[Info-vax] MIcrovax 3100-98

[Scott Dorsey]

Michael Moroney <moroney at world.std.spaamtrap.com> wrote:
>kludge at panix.com (Scott Dorsey) writes:
>
>>What fails in the AlphaStation supplies are mostly the output decoupling
>>capacitors.  Just shotgun all the capacitors out with Panasonic 105C types
>>and they'll work for decades more.
>
>What actually happens when they fail? If I recall correctly the symptom
>of a failing supply was it only happened when powering on, it would come on
>for a few seconds (and appear to be OK) tben it would shut itself off.
>Powercycle and it would repeat (may stay on sometimes initially)

Right.  The output caps are leaking current, and not damping the overshoot
of the signal coming off the transformer.  The supply comes up, and either it
sees too much of a load on the output or a momentary overvoltage on the
output, and the supply goes into protection.  A few seconds later, it tries
again and goes into protection again.

In gthe case of the AlphaStation ones you can usually see physical damage to
the output caps.

>Why do the caps fail, do they dry out? Underrated for the voltage present?

Electrolytic caps are wear items, you expect them to fail.  You can buy 
expensive ones that last longer, and you can overrate them for voltage,
temperature, and ripple.  The warmer they are, and the more ripple across
them, the harder their life is.

If you want the supply to last a long time, you use high temperature caps
with a high ripple voltage rating, then you run them as cool as possible and
you oversize them so the ripple current is reduced.

If you are trying to pack a power supply into a tiny space and you do so by
undersizing capacitors, tucking them under the heatsink in the hottest
possible location, and using lower temperature capacitors because they are
physically smaller, then they won't last very long.  This is very very
commonly the case for server power supplies today and it's why everybody
builds servers with multiple redundant supplies.

>DEC always seemed to overdesign power supplies before then. I know the AS200
>supplies came from an outside (PC?) vendor.

DEC contracted out most power supplies, but they usually contracted them out
to good folks.  But supplies fail.

>Aside: I have a VT220 which seems to have a similar power supply issue.
>Symptom is when powered on it seems to be 100% dead as if no power or a fuse
>is blown, but sometimes it would power on and be perfectly fine until powered
>off.  "Sometimes" became less frequently and now it is "never".  Maybe some day
>I'll dig it out and try to replace power supply caps, or maybe not.

I would replace -all- the caps on the VT220 and I would also immediately
replace the 8.2uF cap in the sweep circuit with a film capacitor.  When that
capacitor goes leaky, it takes out the flyback transformer and you cannot
get replacements for the VT220 for any money.
--scott

-- 
"C'est un Nagra. C'est suisse, et tres, tres precis."