Amiga 500 Rev 8A 1MB Chip Ram Expansion

From day one when I got this board, I wish to expand it to 1MB of Chip RAM and turn it from A500 to a fully functional A500+. I couldn’t find any believable source of information about ram expansion and proper jumper settings. On famous websites, You can find numerous posts about memory expansion. I found them really messy and hard to understand, maybe it’s just all about me. I was postponing this mod for a while until I will have a wider spectrum and all parts needed in my inventory. I was collecting proper components, like sockets, memory chips and a microcontroller witch allows to address this amount of RAM. In meantime, I was still looking for a proper base knowledge and/or a clear manual. When all parts have been collected, the fun begins.

 

The picture below illustrates affected areas on A500 Rev 8A for this modification.

For this modification I used:

 

4x DIL201x DIL16  74F139 PC 8843 Z

and 4x RAM DIP20 1M 256Kx4 CMOS RAM 5V.

 

 

Desoldering vias for new chips is the first step. I was using a desoldering gun and a lot of flux. The trick to removing all solder in one go, from soldering points, even on a ground connection is to use flux on both sides of the bord. After flux has been applied gently move the tip of the solder sucker to the motherboard, wait 2-3 seconds, and hit the trigger. Don’t wiggle the solder sucker tip too much, you will introduce scratches on the traces around the soldering points. I’m Using 480 deg in C when I’m desoldering, but I don’t recommend this for beginners. You can destroy vias and traces. If You don’t have a solder sucker You can use a regular soldering iron and a “got wick”. I hardly recommend using the medium or big beveled tip and a lot of patients. Don’t rush Yourself, do it one by one, with a proper tempo.

 

After all solder points have been clean its a time to mount sockets for new memory expansion and the U32 chip witch is a 74F139 PC chip. It’s a fairly easy process, just position a socket on the board, solder 2 pins on opposite sides, then do the rest. 

When all sockets are in the positions populate them width proper components. To complete the modification we need to change the settings of the JP4A, JP4B, JP3, JP7A, and JP2 jumper. 

 

JP2 is a jumper that changes the position of expansion memory in the memory map between slow RAM and chip RAM. You need to remove the existing bridge from pad 1-2 and creates a connection from pad 2 to 3.

 

JP3 jumper is used to choose whether we use the memory installed on the board or the extension in the slot. On my Amiga motherboard pads have been bridged in a vertical configuration (1-1 and 2-2). The proper configuration for this modification is to create a horizontal connection of those pads in a 1-2 and 1-2 configuration.

JP7A is used to enable or disable the memory slot extension. When JP2 is in a 2 to 3 position connections in JP7A are omitted, so it’s clear that we just need to remove the existing bridge and level a pad opened.

 

JP4A and JP4B. Desolder them, and live them open, clear.

 

That’s it. Before applying power, inspect all modified solder joints in this modification and clean all remaining flux on board. The best way to do it is to use an ultrasonic cleaner with proper fluid. After 10 to 15 minutes dip the board in isopropanol to remove the remaining water. Let the motherboard to dry, then You are ready to play awesome games with a 1MB Chip ram on board 🙂 Have fun. What next? Clock?

A500 with populated Ram in sockets already.

A500 with populated Ram and 74F139 PC Chip Installed.

1MB Chip Ram Extension Gallery

Retrospection Logo Landscape

2 thoughts on “Amiga 500 Rev 8A 1MB Chip Ram Expansion”

  1. perfect, i have been looking for something conclusive and not just 1/2 an opinion on a message board. The zoom in motherboard pictures are gold.

    1. Thanks MT 🙂 just for clarification: to adress more than 1MB BigAgnus must be installed. You can recognize the BigAgnus by the type number
      “8372” on the chip.If the number “8371” is printed instead, old FatAgnusis present in your Amiga. Than You can adress only 1MB of chip ram, in my opinion. If you will install expansion card that exceed the total allowed memory will not work or it will freeze your Amiga. Like A580 Extension. Its needs to have GARY-Adapter to work as well.

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