Re: Need Help Adjusting IIgs AppleColor RGB

[dog_cow@macgui.com (D Finnigan)]

Michael J. Mahon wrote:
> D Finnigan <dog_cow@macgui.com> wrote:
>> Kevin Dady wrote:
>>> 
>>> are you positive the transistor is inserted correctly?
>>> 
>> 
>> Fairly certain: the flat part of the transistor is facing up, and the
>> emitter and base legs are matching as shown on the board.
>> 
>> I recall someone mentioning that when I replaced the transistor, the
>> colors
>> would need to be adjusted. Presumably this would be because of the new
>> transistor having better specifications than its predecessor and
>> brothers.
> 
> ..or just slightly different characteristics, which would be expected even
> for an exact replacement. 
> 

The ECG290A replaced an A673 transistor.

In my original article from a month ago, I incorrectly stated that a C2688
transistor was bad. It wasn't; it was the A673 that was bad.

Schematic:
http://apple2.info/download/IIgsMon3b.pdf


> There is a prescribed order for making color adjustments, to ensure that
> the process converges. It would be best to find the service docs for this
> monitor for the procedure, or, failing that, review service docs for a
> comparable (in schematic) monitor or TV. 

I have been searching for the service manual for this monitor; I have not
found it yet.

> 
> What you have so far described could also result from either an improper
> replacement or additional failures in the circuit. 
> 
> If I were troubleshooting this monitor, I'd start with comparing voltages
> under black screen conditions for equivalent points in the three color
> channels. Big differences would be suspect, and could point to the cause. 
> 

Yes, it definitely seems like a voltage problem. I noticed that turning any
of the color adjustments up too high would result in an over-saturated,
fuzzy screen.

-- 
]DF$
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