24 Feb
2009
24 Feb
'09
12:19 a.m.
On Monday 23 February 2009 09:27:32 am John Floren wrote:
I've got a quick question about power safety
here.
My ADM-3A has needed fiddling for a while, since what I believe is the
flyback is making a very high-pitched whine that gives me a splitting
headache and makes it hard to see straight after 5 minutes. I want to
take a poke around, make sure everything is connected firmly, make
sure none of the components are visibly bad, that kind of thing. How
long does it take an unplugged CRT to discharge?
A rather surprisingly long time.
I did some tv repair work all the way back in 1974, and instead of going out
and buying a "test jig" I got myself a "dud" -- a CRT that had been
traded in
for a new one, because it was a little weak on the green gun. Worked okay
for me, but that HV stayed there a while. I had a connector with the usual
rubber cap and HV wire and all, and an alligator clip on the other end to
connect to the chassis under test. I'd short it to the outside of the tube,
drawing a spark, and then a while later (maybe even many hours later), I
could short it out again, draw another spark.
A good illustration of what they call dielectric absorption. :-)
This hasn't been plugged in for over a month, so I
figure it's probably safe
right now, but if I test it and then want to check something else, how long
do I need to leave it sit?
As long as you don't stick your finger under that rubber cap that's typically
on the end of the HV wire I don't think that you have anything to worry
about. Older stuff, tube sets in particular, going back to the 1950s and
early 1960s had some occasional exposed contacts, but that hasn't been
common for a real long time.
Aside from that, and the primary side of switching power supplies, there
really isn't that much to worry about in there.
Is there a safe way to discharge a CRT when you're
working on it in the
living room?
See above. You can discharge it, and then a little bit later on there's
voltage there again. But you're not likely to come in contact with it.
I'm a little leery of just shorting connections
with a screwdriver randomly,
although that's almost exactly what I did the first time I worked with a
CRT :) (it was 5th grade, and I had just had it on, unplugged it, opened it,
and started poking around with wires. BIG SNAP)
:-)
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin