rebuilding old electro caps

I have some old electro caps in here I would like to try rebuilding before using. Any schems for a nice way of making a 1v-400v psu for this purpose? Or should I try a HV PSU with multiple voltage dividers? Also, what´s the timing should I use for increasing the voltage?

How do you do it, if you do?

Also, I´ve heard that old paper in oil caps should also be rebuilt. I don´t think it sounds right to me. I think those are just plain caps with aluminum plates, paper dielectric, and some poisonous oil stuff for sealing. I just want to confirm that those doesn´t need rebuilding, like any other film caps.

As always, thanks!

A Variac tends to be the standard way of doing this. They are pretty common and not to expensive on ebay, and they can be pretty handy to have around at times. As for time, not sure what the minimum is, I have come across people who spend anywhere from 4 or so hours to whole days just slowly turning up the Variac a little at a time. Personaly I just buy new caps. My time may be free, and my funds rather limited, but I would still rather have new caps that I know will be good for many years of use.

In my experiance paper in oil caps are pretty stable over the years, the few that I have pulled from scrapped gear and tested seemed to be within tolerances. Can not say much about their construction but they do have a sound of their own, I think the paper is actualy saturated in the oil, hence the paper in oil name, but i really have no clue. They have found their way into more then one of my projects. Althogh I would not pay the cost of new ones, they are not that special sounding.

adam

Rafa,

This process called "reforming". Here are first few hits from Google search on electrolytic capacitors reforming:

http://www.vcomp.co.uk/tech_tips/reform_caps/reform_caps.htm

http://www.vmars.org.uk/capacitor_reforming.htm

http://www.nmr.mgh.harvard.edu/~reese/electrolytics/

[quote author="rafafredd"]Also, I´ve heard that old paper in oil caps should also be rebuilt. I don´t think it sounds right to me.[/quote]
It doesn't sound right to me either. Oil caps either work, or they turn into resistors. The old Jensen caps (that you find in old radios etc. here) are normally fine. The Suzuki oil caps are terrible - I had to replace all the caps that were supposed to block DC in my mothers Sansui amp a couple of years ago.

Best regards,

Mikkel C. Simonsen

I don't think reforming always works, but it does work sometimes. If the capacitor is not physically damaged, or the oil (in paper and oil) dried up, you can reform them. I am not sure if they will always be 100% again, but you can get a lot of life out of good older caps this way. It is not too complex of a process but it requires a lot of time. So if time is money it isn't worth it. But if, like for me, time is just time, then it is.

Thanks for all the inputs. It looks like time is just time for me also, and I can hook up a cap on a PSU and let it there for some hours and then change the voltage and let it cook some more, and once more. Not a real problem... Maybe I could make a multiple capacitor reformer... So, that I can plug eight caps in there each time.

Now I´m thinking about the best way to make a variable 4v-400v PSU for this purpose. I have seen a pot driving the base of a transistor type of PSU by PRR. It looks like it would do.

About variacs, I have never messed with variacs, so I will have to do some reading before I consider it...

This may give you and idea on power supply. http://members.aol.com/sbench101/#Power

Gerald Weber (Kendrick Amps) has a big burn in rack for caps. I will re-read the article to see how he does it.
I thought I remembered something about a 1 meg resistor in series with the cap, leakage is high at first, so the cap does not see the actual voltage on the resistor til the electrolyte forms and the leakage drops. So the voltage gradually ramps up by itself. Let me double check this first.
cj

Sounds interesting, CJ. Let us know when you have nore info!
:thumb:

I have some ELNA-caps which seem from about ten years ago.
Any need to do the reforming-dance on caps of this age already ?
They're 'new', so unused so far.

Thanks,

Peter

BTW, if anyone could use some ELNA-caps of big values for cheap, have a look here: http://www.easymeasure.nl/elcos.html

I heard that if lytics sit on the shelf at part houses for more than six months, they heat them up in a low temp oven.
Probably a bunch of B.S.

Thanks CJ.
Even if it's B.S. then al least that 6 months already suggests another ballpark figure before 'something starts' than that 10 years of me.

Would be a sad thought to use in essence good specified but old caps without reforming
vs using 'simple & cheap but fresh recent ones'.
Could imagine the latter would perform better - (depending on actual use in a circuit that is).

Regards, I'll be reforming,

Peter

'Bout time you reformed Peter...

Gerald Weber (Kendrick Amps) has a big burn in rack for caps. I will re-read the article to see how he does it.
I thought I remembered something about a 1 meg resistor in series with the cap, leakage is high at first, so the cap does not see the actual voltage on the resistor til the electrolyte forms and the leakage drops. So the voltage gradually ramps up by itself. Let me double check this first.

Click to expand...

The way I reform my caps is similar to this. I decide how much current I want to draw through the resistor/cap ( ususally use 20mA at 300v ). This gives me a 15k 4W resistor. Put it in series with the cap, & supply 300v to the res/cap string.

I keep a multimeter across the cap & just before it gets to the rated voltage, I switch off. the cap should be reformed. I usually put a variac in front of the PSU so that the max voltage does not exceed the cap voltage.

If a cap never gets near to the rated voltage, toss it. This process can take up to 5 hours, or be as short as a few minutes if the cap is good & fairly new.

Peter

Not a recommended practice, but it is interesting to sacrifice caps out of a candidate batch (or come close to it) by seeing what the resistor-limited "zener" voltage of the cap is. Some good ones can sit indefinitely at 40% or more over the rated voltage.

Wear safety glasses....