[sdiy] judging a cap's age - a troubleshooter's conundrum

[anthony]

When I was working on my R-870 I noticed marks on the PC board for cap 
polarity orientation suggesting that the electrolytic caps had been perhaps 
recently changed.

I looked and they were Nichicons, a brand I like (though not as well as 
Rubicons and some of the Panasonics oh and I like Elnas too...) so I didn't 
look super close at first until I had to search for a specific capacitor 
that had failed (I discovered) and it was quite obviously the original unit.

Then upon closer inspection of the electrolytic caps all over the unit I 
noticed that although they were Nichicons, they were OLD Nichicons: 
10uF/16V. A newer one that size would be 50V. At least that's the usual way 
I age caps. I know scrapping old and used caps is a dodgey venture, but I 
have a big pile pile (tub really) of pre-1990 caps and then the others are 
sorted out into drawers.

So I pose this question: Is judging a cap's age by its voltage rating a good 
rule of thumb? It seems so to me at firt blush, but I always find - almost 
always in some sort of switching supply, a largish for its value/voltage 
cap, usually colored red. Do these have some other special rating that makes 
them more suitable for a critical part in a siwtching supply? Like ripple 
current or something?

I suppose this info is really only useful to myself since I'm probably the 
only one who bothers to look at every single capacitor he scrounges very 
carefully, judging its appearance and specs and name brand and comparing 
date codes. (I wish I could actually read them.) Any sign of gunk around the 
leads and it's tossed. "Spherical" caps get thrown out too - like the 
220uF/400V Daewoo I threw away the other day and wondered what it would look 
like if I took my high voltage power supply outside and slammed it (the cap) 
with reverse polarity. I've noticed more bulges from Daewoos than any other 
high voltage cap, giving me a generally poor opinion of all Daewoo caps so 
they get chucked into the BIG bin. I only actually trow away caps that I'm 
afraid might actually spontaneously pop open like the aforementioned Daewoo. 
I know that if I'd turned on the monitor that this cap had been in it would 
have blown instantly.

And I know that a lot of  you guys think that buying new caps is the only 
way to fly, but really how NEWis that "new" cap you just bought? How do you 
tell? Like I bought some 10uF/50V caps a while ago that were big and fat and 
I figured they were kind of old but I didn't care. But they were Rubicons 
with a translucent yellow label which I think is a unit with higher ripple 
current rating. And quite frankly unless you stick to really good brands, 
new caps are often junk from the day they were made. I have some high 
voltage caps from tube gear that are at least 40 years old and they still do 
their job quite well. I like it that plastic film caps are getting smaller 
and smaller. I like it that a 0.47uF poly cap can take up almost the same 
footprint as its electrolytic counterpart (but not really on a crowded 
board) these days.

If you've made it all the way to this point: welcome friends!

ciao