DIYbanter - View Single Post - My sub-woofer hums...

Don Pearce

On 8 Mar 2007 02:00:42 -0800, wrote:

On 7 Mar, 23:39, Clive Mitchell wrote:
In message .com,
writes

Where do you get the 7 year rule from, and what exactly is it? I've
never come across it, and if it existed I really ought to know.

I've still got 1930s lytics in service, and IME lytics are a long way
down the list of most likely failures in old equipment. The ida that
lytics have short lives seems to be an offshoot of the spate of bad
caps on mobos in the 90s.

The failure of electrolytics over time is due to the drying out of the
electrolyte. The capacitor will generally read the correct value on a
capacitance meter but will develop a high ESR (Equivalent series
Resistance) which causes ripple on a supposedly smooth DC supply. (And
timing problems in old circuitry.)

It seems no-one can backup this 7 yr rule so far. Its far removed from
my experience with lytics, and lord knows I've worked with enough of
them. IME around 4 out of 5 1930s ones have still worked, so thats a
mttf of ballpark 70 x 4 = 280 yrs.

I'm surprised by Andy's experience, I've also fixed lots of valve kit,
and lytic failures have happpened but been fairly rare.

If there had been any sort of 7 yr rule, a lot of the new kit I've
worked on would never have got out the door, as 7 yr life would have
been unacceptably short.

NT

Bear in mind that the lifetime of electrolytics will follow some kind
of normal distribution curve, and the quoted seven years will
represent the very start of the lower end of the tail - probably 0.1%
of the entire distribution, maybe even less. So the average life is
probably more like twenty or thirty years, with the upper tail
extending possibly to a hundred or so.

d

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Pearce Consulting
http://www.pearce.uk.com