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Re: Reforming caps

A few thoughts on old electrolytics:

The quality and longevity of electrolytics, in general improved vastly in the mid '60s. Many of that age or less are probably still OK, especially if they have been used occasionally.

The drying out is accelerated by internal heat. That comes not only from external ambient heat (such as being close to hot-running tubes) but also from the amount of ripple current a filter cap has to endure. For example, a first filter cap right across the rectifier cathodes has a very hard life - the peaks of ripple current are great. If it is a "choke input" filter, with a choke inductor preceding the first electrolytic, life is much easier on the first cap. But few circuits do that because the B+ voltage is increased by using a capacitive-input filter. The filters after the first have very little ripple current to deal with, consequently they have long, peaceful lives. But that first one should always be suspect. If in doubt, I would replace that one, just to get the newer cap technology if nothing else, and leave the others alone unless there was good reason
to replace them.

I have used the Sprague capacitance tester mentioned earlier for years. That tests the cap under voltage, for dissipation as well as capacitance. Dryout is indicated by increased dissipation. There are many new small capacitance/inductance testers; does anyone know of one which also measures dissipation?



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