Upgrade an Atari 400 to 48K of RAM (1-2 hours)

Task: Upgrade an Atari 400 to 48K of RAM

Needed: 48K/52K RAM Card for Atari 400, soldering iron

Time: 1-2 hours


As I have previously posted, the Atari 400 is a stylish computer but limited by RF video, a membrane keyboard, and only 16K of RAM. While there are many 16K cartridges which will run on the 400, there are a number of 48K games made for the Atari 800 which are not compatible with the 400.

The good news is that the Atari 400 is easy to upgrade to 48K of RAM thanks to the 48K/52K RAM Card developed by Jürgen van Radecke (thank you!) and sold in the U.S. by Gavin Haubelt (thank you!) from the Vintage Computer Center. This card makes it possible to upgrade to either 48K or 52K by setting a jumper. It also includes a EPROM which allows you to burn a new operating system. You can toggle the new OS by setting a jumper. This is important because some 48K games will not run on the 400 because they need access to features only found on the later XL or XE OS ROMs.

This upgrade is relatively straightforward requiring the soldering of two wires and the installation of the RAM board which is plug and play.

Atari 400 RAM Upgrade
Atari 400 RAM Upgrade


This is a pretty easy mod which should take an hour or less if you have a workbench with everything set up and ready to go. This easily took me less than hour as I now know my way around the 400 after doing the Super Color CPU Card installation.

The first step is to read the online instructions (PDF) created by Jürgen. These instructions are very good. Consider my instructions below as secondary or complementary to those provided by Jürgen. Hopefully a few of my tips will augment the mod process making it a little easier. This YouTube video is also very helpful to watch before you begin.

The second step is to navigate your browser to the Vintage Computer Center and order the 48K/52K RAM Card for about $40. You will receive the card and a ribbon cable. This mod requires two wires to be soldered on the backside of the main circuit board.

The third step is the preparation of your workspace. You will need a soldering iron to connect three wires along with screwdrivers, needle nose pliers, etc.

The fourth step to disassemble the 400 case. Once you have removed the four screws from the bottom, flip the 400 over and remove the top. Note that the cartridge lid needs to be open to get it off! Lift the top of the case very gently as the keyboard ribbon is attached and will provide resistance. Gently lift the keyboard ribbon from the circuit board. Unlike the 800XL, this keyboard ribbon has delicate pins.

The fifth step is to remove the power board on the right side of the 400 (see photo below). You will first need to remove two screw on the left side of the power board. You will also need to detach the tan bar emanating from the cartridge door. This is what turns the Atari on when the door is closed. Remember the silver switch it connects to on the power board. We will use this switch later for testing. Once those are remove, gently rock the board back and forth to detach it from the pins holding it to the main circuit board. You will need to unplug the RF video cable from the power board to fully remove it. Set the power board aside for now.

The sixth step is to unplug the speaker from the bottom left side of the main circuit board. You can leave the speaker in the based of the case until reassembly.

The seventh step is to remove the main circuit board from the base. It should lift out easily as nothing is holding it the base at this point. Set the base aside until reassembly. You should now have the main circuit board in from of you with the metal Faraday case (i.e. RF shield) enclosing much of the board.

The eighth step is to remove the Faraday cage. Flip the unit over and remove the screws holding the bottom to the top. Set the bottom piece and top piece aside for now. You will need them later for reassembly. The main circuit board will be exposed as shown below.

Atari 400 Circuit Board with Labels
Atari 400 Main Circuit Board with Labels

The ninth step is to prepare the circuit board for soldering. First, remove the CPU card and the RAM card as illustrated above. Note the chips face the back of the Atari. These can be lifted out of their sockets with gentle rocking and pulling up. You can set aside the RAM card as it will replaced with the new card. You can now flip the circuit board over and find the pins to solder the two wires to below. Once the wires are soldered the circuit board can be flipped over to the top side.

Soldered Wires
Soldered Wires

The tenth step is to remove the logic chip (middle of the three) shown on the main circuit board above. There isn’t much space so you will need a tool to wedge it out. You can then plug into this socket the exposed pin side of the ribbon connector provided in the kit. Not that I needed to remove the plastic cartridge guide to make some room to get it in there. It is a little tight. As shown in the figure below, the red wire in the ribbon is toward the front of the Atari and away from the RAM board.

The eleventh step is to set the jumpers on the RAM card to indicate 48K or 52K and which OS ROM is to be used. The jumper options are shown on the back of the RAM board as shown below. I left mine set to 48K and the original OS ROM on the CPU card.

The twelfth step is to plug in the CPU card and the RAM card as shown above. Once seated, plug in the other end of the ribbon cable into the connector on the right side of the new RAM card with red wire side of the ribbon facing up as show below. Reinstall the plastic cartridge guide if removed.

Installed Atari 400 RAM Upgrade Card
Installed Atari 400 RAM Upgrade Card

Finally, we are ready for reassembly. Insert the circuit board into the top of the Faraday cage. Once in you can then replace the paper and screw the bottom piece to the top piece. Flip the unit over and reattach the power board and screw it down. Now reattach the speaker and the keyboard. I recommend removing the keyboard from the top part of the case before trying to plug it back in with the delicate ribbon connector. It is a little awkward and be careful with the pins. Once the keyboard is in it is easy to reattach it to the case. Reattach the top of the case remembering to open the cartridge door first. Close the cartridge door once the top is on and then seat it in the base. Flip the unit over and screw in the four case screws. That is it!

It is now time to test the Atari 400 to make sure it is reading and writing all 48K of RAM. There are two ways to do this. First, I have previously posted about using the SALT diagnostic cartridge to test RAM. Second, find a 48K game which runs on the Atari 800 (i.e. not a newer XL or XE game). I chose M.U.L.E. as shown below. Both tests passed! Note the crisp video on an LED screen. This 400 also has the Super Color CPU Card with UAV composite video converted to HDMI with a Retrotink.



This is an easy mod and well worth it if you like using your Atari 400 as a game machine. This makes it possible to load 48K games such as MULE.

Note that I recommend doing this mod before you do the Super Color CPU mod as the composite cable is attached to the circuit board extending through the Faraday cage. I left this attached but it made the soldering a little awkward.