2 Nov
1998
2 Nov
'98
12:26 p.m.
While we're talking about CRTs, I want to ask if anyone has heard of
any problems with Digital VGA monitors, about 4 years old. We
installed a lab full of them, and when I turned them on, two lost the
magic white smoke, and more have various other problems.
service
Is this
related to what I am about to ask? Could somebody do a short
summary of why Fimi(?) greyscale megapixel NeXT monitors fade after
Fimi monitors seem to be related to Philips, but just try getting a manual for one.
a
20,000 or so hours of use and indicate whether
there are any other
monitors (perhaps some classic) that show similar problems?
The most likely cause is the CRT cathode is losing emission (basically
it's not sending enough electrons to the screen). But it could also be low electode voltage on the CRT, a problem with the
brightness control,
etc.
It's time to mention a trick that's saved me a lot of time over the
years, even though it's too late for you to use it. When I get a
monitor,
whether new, or part of a classic computer, or
whatever, the first
thing
I do is pull the case and note down the CRT electrode
voltages and any
other testpoints I think might be useful (PSU outputs, etc). If I have
a
service manual/schematic, then I note them in there. If
not, then it's
normally easy to find the CRT pinouts at least and I write down the
voltages in a suitable place, such as the user manual. Even a label
stuck
inside the case.
Then, if the picture starts to fade, I can check the voltages again to
see if there are any problems.
OK, if it is the CRT emission (and I've had this happen on large mono
monitors), then you may be looking for a new CRT. There are CRT
reactivation devices (basically they run the heater at about 20%
overvoltage and apply a high voltage, about 200V, between the cathode
and
1st grid), but modern CRTs don't normally respond
too well. Sometimes
overrunning the CRT heater all the time (it's unlikely you'd burn it
out)
will help. There are all sorts of tricks for that,
including wrapping a
couple of turns of wire round the core of the flyback and connecting it
in series with the heater supply. If the heater is already supplied by
the flyback, then you need to try the new winding both ways round. The
way that brightens the heater and picture is the right one.
Incidentally, a lot of micros, video terminals, etc use 12" mono CRTs.
There are basically only 2 types - thin neck, 7 pin, 11V heater and
thick
neck, 8 pin, 6.3V heater. A good source of the former
is old portable
TVs. If you can get a portable TV with a good tube, perhaps with
flyback
or tuner problems, the CRT will probably go into a
TRS-80 Model 3 or a
VT100, or.. with only a few mods, if any.
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-tony