4 Jun
1998
4 Jun
'98
11:24 a.m.
I often work on systems with power on. With PCs it's no big
deal in terms of my safety. The SE, however, is an incredible
pain to repeatedly re/disassemble if my fix doesn't work, so I would
like to work on it cover off/plugged in. I'm only dealing with the
logic board anyway. That's why I originally asked...
By the way, how do people work on machines like the PS/2 Model 25,
in which one can only get to the MB when it's not plugged into the
PSU?
low
voltage
> > But not everyone has an EHT meter. It's
probably OK to use a 10M
resistor
> > (prefereably a high-voltage one) and a
well-insulated screwdriver
to
> > discharge the CRT. Connect one end of the
resistor (use clip leads,
etc)
> > to the CRT earth and the other one to the
screwdriver blade. Then
push
> > the screwdriver under the edge of the anode
cap and hold it in
contact
> > with the metal contact for about 30s.
>
> Most resistors are rated for a 500V or 1kV rather than EHT, so there
is a
That's why I said 'preferably a high-voltage one'. Farnell and Maplin
both sell the Philips HV resistors good to about 10kV. A couple of
those
in series.
> small risk that they'll break down or flash over. Better to use two
or
more in series
if you can.
I'd not trust my life to it, but most resistors break down and go to a resistance when they flash over, not open.
> It's also worth pointing out that even 1mA at 15kV is a fair wattage
--
> another reason to use more than one resistor. A
large colour CRT
might
Yes, but the energy stored in a CRT is not that great. Resistors burn
out
because they overheat, and if there's not enough
energy to heat them
up,
then they'll not fail.
> have a 25kV supply. Of course, as Tony points out, usually the CRT
will
> discharge when switched off. But if it
doesn't, that's just when you
don't
> want to find out the limitations of ordinary
resistors. (I once
proved
this theory on
a Commodore PET).
'Nice' monitors have EHT bleeders anyway, often as part of the EHT
reguation circuit (the feedback loop consists of a potential divider
across the EHT supply). So the EHT should discharge at switch-off. I
check it anyway with the meter, of course.
As this is classiccmp, I think I should mention that some vector
displays, like the DEC VR12, VR14, VR17, etc have a mains-derived EHT.
There's a sealed (oil-filled?) can containing a transformer and a doubler - a schematic is printed on the can. These
things can supply
considerable current for short periods - easily enough to kill you.
They're nasty.
>
> Perhaps this is a good place to mention the "hand in pocket" rule:
if
> you're working on a high-voltage system, and
one hand is in your
pocket, it
> can't be touching a good earth (unless
you're even odder than I am
:-)) so
> there's a reduced chance of current crossing
your body. You used to
often
> see TV engineers with one hand in a pocket and an
EHT probe in the
other.
Nowadays you see computer preservationists with one hand in their
pocket.
I do it whenever I work on HV stuff with the power
applied.
The idea is to prevent the current taking a path through your heart.
Down
one leg is possibly OK, arm-arm is not.
I didn't mention it, because the original poster didn't say he was
planning to do work with the power on. If you are, there's one other
rule
that I'll state - Always have somebody nearby who
knows how to turn off
the power if anything goes wrong. Never work alone.
As a practical point, it helps if that person is not either of your
parents, wife/husband, or girl/boyfriend. The reason is that they tend
to
cut the power on the slightest problem - like a minor
flashover to a
screwdriver, which is very annoying!
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Pete Peter Turnbull
-tony