Re: Need Help Adjusting IIgs AppleColor RGB

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

D Finnigan wrote:
> 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.

Actually, that's pretty normal.  Overdriving the grid of a CRT
causes that.

First, try the easy things...  ;-)

I used generic color TV terms for the controls above.  Using
the terminology of the AppleColor RGB monitor, with no video
input (should be black screen), and the brightness control
set where it should be for a black screen with no video signal,
adjust all of the "cutoff" controls so that the corresponding
color just turns off.

When they are all properly adjusted, you should have black,
and even a small adjustment "up" on any control should cause
the corresponding color to begin to appear.

Then, apply a video signal that generates a white bar, for
example, and adjust the blue drive and red drive to get a
white bar.  You may find that the contrast control needs
adjustment to get an appropriate white level.

If this works reasonably, display a color pattern with colors
you know, including white and black.  Touch up the controls
*slightly* to get accurate colors.

The color of "black" should not vary as you adjust the brightness
control, and the color of "white" should not vary as you adjust
the contrast control.

If this procedure gets good colors, you're done!

-michael

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