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Old 06-08-12, 12:35 PM
martin clark martin clark is offline
pinko bodger
Join Date: Jul 2003
Posts: 11,259
DIY - re-forming NOS capacitors

Inspired and abbreviated from this DIY thread

I have some unused Sprague 3'700uF/40v electrolytics with an 8/97 date stamp - are they still good for me to use in a PSU?

Could well be! Try re-forming a few - you have nothing to lose. You'll need a raw DC supply of about the rated voltage, or at least rather higher than the voltage you intend to run them at, a large-value resistor - 10-100K is about right - and a voltmeter.

snowman_al's notes are:
Old electrolytic caps can develop very high leakage current if not used for a long time. This will cause the cap to heat up and may even explode when first used whilst the cap tries to 'reform'. It is made worse if there is high ripple from the power supply or it is placed straight after the rectifier.

Best practice is to 'reform' any / old caps before using them. You only need a couple of resistors, a volt meter and a DC PSU able to supply the rated voltage (NO More) of the cap you are 'reforming'. Use a 10K ohm 2watt resistor first. Wire the negative lead of the PSU to the cap negative. Put the resistor in the positive lead from the PSU to the positive of the cap. Power on and measure the voltage across the resistor. It will be high at first, but should start to drop very soon if the cap is in good order. After 5 minutes the voltage should be stable and low. If not leave it for 20 - 30 minutes and look again. If the voltage is less it is reforming, if it stays high the cap is dud.
You can try a 2K resistor next, same as above.
Once you are happy the cap is reformed remove it from the PSU and leave it charged for 10 minutes and then measure the voltage directly across the capacitor. If its good it will measure close to the PSU voltage. If it has discharged it is still 'leaky'. Clearly the longer it stays charged the better it is. If it is good remember to discharge it before use / storage. The 10k across the cap is just the job...

I have often reformed caps from the 1940s and 50s repairing old radio sets and seldom have a total failure. So 1990s is new!!
I would say if the cap holds its charge for 2 hours it will be fine to use.

Plenty on the web about reforming old caps too.
Indeed, here are two detailed links provided by another member:
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