Home | Programming | Video Games | Buy My Stuff | Bio | Projects and Repair Logs | Resume and Contract Work

Atari 2600 Composite Video: The Quest For Perfection

Note:  This page deals with taking things apart that clearly state on the label that you shouldn't take them apart, and it involves hot soldering irons and tiny parts.  Should you decide to try any of these circuits, any damage you do to yourself, your Atari, your house, etc. is entirely your responsibility.

Part 1

Once upon a time back in 2005, I realized just how much I'd really like to get out my old 2600 and play some of those games that normally just collect dust.  I've collected video games for eleven or twelve years now (I can't remember, did I first spot those 2600 carts in that thrift store in '91 or '92?) but much of my collection doesn't actually get used that often.  Anyway, I knew I'd have to figure a way to hook the thing up that fit in with my other toys.  Gone are the days of the family 19" console TV which only hooked up to a single antenna.  I've already got a 4-port S-video/Composite switch box cascaded to another 4-port composite-only switch box, between them they handle DVD, VCR, satellite dish, NES, N64, PS1, and a spare port for "whatever".  I have many other consoles, but those are the ones that are played most often, due to 3 particular games:  Super Mario Bros. 3, Mario Kart 64, and Crash Team Racing.  I really didn't want to have a manual old-style switchbox, or anything going in thru RF, for that matter.  A quick Google search turned up several plans for adding composite or even S-video outputs to the 2600.  A not-so-quick expedition into the closet of doom turned up my original and dusty 4-switch 2600,  which I received as a Christmas gift from my parents in 1981.  For a brief moment I considered leaving it in its pristine state... but its an Atari... it was made to be used, not just looked at.  The decision was made, it must be modded. 

I scrounged up 2 chassis-mount RCA jacks, and drilled suitable holes in the back of the 2600.  That was the easy part!  Now all I had to do was figure out what circuitry I needed!  First I read Nathan Strum's comparison of the various Atari composite video mods.  I settled on trying the video driver first, even though he had trouble with it, because it made the most sense to me; just amplify the video signal thats already there, right?  I found a simpler schematic than the one usually passed around, just a textbook amplifier circuit, one transistor and 5 resistors, no annoying bypass capacitors and whatnot.  I found a generic Radio Shack 2N3904 work-alike in my parts cabinet, and pulled the resistors off a junk circuit board.  A few resistor values got modified slightly to accommodate the parts I had on hand :)  I assembled the basic video amp and tried tapping the video signal straight from the input to the RF modulator, but that just didn't work, the signal is too weak.  Alright, what to do then?  I considered pulling the modulator off the board entirely, so it wouldn't load down the signal, but instead I decided to try tapping the video Crackers style: connect LM0, LM1, LM2, and Sync all together and shove them in the video driver.  I got output, but the picture on my TV was just unusable.  No color, and the playfield was horribly skewed to one side.  I was about to try Ben Heckendorn's mod next, but a little more research turned up Saundby's 7800 mod, which came with a nice explanation of how 2600/7800 video works.  Apparently the TIA outputs are digital, and rely on external components to assemble the analog video signal.  Now I was on to something.  The Heckendorn mod uses variable resistors on the TIA outputs, but with the new 7800 mod info, no need for that, we know its just a simple resistor ladder D/A converter so we can use fixed values.  To quote a popular movie, "confidence is high", so I went ahead and hot-glued the little perf-board video amp to an empty area on my 2600's main board.  I also cut the base lead to the sound mixer transistor to clean up the video signal, and removed the big pink sound mixer coil to make it easier to run the wires out of the shield.  Yes, I put the shields back on when doing projects like this, always.  I like to be RFI free :)  At this point the plan was to use the resistor ladder D/A converter idea, and hook it to my simple video amp I'd already built.  I dug in the parts bin and found 3 resistors which were roughly double each other in value, and still in the ballpark of the 7800 mod design.  I soldered these directly to the tap points on the top of the board, connected the other end of them together, and fed in sync thru a diode as shown in the 7800 mods.  I fed this signal to the input of the video driver, and routed the chroma straight from the tap point on the 2600 to the output of the amp, bypassing it, as everyone said it was strong enough as-is.  Now, back to the TV for a test!  Closer this time, but still an awful picture.  I paced around the house a few times and thought about it.  The color was there now, the picture wasn't tearing as bad but still too bad to play a game, some games refused to give any video, and the ones that did still had skew at the very top and bottom.  It occurred to me that it must be a sync problem.  Most of the 2600 mods did not use a diode, and that diode would be dropping 0.6V from a presumably weak sync signal.  Back to the bench, remove the diode, replace with straight wire.  Back to the TV for a test and... SUCCESS!!!  What an incredible picture from my old 2600!!!  A little tweaking of the color potentiometer with Pitfall 2 in the slot (as recommended by most everyone) did wonders.  Actually what was most helpful to me was the Activision "rainbow" logo, I used that to get the colors just right.  One last test before the cover went on... should I put a potentiometer in place of the fixed resistor at the input to the video amp, for tweaking it just so?  I tried jumping a 1K across the 5.6K just to see what would happen.  The brightness increased somewhat, but it also made the picture start to distort.  I decided to leave the fixed resistor in place.  All done!!!  I tried Galaxians and Midnight Magic.  The sharp lines show up with amazing clarity!  I was very happy with the result, for about 24 hours... 

Below is the schematic of what I had at this point in the story.  The 22 ohm resistor was a 15 in the original amp schematic I started with, the 500 was originally 560 but all I had were two 1K's to parallel, and the 100 was originally a 75, no doubt to match the standard 75 ohm impedance of composite video ports. 

Part 2

Ok, so now I had a super sharp picture coming from the Atari, and I played Missle Command, Galaxians, and a couple of other favorites well into the night.  The next morning, I was playing Air and Sea Battle and Ms. Pac, and I started to realize that the colors just weren't very vivid.  In fact, everything was rather pastel.  Well, despite my fiance's insistance that it looked quite nice, I soon had the cover off of the Atari and the soldering iron warmed up.  First thing, I tried injecting the chroma right after the D/A ladder, so it would get equal amplification.  Quite an improvement in the color!  Unfortunately the colors were now smearing a bit.  Especially blue.  At this point the fiance informed me that I am too much of a perfectionist ;)  I looked at the 7800 mod again, as well as doing a Google search on converting S-video to composite... and I noticed that other people have chroma going thru a capacitor before combining it with the luma... hmm, isolate any DC component of the signal, good idea...  so I sent the chroma thru a capacitor to the amp input, things were looking better... just for fun I tried isolating the luma thru a cap as well, and varying the levels of both with potentiometers.  I also tried different capacitor values, the DVD and VCR crowd recommended 470 pF but that resulted in horrible smearing of the bright colors, 0.1 mF worked far better.  Both capacitors made a noticeable improvement, but changing the levels of the signals didn't help, so I ended up with the circuit below as the best result of lots of experimenting.  The blue still smears, but not so badly.  I decided to live with the slight smearing, for the time being, to get those bright colors.  I can play those old games on my new TV and I still think the picture is pretty darn good.  Just not quite perfect.

Part 3

There is no part 3!  I did design a Mark III Raiku Special Composite Mod, using separate transistor amps for luma and chroma, and then combining the signals after amplification, but it failed miserably!  After due consideration, I decided that the second revision of the circuit did well enough, and certainly far, far better than anything you'd ever get with the original RF out connection.  This project is done!

Did you find this information helpful?  Or did you find something that was just plain wrong?  Or perhaps even *gasp of horror* you found a better way?  Let me know! 

Comments?  You can email  me here: linermonkey at yahoo dot com