19 Nov
2010
19 Nov
'10
4:14 p.m.
--- On Thu, 11/18/10, Jules Richardson <jules.richardson99 at gmail.com> wrote:
Is it? I am not convinced. I think in all my classic computer repairs
I've had one low-emission CRT, and that was in a Mac+. The picture coul;d
be made bright enough, but there was a curious shaddowing to the right of
objects on the screen. There as nothing wrong with the video amplifier
circuit, and a new CRT did cure it.
But many times I've had CRT electrode voltages drift, upsetting
brightness or focus.
My desktop CRT (over 10 years, but not what
I'd consider
vintage!) has been ailing for some time, with the picture
getting darker and darker despite having the brightness at
100%. It's still good for high-contrast stuff such as black
text on white background, but forget trying to pull details
out of most photos, for instance.
A couple of things can cause this, but yes, most common is a weak
picture tube. The tube in my monitor here at work is pretty worn out too It'll help a lot, usually. Start with maybe 10%
increase. You can
create a higher heater voltage by making your own turns around the
flyback core - a couple of turns of insulated wire should be enough.
Filament voltage does not have to be DC. Another thing to check before
boosting the filament would be the monitor's main B+ voltage. If it's
sagged too much due to worn out electrolytics, you'll have a dim,
shrunken picture. Similarly, faulty electrolytics in the signal path
will cause smearing and dim colors. So, start by checking voltages - the
main B+ and the supply voltage to the video amplifiers. One thing I have
seen is a monitor where the video signal from the computer was dim and
faded, even with the brightness cranked up - but the OSD menus were
really bright and clear! The problem was related to the video amplifier
for the signal input.
The other thing to distinguish betwene is low brightness and low
contrast. They are not the same. Contrast problems are almost always due
to video amplifier problems.
Oh, and clean the screen :-). Seriously, I once spent a day tracking
down a nonexistant video amplifier fault in a VT100 with a dim picture
which turned out to b caused by a filthy CRT screen...
If that all looks good, then crank up the heater voltage a bit. Worse
case, you burn out the tube, but hey, it was already unusable when you
started, you haven't much to lose. You should be able to get several
Hmm.. Sure, if the CRT is defective, there's no harm in trying to boost
it a bit. But if the fault is elsewhere, then boosting the heater voltage
will not help, and may damage the CRT. Then you have both the original
fault to find and cure, and a CRT to replace.
-tony