18 Nov
2010
18 Nov
'10
9:14 p.m.
--- On Thu, 11/18/10, Jules Richardson <jules.richardson99 at gmail.com> wrote:
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 - to the point where most photos lack
all detail, appearing as blackish patches. It's still fine for text though, and since
that's all I really ever work with, it doesn't bother me.
Heater voltage seems good at 6.4VDC / 350mA (it's
derived
from the PSU in this monitor rather than the flyback
section), but I'm considering boosting it a little and see
if it improves things, obviously shortening the life of the
tube in the process.
Yes. That is a common method if improving the picture. Boosting the voltage causes the
filament to heat up the cathode more, thus emitting more electrons. A standalone picture
tube rejuvinator can also usually "zap" the cathode to help improve emissions as
well, by stripping off the coating. If you have access to a rejuvinator, try that first.
Another common trick that helps in some cases is to turn up the SCREEN control on the
flyback. This increases the voltage to the screen grid on the tube, and will produce a
brighter picture. Sometimes this is enough, but if the tube is real weak, it'll just
wash out the picture.
Question is, what's a sensible amount to over-run
things
by? Say I aimed for around 10%, is that too much and going
to kill the heaters in next to no time, or so little that
unlikely to really make any useful difference?
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.
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 more years out of it though.
-Ian