Can't upgrad rev 5 A500 to KS 3.1

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perhaps you need to install the missing resistor pack near the CPU.

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I just tried this again myself as I too had a grumpy A500 rev 5 that would not take a KS3.1 ROM.  Mechy's advice is correct, there IS a difference on how the jumpers are done.  I tested the usual Pin 1-31 and 31 to 21 mod just now - did not work.  So, I then did the one Mechy described above and it works.

Also, on this machine, the Motorola 68000 would not work - ROM would start up and then guru.  Replacing the Motorola with an ST68000P8 clone and no problems.  Perhaps they have a lower tolerance with the bus noise.  The 68HC000s are also much more forgiving.

Anyway, try Mechy's instructions, they worked here just now.

iirc, resistor pack helps with bus noise.
anyway, OP should check out this old thread: http://www.amiga.org/forums/showthread.php?t=69292