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In Reply to: Reforming caps posted by SteveBrown on December 27, 2003 at 14:25:44:
Steve,
I always replace all small electrolitics (under chassis) in vintage amps, but not necessarily the big filter caps over the chassis.
About Fisher, in the directions that come with his Fisher 400 Resto-Pack, the FisherDoc (Al Pugliese) writes:
"Generaly speaking, and contrary to popular belief, power supply electrolytics should NOT be changed unless they are leaking. They are remarkably long lived, and the more they are used, the better they behave. If the originals are in fairly good condition, the unit should be run with the existing filter caps in place. Often a flter cap will exhibit a small degree of leakage when the set is powered up. This "ripple" will often disappear as the set is used. Sets that have been idle for a year or more need to be run for a while which allow the filter caps to "re-form". If after a few months of steady use the ripple is still there, and if it's AUDIBLE, then the failing filter cap section should probably be by-passed by wiring in individual caps under the chassis, unless a suitable replacement is obtained. Generally speaking, ripple should be less than approx. 1cm with the scope set to 2v/cm.".
I hope this help
Raul
Follow Ups:
I actually lifted one of the cap leads yesterday to measure the current through it.. came out to about 40 uA. I didn't think that was too bad. One cap that does kind of bother me are the two used for the voltage doubler - and that's primarily because they seem to run so close to their max voltage. Each of the two caps are rated for 250v, but on power up, before the output tubes start to draw current, the B+ hit about 495v. I might replace those two with 350v units.
To limit the potential damage at start-up you could install an in-rush current limiter, the voltage won't change much, but the reduced current will help. They use such in some MacIntosh amps. I installed a Keystone CL-80 in my Fisher 500C. If you don't use the phono input, pulling the 12AX7s will allow the power tranny and caps to run much cooler; I did this on my 500C without any ill effects, AFAIK.
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.
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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