The Life Expectancy of a Mylar Capacitor

[Rectangular]

I've been trying to resurrect an old 'Rhythm Ace' drum machine. I replaced all of the electrolytics, and got it working to the point where I can hear the voices (snare, cymbals, bassdrum, etc). however, I'm not sure if its sounding quite right. the circuit board has at least 40-50 big green Mylar Capacitors,  and I'm wondering if I should consider replacing these, too.  The pcb is about 40 years old.  do mylars degrade over time,  the way electrolytics  are guaranteed do ?

when people are restoring old fuzz faces, and amplifiers, do they replace the film caps, along with the electrolytics ?  what is the life expectancy of a mylar capacitor ?

[John Lyons]

I don't think they degrade much if at all.
There may be a leaky cap but on the whole
I doubt that they degrade just on age alone.

John

[R.G.]

I don't think they degrade much if at all.
There may be a leaky cap but on the whole
I doubt that they degrade just on age alone.

Correct, John. When used within their ratings and not subject to transient overvoltages or overheating, their life expectancy is longer than yours.

[Rectangular]

fantastic news. now I just need to work through all 200 of the resistors...

[amptramp]

If you want to calculate the reliability of components, download MIL-HDBK-217F from the following source:

http://www.sqconline.com/download/MIL-HDBK-217F.pdf

This gives the failure rate in failures per million hours veersus temperature and derating as an equation showing the effect of variations in the applied stress.  Derating is the stress in operation as a percentage of the rated stress.  If you have a capacitor rated at 100 volts and it is used at 9 volts (fairly common for a stompbox), it is derated to 9% for voltage.  Active devices have separate derating values for voltage, current and power.  Section 10 of the handbook is about capacitors.

Note: this is a big book (205 pages) and is almost 14 MBytes to download, but you will get a lot of insight as to what you should use.  You can also check Change Notices 1 and 2 that amend some items.