Quote:
Originally Posted by McMurdo
I suppose modern film capacitors at 0.1uF or so have very little leakage at all.............. That would be true, even at 1000volts from a modern megger they show zero leakage.
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I agreee with the above, and personally I think that there is much more fuss being made of this than necessary. If you are restoring vintage equipment, it will have had waxed paper caps or other types that all leaked a little from new, and the circuit impedances/resistances were such that no problems arose unless one cap developed a significant leak, i.e. was actually faulty.
Realistically, using modern caps there is no problem at all as there would barely be nanoamps of leakage and I can't think of any vintage circuit that would be that sensitive, its designers had to cope with far greater leakage from new.
Just use good quality caps, keep the voltage rating suitably higher than the working DC voltage and go higher again for dynamic circuits such as output stages that may generate voltage peaks, using 500 volt types if you need to. If you don't have an 'equal value' situation, look at the application and if it is fundamentally DC then don't worry. I'd be a bit more careful if it was off a loptx or the switching stage of a switchmode power supply, but all I'd do is make sure the values were equal and the quality appropriate. The same applies to electrolytics, just use common sense and replace a crook 450v with two 350v in series (but double the capacitance value) if you don't have the right value to hand (say 8uF 450v) but do have two 16uF 350v.
I've been servicing for nearly 50 years on radios, TVs, transmitters big and small, and industrial electronics and I've never met a series cap problem, but I've used a few when I needed the voltage rating or a cap value I didn't have in my kit at the time.
Most of the caps fitted during restorations are probably only going to see a few hundred hours of use at the most anyway. Only a small percentage will see much more than that.
Cheers
Billy