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In Reply to: Re: Reforming caps posted by Leestereo on December 28, 2003 at 08:05:06:
Thanks for the advice. The in rush current limiters sound like a good idea. As for the phono tubes, yes I do use that input, but in addition, all four of the input tube heaters are tapped off the cathode of the output tubes (cheap way to get DC for heaters), so removing one (of a series string) would not work.
Follow Ups:
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?
And about coming up slowly with a Variac -No point in leaving it overnight at one setting. If the caps are going to re-form, they will do it within just a couple of minutes at each lower setting.
For those without a Variac (or equivalent; Variac is a brand name), if the amplifier can be connected for 220/240 volts, put it on that connection for a while; it will run at half voltage. Transformers are sold for overseas travelers which will reduce 240 to 120 - that would accomplish the same thing. Half an hour at half voltage is more than enough. Better to bring it up in closer steps, but half voltage before full tilt is some improvement. Your old caps will appreciate it.
Thanks for your input. I did have an electrolytic fail on an old Lead Stereo 20 (circa 1959) and it caused tremendous current draw from the transformer. Fortunately, I caught it before the transformer suffered too much. But as you've noted, I have not really any problems in the 8 or 9 amps I've worked on from the mid to late 60's vintage. I think the advice to replace the first filter cap is a prudent one - I'll do that in this Fisher I'm working on. As for the other electrolytics, I've measured the current draw through them and it is very low in all cases (10's of microamps).
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