Re: AppleColor Composite schematic?

["Michael J. Mahon" <mjmahon@aol.com>]

On 3/20/2013 2:52 PM, retrogear wrote:
> On Monday, March 18, 2013 2:36:38 PM UTC-5, D Finnigan wrote:
> > Anyone have the schematic for the CRT controller for the AppleColor Composite display (NOT the IIgs)? Someone on AppleFritter seems to have a display where there is no green. I think it might be from a failed transistor. -- ]DF$ Apple II Book: http://macgui.com/newa2guide/ Usenet: http://macgui.com/usenet/<-- get posts by email! Apple II Web&  Blog hosting: http://a2hq.com/
>
> most rgb displays of that vintage had an output transistor for each color channel probably with a heatsink attached. with a voltmeter, measure the voltage on the collector of each which would be the center pin or the metal tab on top. if one voltage is significantly higher than the other two then good possibility transistor is bad. if one voltage significantly lower than the others then more likely resistor, solder connection or (sigh) the crt itself.

Just swapping the red grid with the green grid at the CRT would quickly 
rule out the CRT if the green gun now worked but the red was off.

That's the kind of simple swapping that can isolate things easily.

If a 0.1uF capacitor is used to bridge equivalent points in the green 
and red color channels, injecting the red signal into the green channel,
then it is possible to determine how far back from the CRT is working.

As soon as the injected signal doesn't make it to the green gun, the
immediately following stage is where the signal stops.  Then a scope,
a voltmeter, or (power off) an ohmmeter can often isolate the failed
component or connection.  In a single stage, that won't be more than
a few components.

(BTW, most transistors have an E-B-C pinout, with the collector on one
end.)

-michael

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