Re: Problem with AppleColor composite monitor

[Rubywand <rubywand@swbell.net>]

sirarlias@webtv.net writes ...
> 
> Hello all.
> 
> 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.
> 
> 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 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?
 ....

     Depends. This is a different monitor than the one discussed in the FAQs
(a IIgs RGB monitor).

     In the case of the IIgs monitor, the blown inductor (in the Red circuit)
removes Red control and the Red output transistor runs wide open. This swamps
the display in Red.

     In your case, it does sound like something on the chroma board in the
Red circuit is messed up. A blown inductor might turn off Red if the rest of
the circuit (up through the output transistor) runs nearly full-off as a
result. (Or a shorted capacitor somewhere in the Red circuit could turn off
Red.)

     One fairly easy check (with everything OFF and unplugged and following
safety guidelines) is to use an Ohm meter to compare resistances to Ground at
different points in the Red circuit vs. the Blue or Green circuit. (Usually,
the red/+ Ohm meter lead is the one connected to Ground. R x 100 is usually a
good range setting.)  If you find a big difference at some point, the nearby
parts are suspect.

     A blown transistor, like the Red output transistor, is a decent suspect.



Rubywand