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Henryk Richter

Amiga 500 Restauration

Revitalizing an Amiga 500

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While seriously on a retro-trip I thought it to be cool having an Amiga 500 again. I found a well treated machine on EBay that even came along with an original Commodore A590 disk drive. Needless to say, a machine from the 80s might well be in good optical shape but a lot can happen in a time period of nearly 30 years.

The first thing I noticed after switching on the machine and loading old demo and game disks was the distorted sound, along with a missing color channel. At least I was grateful for the still working disk drive. Re-soldering the SCART-cable solved the video issues.

In terms of audio issues, I suspected a broken -12V line. Some measurements confirmed that the -12V line was indeed silent. So I opened the PSU and replaced the transformer based PSU electronics by a switching power supply. Guess what I could fit into the Amiga PSU enclosure: the PSU from the old Cisco 2501 router. That one provides sufficient power rating and the needed voltages. As it turned out, the PSU was not the main culprit. Before connecting the power supply to the Amiga, I routinely checked the power connector and found a loose wire in it. Anyway, the modified PSU works well.

For people who buy an Amiga without further accessories, it might be tough getting software onto it. PC floppy disk drives are becoming scarce and usually can't write Amiga disks anyway. On the other hand, one needs at least 1 DD floppy to boot up an Amiga to get ready for other data transfer means (via parallel or serial ports). A more convenient option might be the purchase of an inexpensive Gotek floppy emulator, along with the installation of an Amiga compatible firmware.


1 MB ChipRAM conversion: Agnus with isolated Pin41, Jumper at JP2

The board came with the original 8371 Agnus chip. Furtunately for me, I still had a 8372A among my old spare parts. The conversion for Rev.5 boards requires a 512 kB trapdoor expansion. In my case, I had an original A501.

The 1 MB ChipRAM conversion consists of three steps (you might want to look also here for instructions with exceptionally good pictures):

  1. Swap Agnus chip for a 8372A (isolate Pin 41 beforehand for european machines to force the chip into PAL mode)
  2. Change JP2 (between CPU and ROM socket) from bottom to top by cutting the trace between the two bottom pads and connecting the upper two pads
  3. Disable "ExRAM" line on 512kB internal memory expansion (cut JP1 on A501)

As you may see from the pictures, I actually converted the JP2 pads into a true jumper. For this to work, I just used three chip socket pins which I soldered directly to the pads. This way, I can easily revert to 512kB Chip + 512 kB Slow RAM.