To replca or not to replace

[Thomas Giz]

I have on my bench completely dissembled, over ten years old, amp with all Nippon Chemicon caps. Seem to be good brand but I thought maybe it is a time to replace all electrolytics with same brand and series, fresh ones since amp has been used pretty often. Power supply takes 2200uf/80v and 10000uf/80v caps and there are bunch of 33uf/100v all over the amp. First 2200uf cap unsoldered to take capacitance measurement - 2160uf, still good. New ones measure only around 1950uf - odd thing comparing to used old ones. BTW old ones are general use 85 degrees SMH series, cheapest regular caps.
My question is whether it is really time to replace those caps and why old ones measure much better than new ones ?

[Mooly]

Measuring the value of an electroylitic doesn't tell the whole story...

Caps commonly fail or deteriorate by showing an increase in E.S.R. or equivalent series resistance.

An analogy is to think of a 9 volt PP3 battery. Even when flat it still reads nearly the same as a new one off load. Its similar with old caps. The value in uF is often unchanged, but its as though there is a non linear "resistance" in series with it too.

At 10 years old I suspect they will all be just fine tbh.
Heat is a killer of caps so if any are near a hot transistor or resistor then maybe look to replace those.

[Thomas Giz]

... same thinking here but it never hurts to ask... thanks

[sofaspud]

What Mooly said... And for power supply electrolytics the tolerance may be no better than 20% eg 1760uF to 2640uF for the 2200uF caps. Not normally an issue because a more precise value isn't required as with timing & filter apps.