Problem with 2600 A/V mod

[landsmarra]

Hi again guys,

 

Well, I've had someone perform this mod as I am technically challenged. Anyway, the modded unit now has faint vertical stripes on the screen with all carts that are more noticeable with a lighter-colored background. Anyone familiar with the Tristar/Super 8 converter for the NES will know what kind of stripes I am referring to.

 

I sent it back to the modifier who apparently tore his mod apart, couldn't eliminate the stripes and concluded that it has to do with the way the 2600 builds the picture. If it's true, how come they are not really noticeable through RF? Could it be that RF interference/haze masks them?

 

Might he have done something he shouldn't have or overlooked something on the mod? I know it's a little tough without seeing the mod or the stripes but I thought someone might know of some general reason why it might be doing this.

 

If anyone wants to see the unit, I'll send it to them.

 

Thanks.

[tearex]

If it is what I think it is, basically it's the processor clock that you see. I can see it (lightly) on my unmodded 2600 and it is also a well known "feature" of Commodore 8-bit systems.

 

My suggestion would be a bigger "main" electrolytic capacitor for the 5V line (the large one on the daughterboard with the switches) and then also adding some set of both larget and smaller caps (say 100 nF, 1 nF, and 15 pF, the latter should have a very high frequency rating) between +5V and GND next to each chip. This ought to stabilize the voltages a bit, based on my admittedly private experience. If you feel technical, adding a metal housing (with heat sink!) around the TIA chip might also help.

 

Also if you want to sacrifice some sharpness you might low-pass-filter the Luma line, but that would sort of defeat the purpose of the A/V mod.

[Bryan]

What you are seeing is either noise from inside the 2600, or the chroma signal is too strong and is visible as changes in luminance. The original video circuits are very simple, but the best way to get a good AV signal is to copy the Super Video circuits for the XL/XE. It's a lot of work, but the result is very sharp & clear.

 

-Bry