Dying VaxStation 4000 VLC - help?

From: Mark Tapley <mtapley_at_swri.edu>
Date: Fri Nov 15 09:19:01 2002

All,

Just catching up on the digest. I think next time I beg for advice I'll
take digest mode *off* first. The delay is frustrating. Tony was way ahead
of me last night, but I didn't get the benefit of his advice until this
morning.

Tony D. said:

>I will bet that
>Red = +5V
>Black = Gnd
>Orange = +12V
>Blue = -12V...

You'd win, as far as I can tell with my meter, with the minor variations
I've noted.

>> Blue is one wire, and goes to appx. 12 V. when the system is on.
>
>I would think the last one is actually -12V ?

Correct, typo on my part.

>More intersting would be to
>use a 'scope to look for noise (particularly spikes at the PSU switching
>frequency). Such spikes can make a supply rail look to be too high on
>some meters.

Yeah. Sigh. What's a likely frequency for the PSU switching? Is my
(primitive analog) VOM likely to yield any useful info if I set it to the
"AC Voltage" ranges?

>> M9124
>> LM393N
>> QST
>
>Aha. A dual voltage comparator. This could be used to check if power is
>OK, and to generate the reset signal as appropriate.

I don't know the causality yet (failure causes comparator to switch, or
comparator switching causes failure), but the comparator (at least output
A) does switch when the failure appears. Output A is Pin 1 as you recalled
and Output B Pin 7, thanks to Toth for the link to the data sheet.

Now *I'll* bet - Tony wants to know what the comparator *inputs* are
connected to. Tonight's work is cut out for me. But in any case, I'm still
guessing the power supply is a likely culprit. It looks like the comparator
is properly protecting the mainboard from an overvoltage on +5V or +3.3V. I
don't know what's going on with +3.3 being referred to something other than
ground. In either case, a lower +5V supply (= appropriate repair to the
PSU) would solve the problem.

Is replacing all of the PSU output electrolytic capacitors just on
suspicion a good move? That's what I plan for the weekend. (Lucky for me
capacitors are not entitled to due process.. :-) )
                                                        - Mark
Received on Fri Nov 15 2002 - 09:19:01 GMT