7800 Composite Video Mod problem - Video distortion / Dim

[Barakandl]

Hi AtariAge

I recently got my classic consoles out after sitting for many years. I decided to do the video composite mod on the 7800. I got the kit from vintage gaming and installed per the website instructions including the optional audio step with resistor.

 

My problem is the video looks really bad. There is a pixelation pattern across the screen and everything looks really dim like the signal is too weak. I hooked up my NES and it looks great on the same TV and same composite wires. I am going to open the console back up and check my work, but i think i got it installed correctly. The ground for the RCA jacks traveled through the small PCB included in the kit, should i tie that to a master ground on the PCB?

I suppose it could be a problem with the console itself. I am highly skilled at PCB rework, so i can change parts / test if i know where to look.

 

Thank you

Andrew

[R.Cade]

I would contact the support where you bought the mod- maybe they have some tweaks to improve the picture.

 

I usually do a self-made mod similar to this from the RF box input signals, and I recall I had to do a lot of work on the 7800 board to get the brightness up and colors richer. By default that signal is very weak and has very low bland colors without some work. I'm not sure that transistor amp has enough gain with those bias resistors, personally.

 

It could be that many 7800's are different also... and televisions.

Edited March 11, 2016 by R.Cade

[Barakandl]

I sent an email to vintage gaming.... i should say again, i don't think the kit is bad or anything... it very well could be my 7800 itself. It hasn't been tested in probably 10 years.

 

So if the 2n3904 isnt amplifying enough, how about a darlington NPN like 2n6427?

 

Do you have any links or information about what you say about getting the brightness and colors richer? A schematic diagram or service manual would be nice.

 

Thank you

Andrew

[R.Cade]

It is enough amplification if you bias it hotter - you can do this with a pot so you can adjust until you get less bleeding. For the color changes, you'd need to search the forums for my comments. Off the top of my head, you connect pin 6 and 9 on the TIA with 200-800 ohm resistor to increase chroma. There is a similar adjustment for the MARIA, but it's not the pins that it should be (seems like the official known pinout has it one pin off).

[Barakandl]

I uploaded a video to youtube that shows my problem.. It looks like a chess board pattern where half the pixels light up white.

 

https://youtu.be/1jidZxA-fX4?t=13s

Anyone recognize what causes my distortion? Im not sure if it is analogue distortion or somewhere in the digital side.

Edited March 11, 2016 by Barakandl

[R.Cade]

That's bizarre. It does that on another display, such as CRT?

[Barakandl]

I figured it out... i overlooked a step in my haste that had me cut out a resistor. I didnt notice it until I read a different installation guide. Stupid mistake on my end.

 

It looks OK now. Playable for sure. I still have some slight ghosting type effect and the colors don't seem to be that great, but I think I am happy enough for an Atari 7800.

 

Now to figure out if it is easy to open up a donor 7800 and change the EPROM / masked ROM out for a homebrew one. I have all the blank EPROM sizes on hand for pinball machines.

 

Thank you

Andrew

Edited March 12, 2016 by Barakandl