Task: Upgrade the Onboard Memory of an Amiga 500 to 1M of Chip RAM
Needed: Four 256×4 DRAM Chips, Four 20-Pin DIP Sockets, Four 100nF Capacitors, Soldering Iron and Solder
Time: 30-60 minutes
Introduction
I have previously posted about purchasing or building an Amiga 500 16-bit computer. The Amiga 500 came stock with 512K of onboard RAM that was accessible by all the main ICs. As such, this memory is referred to as “Chip RAM” because all the chips had access to it. Importantly, this memory was used by the Agnus chip which had direct memory access (DMA) for graphics. Unfortunately, the older Agnus chips could only access 512K of chip RAM. The later motherboard revisions (e.g. 6A and later) featured a “Fat Agnus” chip from the enhanced ship set that could access 1M of chip RAM that was needed for more advanced games and software.
Kudos to Amiga for including empty spots on the motherboard for an additional 512K of chip RAM to add to the stock 512K for a total of 1M. It is relatively easy to solder in four new 20-pin sockets and drop new 256×4 RAM chips in along with corresponding 100nF capacitors next to each socket. The only other thing that needs to be done to complete this upgrade is to set a jumper.
Instructions
The first step is to source the sockets, RAM, and capacitors. I purchased my 20-pin sockets (n=10) from Amazon for $7. I purchased my chip RAM (n=4) from eBay for $17. I went with the HYB514256B60 DRAM chips made by Siemens. For the ceramic capacitors I went with a kit I purchased on Amazon for $13 that included the 100nF capacitance model. In all, the upgrade cost me about $30. Note that some sellers offer the sockets and the chip RAM ICs as a kit. The TI TMS44C256-10N is another compatible chip RAM you can look for.
The second step is to solder in the 20-pin sockets noting the notch orientation toward the back of the board and matching the existing RAM. You will very likely need to first remove the solder from the holes (see below) using a desoldering gun or soldering braid. Same for the holes where the capacitors go. I first reflow the holes with some new solder to loosen everything up for a clean removal with a desoldering gun which is very effective.
You are now ready to install the sockets. A common strategy is to start by soldering only two pins from the top and bottom of the socket to make sure the socket is flush with the motherboard before taking the time to solder the rest. This prevents a more time-consuming desoldering session if it goes in wrong. Here is the soldering station I use.
The third step is to seat the chip RAM noting the notch orientation toward the back of the board. Check to make sure no pins are bent or out of the socket.
Fourth, it is time to set the jumper. Identify the JP2 jumper near the Kickstart ROM (IC shown to right below), CPU, and a clock crystal (silver component shown below). You must first use an X-acto knife to cut the connection between the middle pad and one to the right (facing toward the front of the board). Be careful not to let the knife go too far and cut those traces as I almost did (see scratches). Use a multimeter on continuity mode to make sure there is no contact. Then apply a solder blob to connect the middle pad with the left one (toward the back of the board).
Finally, reassemble the Amiga and boot to Workbench. You should now have 1M of chip RAM. I included a screenshot of my Amiga 500 booted into Workbench 1.3 showing 1M of chip RAM (labeled graphics memory) in the header. Ignore the second huge memory number (labeled other memory) to the right. That comes from my PiStorm and is RAM allocated from a Raspberry Pi. Will cover that later. Also shown below is the chip RAM memory summary from the Sys Info program.
Comments
This is an easy upgrade if you are comfortable with a soldering iron and will allow you to play some games that need more graphics memory.
Note that an alternative approach is purchase a RAM expansion such as this one for the trapdoor. This also requires setting a different jumper. I wanted to get my RAM onboard.
