chroma/luma output for 4 switch 2600

[+johnnywc]

Another quick question: is this modification Chris Wilkson's mod that I've heard such good things about? Does anybody know?

 

Thanks,

[jsoper]

I've done it a couple times now and it looks great on a Commodore 1702 monitor (don't have an S-video tv handy). Just mounted RCA jacks onto the 2600 and used standard RCA patch cables to go into the 1702 chroma luma jacks.

 

Tried hooking luma and chroma together to make composite but the edges had a smear to them. Experimented with different capacitor and resistor values on the chroma output but nothing helped.

 

Made one 2600 with chroma and luma going to the same transistor and it looked decent. So I think the issue involves the two transistors interacting.

 

One thing that helps is to keep the signal paths short. I clip my luma and sync resistor leads down to 1/4 inch and solder them standing up on the board and join the other sides together right there. Then you only need four wires for video: luma,chroma,vcc,gnd. Clip the base lead of that one transistor that used for audio mixing or it causes interference.

 

Oh yeah, the FAQ may have some luma resistors switched. Just install them in the same order they are on the board, smallest to biggest resistance. Try just one luma resistor without chroma and make sure you get a b/w picture with that first

 

Another thing is that it looks pretty good even if you don't use a 4050 buffer. If you're modifying a 6-switch or junior it already has a 4050 so you can tap onto it.

 

You're right, those other video mods aren't very good. I wish people would take them down.

 

BTW, I don't have any pictures to post (someone is going to ask for them). John

 

[ 01-29-2002: Message edited by: John Soper ]

[jsoper]

Sorry, just a couple more things than I'll shut up

 

The chroma luma mod was developed by studying the circuit on the atari 8-bit boards. So you know it should be good.

 

The 4050 is just a standard digital buffer IC. The FAQ shouldn't refer to it as a video buffer. The transistor is already acting as a 10x impedance buffer (the 2600 sees roughly 750 ohms instead of TV's 75 ohms) so I still don't know why it's needed.

 

A similar chroma/luma circuit works for the 7800 and 5200. If you block the luma transistor input with a capacitor, you can use it for an intellivision and colecovision.

 

Don't know the circuit on the Wilkonson mod, but saw it working last CGE. It is a small board with a TIA IC attached to it. The board plugs into the empty TIA socket. I don't know how it would work in a junior, the TIA isn't socketed and the case doesn't have much vertical space. Haven't heard anything about it in the six months since.

John

 

[ 01-29-2002: Message edited by: John Soper ]

[+johnnywc]

Has anyone tried this video mod described in the FAQ using the CD4050 video buffer? I already did Ben's mod to my 4-switch but was not pleased with the results. The picture seems to bleed and have ghosting problems and the video signal is too weak, requiring amplification through a VCR (ugh!). It appears this new mod, although more complicated, amplifies the video signal and even uses a video buffer for a clearer picture. It also provides separate leads for luma and chroma (for S-video out).

 

Before I take this on, I was hoping someone else on this board had tried this and could confirm whether it was worth doing. Also, I had a few questions on the schematics and combining the luman and chroma outputs to a composite signal.

 

Any help would be greatly appreciated. I plan on posting my findings (with plenty of pics and step-by-step instructions) once I'm finished.

 

Thanks,