On Sun, 18 Apr 2004, Lee Fastenau wrote:
> * Twitchy colors may be the result of a dirty color potentiometer (simple,
> but not talked about in the context of a noisy picture as far as I could
> tell). Clean it with electronic contact cleaner (not WD-40!), give it a few
> spins back and forth, then calibrate it with a game you're familiar with or
> a color bar generator cart.
I've mentioned this a couple of times over the years, but I don't think it's
posted anywhere. Good to have it archived!
> * There looks to be two other potentiometers that may be adjusted with a
> small allen wrench. I didn't touch these and don't know what they do, even
> after checking the web. Anyone know?
Those are variable inductors. One is for tuning the sound mixer, one is for
tuning the broadcast frequency. Many VCS motherboards only have the broadcast
tuner.
> * I had removed the metal shield from the top and bottom of the board during
> cleaning, not realizing exactly how indispensable it was for attaining a
> clear picture. I had vibrant colors and a still frame from cleaning the
> color pot and cable connection, but noticed (this is going to sound weird)
> game-specific noise... almost like ghosting of the game logic across each
> scanline. When I replaced the shield, the picture was VERY crisp and very
> colorful. The only artifacts remaining are very faint vertical, but
> equidistant bands, but they are hardly a distraction.
This is normal. It is in fact logic ghosting. Trying pluggin in Kaboom!
and moving the paddle to see this effect in a *dramatic* way.
The vertical bands are caused by the chroma signal affecting the luma level.
Very hard to irradicate. Try to ignore it. ;) If you can't, then you can
increase the value of the resistor that goes into the video summing point.
If it's too large, you'll lose color altogether, but increasing it a little
won't hurt much.
> I considered doing a composite mod until I cleaned everything, but there's
> not much of a need now.
It's amazing what proper calibration can do for old electronics! :)
-Chris
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