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Old 18th January 2017, 10:14 AM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
Posts: 4,651
Default Re: Longevity of modern passive components

Long answer Phil ,

I agree with Bob and John about modern film resistors and plastic film caps. A lot of the long life is to do with newer materials (plastics, epoxy, plating etc) sealing the device. Care should be taken not to compromise that by bending the legs close to the body or leaving the gear in damp or humid storage etc.

Electrolytic caps though may have a shorter life which is usually defined in their specification. Whilst construction is better these days, sizes have become smaller, so we should check the spec.

We are familiar with seeing 85 or 105 degrees C on them and the spec will also say such as 1000, 2000, 5000 hours life.

The temperature shows the cap will work OK at that level and the hours will say for how long at that temperature. If they are de-rated by running cooler their life doubles for every 10 degree drop in temp. (Thus a 105 cap of 1000 hours rating would have equal life to an 85 cap of 4000 hours rating, all else being equal, but bear in mind an 85 cap may not work at 105 if you plan on using it that high!)

In the larger uF sizes, a good high temp 105 cap might be 2000 hours at 105. If our gear runs at 45 degrees then it would do that for,

2000 hours at 105
4000 at 95
8000 at 85
16000 at 75
32000 at 65
64000 at 55
128000 at 45

divided by 8760 hours = 14.6 years at 45 degrees

If it were the more common 85 cap in a large mF size for smoothing rated at a similar 2000 hours it would only last 3.6 years. Some are 5000 hours at 85 which would be 9 years etc etc.

Then the gear most likely will be off for a lot of the time so the temp will drop to 20 ambient and life will increase.

Putting this into practice I've recapped quite a lot of 90's solid state gear over the last few years. On several there were already blown or bulging caps and the one time I didn't do it a cap blew. But yes you can also see how some almost-unused gear from a dry cool home can have good caps at 40 years if it has been turned on every week or two to let the caps reform.

Valve gear runs warmer. I measured 48 inside my Kit88 so look to be changing electrolytics at anything over 10 years if used for long hours. Smoothing caps certainly, and cathode bypass caps also, have a hard life
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