3 Nov
2005
3 Nov
'05
10:42 a.m.
Yeah, I've replaced CRTs and flybacks before.
As, of course, have I.
A better idea is to place a current-limiting resistor in series with the
alligator clip to ground (twist one lead around the screw driver, and
tape it on, attach the allegator clip to the other lead). 100k to 1M
ohm should do the trick, and hold it on the metal part of the anode cap
for 5 seconds or so. Not using the resistor will still discharge the
tube, but it runs the risk of damaging it.
Agreed. Actually, I have a Heathkit EHT probe. It's just an 800M resistor
and a microammeter in series. I connect the ground lead of that to the
CRT earth, then hold the probe end on the anode connector for a minute
after the meter has got to 0. That seems pretty reliable.
If you have the tube powered off for a couple days, the charge will most
likely be quite dissipated. In fact, having it off that long means it's
The semiconductor rectifiers used in computer monitors, etc will leak
enough to discharge the CRT. Watch out if you work on old TVs with valve
rectifiers, they are much less leaky.
unlikely to have any charge on the tube at all, but
it's still best to
still discharge it, just in case.
Of course, I've worked on monitors live before. Trick there is to stay
clear of the PSU and HV parts alltogether, and use insulated tools.
I once had the 30kV EHT in a colour monitor arc over to my fingers.
Suffice it to say I then used langauge that I would not repeat on this
list :-)
If you're working on a 'live' unit, don't work alone. Have somebody
nearby who knows how to turn off the power if things go wrong.
And rememebr that (at least in Europe), the mains is a lot more dangerous
than the RHT due to the former's much lower impedance. Don't take great
care to avoid the (generally well insulated) EHT and then put your hand
on a live heatsink in the SMPSU.
Once you do it once or twice, you'll start feeling
a bit more
comfortable doing it. It isn't a good idea to *replace* the flyback
with the terminal live, but you'll probably need to have the power on
when you adjust the screen/focus controls on the new flyback you
install.
Actually, in a VT100 the presets are all on the video bard. It's still
worth rememebring there's about 800V on some of them, so use a
well-insulated screwdriver to tweak them.
-tony