On Jun 4, 0:52, Tony Duell wrote:
[useful stuff]
> 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
small risk that they'll break down or flash over. Better to use two or
more in series if you can.
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
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).
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.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York
Received on Wed Jun 03 1998 - 20:36:15 BST