2MB RAM Amiga 500 revision 3 capacitor mod

Information

Amiga 500 revision 3 motherboards suffer from poor signal quality and have no filtering on the clock line which goes to the memory expansion connector. This memory upgrade requires a modification to make it function in a revision 3 Amiga 500.

Commodore improved the layout of the PCB with revision 5. The cap mod is optional for revision 5 computers and may improve reliability if your computer is unstable.

Commodore again improved the PCB with revision 6A and added RC filters to the system clocks. No cap mod is needed for a revision 6A or later machine.

The fix for revision 3 is to add the filter Commodore installed on the 6A and later machines. It is a simple resistor and capacitor (RC) filter. The memory board can be easily modified to include this filter without modifying your Amiga.

Revision Cap modification
3 required
5 optional
6A and later none

 

If you have a revision 3 Amiga and are unsure about performing the modification yourself then contact me before you buy. I can arrange a board which has already been modified for revision 3 compatibility.

Mod

  1. Cut the track labelled and circled on the bottom of the memory board.
    2MB-rev3_mod_cut.jpg
  2. Fit and solder a 47pF ceramic capacitor in the C7 position.
    2MB-rev3_mod_c7.jpg

Technical FAQ

Q: What does the mod do?
A: The mod adds and RC filter to the _CCK system clock line. This cleans up the signal and crucially delays the clock so the timing matches that of a revision 6A.

Q: I have a revision 3, what happens if I don't do the mod?
A: Your revision 3 Amiga 500 will likely not boot. If it does then you may suffer memory corruption.

Q: Do I need it on a revision 5?
A: Revision 5 will work with or without. Perform the mod if you have any reliability problems.

Q: I have a memory board with the cap mod done. Can I move it to a rev 6A or Amiga 500+?
A: The clock signal will be delayed twice (by the Amiga and by the cap mod). This is not recommended but in my tests it functions. If you have reliability problems try reversing the cap mod.

Q: What problem does the bad timing cause?
A: The poor clock signal interferes with the DRAM memory refresh cycle so the memory contents become corrupt. This causes the computer to glitch or crash.