Re: Problem with AppleColor composite monitor

[mjmahon@aol.com (Michael J. Mahon)]

sirarlias@webtv.net wrote:

>I recently found an AppleColor composite monitor (manufactured 11-88 in
>Korea) with a red-deficient picture. Blue and green come in fine but
>there's little-to-no red in the picture. At best there is a semi-orange
>where there should be red, so it seems a trickle of red is getting
>through. Otherwise the picture is crisp with no screen burn. Pushing the
>color kill switch gives the expected black and white picture, and all
>the external controls behave as expected.

Hmm.  If you get black and _white_ (not cyan), then the red gun
of the CRT is working fine.  I would start looking at the cable and
connectors, then the red driver transistor (in the computer and in
the monitor.  Oh, BTW, does another monitor or computer "move
the trouble"?  If so, you've localized the problem.

>I cracked the case and tried adjusting the bias tabs; blue and green
>affect the picture as expected, but not the red. (Ha! Of course it
>couldn't be that easy, now could it!) The only thing that happens when
>adjusting the red tab is a slight vertical compression of the picture. I
>checked with a multimeter and the tab seems to be in order.

I'm pretty sure I don't understand what you mean by "tab".  Are
these potentiometers you're adjusting, or are they the gun alignment
magnet tabs around the neck of the CRT?

If they are the pots, then I'm having trouble seeing how the color
killer could produce white.  If they're the magnets, then I'm still
happy with my red-signal (but not red gun) diagnosis.

>I also checked for cold soldering/cracks and found none. Also, I pulled
>the entire thingamajig (chroma unit?) off the back of the CRT and
>checked the pins; they were clean, straight, and seated tight. There was
>no change in picture when I turned the monitor back on.
>
>This problem resembles the example of the blown inductor on the chroma
>board in the FAQ, except in this case there is no red tint. Do you think
>the inductor could be blown in this case as well? Something else?

Yes, it does sound consistent with that diagnosis to interrupt the red
channel, but I's still suspect the cable and connectors first.

>The monitor is in excellent condition otherwise and I'd like to salvage
>it.
>
>Your expert advice is greatly appreciated!
>
>-RPM
>
>


-michael

 Email:  mjmahon@aol.com
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