Re: AppleColor RGB "yellow-tint" problem

[dynasor@infi.net (Dennis McClain-Furmanski)]

Mitchell Spector (spector@vax2.concordia.ca) wrote:
: 	In the past, I've posted questions concerning various problems 
: I've been experiencing with my AppleColor RGB monitor. Recently though,
: I think I've finally narrowed down the source of the problem, or rather
: one major problem causing the rest.

: 	The entire screen (including border areas) has a kind of 
: yellowish-tint overlaying it. Almost as if a fine pale-yellow screen mesh 
: had been placed over the picture tube. The result is, colors seem to
: be mixed with this yellow. Pure white has a kind of yellowish tone
: to it, greyscales come up in shades of baige, blue turns out slightly
: purple, etc. Another problem is the brightness is too low, I usually
: have to let the screen warm up for quite awhile or else all is too dim.
: This is really been annoying me lately, windows in the Finder look as
: though they've been soiled yellow and the desktop is a blue-purple color!

: ..Does anyone know the cause of this "yellow" tint, or perhaps if it's
: possible to get it fixed? The person who was going to sell me a replacement
: RGB backed out, so now it seems I'm stuck with this one for the time being..
: :-/

: Mitchell Spector
: m_spect@pavo.concordia.ca / spector@vax2.concordia.ca

The problem is almost certainly the screen temperatures. This is
the particular voltage and/or the response to that voltage on the electron
gun of the CRT. One solution is to have the CRT 'cleaned' with a CRT
brightener. This is built into some tube testers, and involves buring the
oxidized metal off the cathodes by jamming excessive voltage through them. I
know that sounds unappetizing, but it works. It can shorten the full life of
the CRT, but make it perform better in the mean time. This is the idea
behind the old 'picture tube brighteners' you used to be able to buy for
TVs.

The other option is adjusting the screen temp pots. These are on the board
which hangs on the back of the CRT. (I *believe* this is where they are;
I've not paid attention to the location because I've never needed to adjust
mine. But they're in there. I'm going to go with this assumption, and
someone who has one open or has noted the location can correct it if need
be.)

Set up the monitor to run attached to the machine, with the back off. Set up
a mirror in front of it so you can see the screen changes without having to
reach around the front, or otherwise get yourself in a position where you
can't see your hands. (They're going to be fiddling around with thousands of
volts -- you don't want to let them wander around unattended). Note the
positions of the pots, labeled Rsomething, Gsomething and Bsomething,
*before* you turn anything. Mark them and the board with a pencil if need
be.

With a multihued screen running, Finder is fine, whatever it takes so you 
can see the affected colors, adjust the pots with a _plastic_ screwdriver.
SLowly, a little at a time. Since the screen is too yellow, you'll probably
need to add blue, so start there and turn it up. Which way is up? The way
that doesn't make white turn more yellow.

Changing one might change others, i.e. you might have to change other pots
too. Just keep track, do one thing at a time, note what you do when you do
it, and be certain you've marked the original positions before you do
anything.

If changing these doesn't work, a trip to a tech with a brightener is the
only hope. If the CRT is shot (the other primary possible cause) it'd cost
more than a monitor to replace the tube.

If fiddling with a few thousand volts makes you nervous, find a tech to do
it. And just to make sure, look on the mamaboard too before you start, since
the screen temp pots might be down there. There *are* other pots which could
be on the CRT board.

If you want to try yourself, and you get it open and don't see what I'm
talking about, email me and I'll try to walk you through.



--
dynasor@infi.net                                       The Doctor is on.