supply at a low voltage and gradually
winding it up to the normal value over a period of several tens of minutes.
If they've completely dried out, they're dead anyway. Stop if the current
shoots up, or seems too high -- you probably have a short. Is an Altair
board like a typical S100 board, fed from an 8VDC unregulated supply, and
with on-board regulator? If so, just use a bench supply as above.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Mon Jan 14 2002 - 16:34:50 GMT