Final blog Modding of an Atari 7800

Atari 7800 composite mod

First, sorry been a while since the last post on the 7800. I got a series of tech contracts that are taking up all my time. This and also my Atari 2600 Parsec program I have been wanting to get back to (among other things). But, enough of the excuses. Now on to the mod.

The Atari 7800 mod enables the 7800 to transmit a, much sharper, composite single instead of it’s standard, fuzzy, analog signal. The mod is a rather simple and inexpensive upgrade and is well worth the trouble.
Here is the inside motherboard of a 7800.


First thing I did was remove some stuff. I removed the analog box and the channel switch. It is optional to remove these parts but, in my mod, I was going to use these places for other things.
See the silver box in the upper right. It is the analog circuitry and will be useless. The analog box is connected to the motherboard via the small board jutting out of it in the front and has the four white connectors. Desolder the four connectors from the board. The next thing you need to do is flip the motherboard over and there are four points where the analog box is soldered to the board. Theses are not connection points, Atari just used these points to attach the analog box to the board. Desolder these points. The analog box will now come off, sorta, easily. Making sure you have all the points desoldered was the hardest part.
The next thing I removed is the black switch that is next to the analog box. It’s the switch for channel selection and is also useless. There are three points under the switch that need to be desoldered. Flip the motherboard over and desolder these points and the switch comes off easily.
Here is what it looks like with the analog box and switch removed.


Notice the blanket the motherboard is setting on. I set it on fire by absentmindedly putting the soldering iron on it. I would not recommend doing this.
After you have removed the hard stuff it’s time to clip out a couple of resisters. Depending on who’s blog you read I have seen anywhere from two to three resisters need to be removed. I, personally, found that only two need to be removed. The square black boxes show where the resisters are. Just clip them out. Ignore the wires at this point.


Now that you have the old stuff removed it’s time to add the new stuff.
Below is the little composite board I bought from a seller on ebay. The board is about the size of an American quarter. There are many sellers on ebay who sell this, or a version of this, board at about $10 to $20. My package came with the composite board, a piece of double sided stickem and the wires pre-soldered onto the board.


The left side of the board is the input from the motherboard. The right side is to the output connectors that to the (yet to be created) plugs to your TV.
Notice on the right the +5v (red), audio (green), video (yellow) and the ground (black). On the left is the audio out (green), video out (yellow) and ground (black). The 7800 is mono only. This is a change from the 2600 which, could be stereo. That’s why there are only one audio out instead of two. As you see the circuitry on the board is fairly simple. The 7800, as well as the 2600, had the composite already built in and is fairly easy to tap. I guess it was just the times that Atari didn’t elect to include a composite connection on the back of the 2600 and 7800. Not many people back then had TVs with composite connections.
The connections to the motherboard are fairly straight forward, except the connections that are needed to support the cartridges that have a the Pokey sound chip, such as Ballblazer. There are only two cartridges that were released with the Pokey chip but I wanted to have a complete setup. I will go more into the Pokey setup and the problems I had later.
Here is a picture of the connections that need to be made on the motherboard, excluding the ones for the Pokey chip.

Here is were the Ground (black), video (yellow) and 5 volt (read) are soldered to the board from the left side of the Mod board (picture above).


The primary sound is soldered at this spot mark with the black box. Notice where the other cables were soldered as well.


The 7800, even in the 7800 enhanced mode, still uses the sound chip of the 2600. To increase the sound quality of the games a POKEY chip from the XL line can be included in the game cartridges. If you just attach the main sound line, shown above, all the games will have sound except the two games that used the POKEY chip. You have to attach another point on the board. This point DOES NOT go through the mod board but is connected directly to the external sound connector. The black box below shows where the solder point for the POKEY line is.


Notice that it’s one of the points where you removed a resistor. For some reason, finding where and how to attach the POKEY line was almost none existent on the web. I assume that not many people bother with attaching the POKEY line. NOTE: the sound from the POKEY games will be a bit quieter. One fix that some do is put a resistor on the main line to make it quiet too. I didn’t do that. I didn’t want to hobble the 98% of my games for the 2%.
I merge the POKEY line and the main sound out line from the mod into one line. I also split the ground wire to go to each, the video and the sound separately. I tried just hooking the ground to the video then jumpering it to sound and I got a hum. I found splinting the ground wire to each made the hum go away.


You can see now why I removed the analog box and the switch. I wanted to use that place for my connectors.


Here are what the connectors look from the back. I put them there to cover over one of the cracks.
I have got to say the picture quality is stellar. The build is also a simple one, except maybe removing the analog box.
Since removing the analog box and switch is optional and there are other place you can put the connectors, I would give this project an advanced novice grade.
NOTE: All the information is supplied with no guarantees. Follow the instructions and things will go right. If you don’t they won’t. Either way, I’m not responsible to what you do to your 7800.
That’s it. Happy tinkering.
Field Mouse


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