2600 games not so clear after composite mod on 7800

[stuartmcivor]

Hello,

 

This week I installed a composite mod in a working NTSC 7800.

 

 

Everything seemed to go well. There is picture and sound for both 7800 and 2600 games.

 

 

What's bugging me is that the 2600 game's picture does not seem as clear when compared to my similarly modded 2600 JR.

 

 

On the 7800 the picture look blurry, cloudy, like the actual TV screen is dirty (underlined in red). It is less obvious in the pictures, sorry.

 

 

The same game cartridge in the JR, on the same TV, looks noticeably cleaner and sharper.

 

 

To mod I completely removed the RF box and resistors R3 and R5. Ground (black), +5 (red) and Video (yellow) are pulled from where the RF box pins connected. Audio (white) from above the Maria chip. I did not do the Pokey audio. I have no games for it. I also didn't change R11. Power is from the original 7800 PSU. I see no damage to the board. The capacitors and voltage regulator are original from Atari. My solder connections seem good and solid.

 

 

I tried three different mod boards. The first (and the same as in the 2600 JR) uses one 2N3904 transistor, one each 3.3k, 2.2k and 75 ohm resistor. This board was very dark and fuzzy (sorry, didn't think to get a picture). I assume the 75 ohm resistor was the cause and quickly moved on to another board. The second board was similar to the first only no 75 ohm resistor. The picture was much better but still has the cloudy, dirty screen issue. The third and final picutred board uses two transistors 2N3904(6), 8 resistors, one 47pf cap and one adjustable 5 ohm pot. Turning the pot (a lot) makes the picture brighter/darker but not cleaner The results are the same as the second with a seemingly cloudy, dirty screen.

 

Might anyone have an idea why the picture is lower quality on the 7800? Thank you.

 

[+-^CrossBow^-]

I don't have any immediate answers, other than I've seen it before on 7800s sent to me that had the simple 1 transistor upgrade board in them and was requested to swap it out with a UAV. It is why I only use UAVs for composite and s-video for all the Atari systems at this point. So while I know you have tried a few other boards, you haven't tried the UAV and should give that a shot next. However, while you may not own any pokey games currently, that doesn't mean you won't in the future, or whomever should get that console after you. So I would advise redoing the audio section for proper balance to handle both the TIA and POKEY audio in the future.

 

[stuartmcivor]

Thank you for your input -^CrossBow^-. The UAV mod never really caught my attention as I don't have the ability to use s-video. Now, looking at it harder I see it can also output composite. There is a bit of a cost increase from the small transistor/resistor through hole boards I soldered together myself, which was part of the fun. I do like that the RF can be kept.

 

I can do the Pokey with the parts I have on hand. Run a wire from the north side of C10 to the audio out on my mod board. I am confused as to the source of the other audio channel. Do I keep the source where it is now or this that I found in your comments in other thread...

 

"Just have to lift the north legs of R5 and R6. Attache them together and then solder on a 10µf cap to the center tab of the audio RCA jack. + of the cap from the resistors with the negative lead to the center tab of the RCA. That would essentially replicate the proper mixing between the TIA and external audio line (Pokey), to maintain the volume balance between them and also be compatible with all cartridges."

 

and again here...

 

"- replace the cap you removed at C10... or just jumper across the vias as it isn't that critical

- replace the resistor you removed at R5. It was a 6k8 (6.8k) 1/4w resistor. But only solder in the south lead of it back in. South being the side facing towards the front of the console and North being towards the back of the console in this case.

- Desolder the north leg of R6 (It is directly between where R5 was and the red audio tuning coil). Or clip the north leg as close to the base of the mainboard as possible if that is easier.

- You should now have the two resistors next to each other soldered onto the board at only the south ends of them. 

- twist the two north side leads of R5 and R6 together and apply a dab of solder to attach them together at that top.

- get a 10µf 16v there abouts electrolytic capacitor and solder the + lead of that cap to the two soldered together ends of R5 and R6.

- solder a length of wire from the - lead of the cap and solder the other end of the wire to your center posts on the RCA jacks."

 

This would eliminate the "low volume" from the Pokey channel, correct?

 

Thanks again.

[+-^CrossBow^-]

You are correct. But as you want to keep the RF intact, you actually need to replace R5 and the other resistor you likely removed from the composite upgrade you installed earlier. Then you need get another set of resistors 6.8k and 18k to attach one lead to the south legs of R5 and R6. Then twist the leads of the new resistors together on their north leads and attach the 10µf cap + lead from the twisted leads all soldered up. Run a wire from the - lead of the cap back to the RCAs.

 

But you will need to replace out everything you removed if you want that RF fully working again.