On 19 Nov 2006 09:20:36 -0800, Simoc wrote:

You are probably thinking, that the 2 mins would be normal when they're
discharged. But not, that's not the point. If the caps were good, they
would charge virtually immediately when powered, even if they're fully
discharged. That explained your last sentence (as then the caps weren't
dried out yet).

But as they are now dried out due to aging, their ESRs have increased
and that reduces both the charge and discharge current, causing too
much ripple voltage (and thus preventing the device from working). They
probably get discharged within 24h, but the point is there, that those
caps get _temporarily_ "rejuvenated" (ESRs decrease) when power is
applied for long enough time (the 2 min), by causing them to heat up.
But once the device is turned off, the caps cool down and then the
"rejuvenation" begins to lose its effect (ESRs begin to increase again)
_slowly_ (not immediately when cooled down and discharged, but within a
few days).

So the dried out caps should be replaced with new ones. Easy repair :-)

I understand that now, just like a car battery that's duff could have
virtually no charge when cold but when warmed up works. I wasn't
looking at it from that point of view, I was just thinking how could a
change of capacitance affect performance depending on how long it had
been off, as you guessed :-)

There is only about half a dozen electrolytic caps and it's only a
single sided board so I'll take them out and test them. You said not
the mains filter cap(??), there is a 400V 1000uF (or 100uF can't
remember which) in there and I assume that's the mains one and it may
have a slight bulge on the top. Any particular reason not to go near
that one?

Thanks again, I'll go inside it again sometime, hopefully before I need
to print something.

--
Regards - Rodney Pont
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please send any emails to the address below
e-mail ngpsm4 (at) infohitsystems (dot) ltd (dot) uk