Re: Problem with AppleColor composite monitor

[Rubywnad <rubywand@swbell.net>]

sirarlias@webtv.net writes ...
> 
> Hello everybody.
> 
> I probed the R,G,and B circuits on the chroma board as Rubywand
> suggested and found no differences in resistance at each respective
> component. I was hoping something obvious would show up here.

     The difference may not be easy to spot unless you pick the right test
point with the right Ohm meter leads and experiment with different Ohm meter
range settings.

Note: Ohm meter leads are, in effect, 'polarized' because R is measured by
passing current through the circuit/component being checked. When
semiconductors are involved, you will usually get a different R between two
points depending upon which Ohm meter lead goes to which point.

Another variable where semiconductors are involved is the voltage the Ohm
meter uses to create the current flow. If it is less that a Volt or so, you
are likely to miss detecting differences in many semiconductor devices. In
general, you are probably better off using an old analog style meter than a
digital meter for spotting differences.

     With everything Off and unplugged, try same-measurements (same leads
going to same points) on transistors in the Red and Green circuits. For each
transistor, check resistances between each of the three leads. Swap around
the Ohm meter leads and repeat the check. Especially, check the Red vs. Green
output transistors.

     Probably, there are some mini adjustment pots on the Chroma board. Check
these carefully, too.


> 
> Also, I examined the screen on "B&W" with a loupe and found no red
> pixels.
> 
> As for using a scope, I don't have one, nor would I know how to use it
> if I did. (I'd like to learn, though.)
> 
> If it is the red gun is there any way to fix it? Or would the entire CRT
> need replacement? Any other ideas?
 ....

     There may be a fairly easy check for Red at the CRT. With everything Off
and unplugged, find the output leads from the Chroma board for Red and Green.
Label or mark them. Cut the leads and swap them around.

     If you get Red in the display (probably, where you should get Green),
then you pretty well know the Red problem is not in the CRT. (If you do not
get Red, you can not really be sure that the opposite is true.)

     If you get Red, suspicion falls back on the Red circuit on the Chroma
board, although, something like a blown Red level control pot or a broken
trace, etc. for the Red circuit _before_ the Chroma board is possible.

     In fact, if there are any socketed IC's, try pulling and resocketing
them. (The same goes for plug-in cables going to the Chroma board.)  It would
just figure that the whole problem is a bad contact for the Red signal pin of
some IC.



Rubywand