"ScottW" wrote in message
...
"Trevor Wilson" wrote in message
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**Actually, I suspect you do. Electrlytic capacitors begin wearing out,
as soon as the equipment is turned on. Many caps destined for domestic
products have a very short (published) life span. Certainly less than
5,000 hours.
Couple of points.
1) These short lives are at very high temps. 80c.
No domestic product has temps that high...they'd have to
provide thermal shield to meet U/L safety requirements.
So the cap life doubles with every 10c decrease in temp.
Get down to a quite reasonable 40c and you're looking
at 40,000 hours...
Agreed.
2) The primary wearout failure is loss electrolyte causing a
a drop in capacitance with failure usually spec'd at 80% of
rated value. Most electrolytics are applied as filters which
aren't terribly value sensitive.
Electrolytics in a crossovers value sensitive applications would
be very poor design.
Agreed.
If I was presented with a 20 year old piece of equipment, with a large
number of caps, some of which had failed, I would advise replacement of
most/all electros as a matter of course.The reason for this is that I do
not like to see my clients return with the same equipment, for similar
faults, within a year or two of service. I'd much prefer that my repair
last AT LEAST as long as the equipment had lasted to date. Preferably
longer. As a rule, I always use superior quality replacement caps, where
possible.
Agreed up to the point where a ludicrous number of parts are involved, like
150 caps in a simple stereo active crossover.
The still-excessive failure rate may help explain Linn's continual
redesign of its active crossovers from discrete components to circuit
boards with plug-in modules to plug-in modules placed directly in the
power amp.
Failure of 150 parts in an electronic crossover, even after 20 years of
24/7 use is characteristic of junk-as-built.
**Nope. It is quite common. Ten years is about what we expect from a
permanent power supply in an old VCR.
Irrelevant because VCRs aren't comparable to electronic crossovers. Every
VCR I've scrapped had worn-out heads.
Cap failure is a normal event. Particularly for products which operate
at elevated temperatures. Even quality brands like Sony and Panasonic
are so afflicted. Quite a large number of engineers appear to treat caps
as if they are 'blcak box' passive components, which are impervious to
heat. They're wrong, of course.
Come on Trevor, this is a high end electronic crossover, not a power amp or
piece of consumer mid-fi.
Interesting that 'same price for better specs' translates to
'fantastic
markup' for you.
Stephen, can't read can you? Or is it that context means nothing to
you?
The above is irrelevant to what I said, and more strikingly, it is
irrelevant to what you said.
It's a paraphrase of what you said in reply to what I said. I agree
that
it didn't make sense in context.
You can buy motor oil cheaper in the store than at the
mechanic's, too.
Of course, but irrelevant.
Exactly relevant to the cost of replacement parts "obtained
competently"
by the tech vs the cost to the consumer installed as a repair.
That wasn't what I was talking about. I was talking about the
incremental
cost of a higher-spec part which is what you started out talking
about.
Yes, I said I was able to get the higher-spec'd parts installed for the
supposed cost of a Linn renewal to original specs. Then you said the
repairman got a 'fantastic markup' which doesn't make sense,
It makes as much sense as you care to understand.
As things stand, the whole story is strange.
The number of parts (150 capacitors!) that were replaced was way out of
proportion to reality for either a passive or active crossover.
**Not necessarily. I've not sighted the crossover in question, but it is
entirely possible that Linn have used a large number of decoupling caps
in their design.
All things are possible, but 150 decoupling caps in an simpe 2-channel
electronic crossover is ludicrous.
I've worked on a few crossovers and some use rather prodigious numbers of
eletros in their design (usually as decoupling caps).
I guess some people do crazy things. For example, look at the power supply
caps in a ME power amp!
Why is a dc block needed in an active crossover?
No, these are decoupling caps, for the power supply. A large number of
electrolytic decoupling caps are not needed - the decoupling caps that show
up in any volume at all are ceramics or film caps, and those typically have
much longer lives than electrolytics.
Anyway...electrolytics not being symmetrical with polarity are
an awful choice for an ac coupling/decoupling application.
Not true.
See this guide.
http://www.electrocube.com/support/bullet3.asp
I don't see where this document supports your claims about electrolytics
being bad for coupling, Scott.