Re: messed up colors on Apple Color Composite Monitor?

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

sirarlias@webtv.net wrote:

>I have two AppleColor monitors that exhibit the same problem, one with a
>bad green and the other with a bad red. As far as I could determine in
>both cases the little chroma board on the back of the CRT turned out to
>be the culprit. I was told it was probably one of the transistors.
>
>I tried replacing them with known good transistors from the other
>monitor with no luck. Eventually I grew tired of messing around with it
>and ended up replacing the whole chroma board on one with one robbed
>from a third working monitor in poor cosmetic condition.

The best way to proceed is by doing some signal tracing.  There isn't
much circuitry on the chroma board, and it is pretty much equally
divided between the three color channels, two of which are working,
so comparing the green channel with the others should quickly
reveal the point where the green signal vanishes.  I would also suspect
the little chokes (peaking coils) on the board.  If one opens up, it
would kill the channel.

Signal injection is another good choice in this situation.  Use a
capacitor (say, 0.1 at 250v) to jumper corresponding points in the
circuit from a known working channel to the non-working one.  You
won't see "correct" colors, but you will see the obvious presence
of the heretofore missing color if everything in the channel is working
from the injection point forward to the CRT.

It's even possible (!) that one gun on the CRT is out--for which the
remedy is another monitor.  ;-)  This could be diagnosed by jumpering
another guns voltages and signals to the suspect one.

If the CRT is OK, and you are just missing a color, you are virtually
assured that it is caused by a faulty component that costs less than
a dollar.

>Hopefully in your case it's just a loose cable. You also might want to
>try adjusting the "green" trimpot on the main board.

Yes, a faulty wiper contact on the "green" level would be a nice easy
problem to fix!  ;-)

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/