Prophylactic replacement of electrolytic capacitors?

From: Rob Kapteyn <kapteynr_at_cboe.com>
Date: Tue Jan 15 15:14:36 2002

Tony:

You first question is answered by your second question.

The reason that I propose to "insulate the computer from the world" is to solve the first problem that you pointed out:
"applying the reforming voltage ... to random points in the logic"

If the computer itself was "insulated from the world", the chasis would be able to float so that no actual
voltage was applied to any circuit other than the capacitor being reformed.
You would need to be sure that you did not even touch the computer during the process.
 
If my theory actually works -- it could save a lot of trouble for computer restorers.

My assumption is that the two sides of the capacitor are isolated from each other.
This seems like a reasonable assumption to me.
But, just in case, I thought I should check with those of you that might have more experience rehabilitating old computers.

Perhaps I just need to find something less precious than my Altair to try it out on ....

Why don't I want to unsolder the capacitors ?
My experience is that I can do a lot of damage unsoldering things.
My soldering skills are not as good as they were when I put the Altair together 25 years ago.
My fingers and eyes do not seem to work as well as they did back then either.
Perhaps I just need to get myself one of those new high-tech temperature controlled soldering irons.



Tony:
Reading your later messages, I notice that you recommended using a VOM meter in the Ohms mode to test the caps.
I have always avoided the Ohms mode of a VOM when working with TTL circuits.
The VOM applies a voltage to its test leads when it is in Ohms mode.
As I understand it, delicate circuits an actually be damaged by these meters.

I have not worked with hardware in years though. Maybe the modern VOM meters are safe.

-Rob

-----Original Message-----
From: Tony Duell [SMTP:ard_at_p850ug1.demon.co.uk]
Sent: Monday, January 14, 2002 6:09 PM
To: classiccmp_at_classiccmp.org
Subject: Re: Prophylactic replacement of electrolytic capacitors?

>
> 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.

OK, yes, reforming can work...

>
> I want to reform the original capacitors in my Altair "in place" --
> i.e., without unsoldering them.

Why do you not want to desolder them? It'll do a lot less damage than
applying the reforming voltage (a little above the working voltage of the
cap) to random points in the logic.

For example, there are likely to be some 16V electrolytics between the
+5V line and ground. Do you seriously want to try running TTL at 18V?
Desolder the darn thigns!

>
> I have the following setup in mind:
>
> 1. Insulate the computer from the world -- unplug it and put it on a
> rubber table.

What's the point in doing this?

> 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)

I would include a current limiting resistor (a few k) in series with the
cap. And please desolder it!

-tony
Received on Tue Jan 15 2002 - 15:14:36 GMT

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