🚀 go-pugleaf

I would rather doubt a MMU chip problem.  It is MUCH more likely
that you are seeing ringing and other noise problems.  A good way
to check for this is to run a decent memory tester program when
the board is set up for 4MB, and see where it gets errors, and what
the patterns of error bits are.  If you get randomness, it is almost
definitely ringing or other noise problems on the dram control lines.

One example to show this... Recently I upgraded a Rev. E 520 ST
board to 4 megabytes using the "New World" board.  Initially it showed
exactly the same symptoms you got; it worked fine at 2.5megabytes,
but got memory errors at 4megabytes.  I eventually got it to work
at 4 megabytes by shortening 3 of the interconnecting cables to
about the minimum workable length (about 9 inches -> 6 inches),
added inline 68 ohm resistors to the RAS/CAS lines for the upper
bank, and an inline 33 ohm resistor to the MAD9 line from the MMU.

dgc