Replace all the 4116 RAM with 4164 RAM

Replacing the odd RAM chip in a Williams set with a 4164 is all well
and good, but what about if we want to replace the whole lot? All the
modification to each chip is going to get a bit tedious... fortunately
there is an easy way to modify the board to use ALL 4164's - it will
not work with a mixture of 4116's and 4164 or with all 4116's.

Let's have another look at the pinouts of the 2 RAM types:

       ___  ___                ___  ___
 -5v -|1  \/ 16|- Gnd    n/c -|1  \/ 16|- Gnd
 Din -|2     15|- /CAS     D -|2     15|- /CAS
 R/W -|3  4  14|- Dout   /WE -|3  4  14|- Q
/RAS -|4  1  13|- A6    /RAS -|4  1  13|- A6
  A0 -|5  1  12|- A3      A0 -|5  6  12|- A3
  A2 -|6  6  11|- A4      A2 -|6  4  11|- A4
  A1 -|7     10|- A5      A1 -|7     10|- A5
+12v -|8      9|- +5v    Vcc -|8      9|- A7
       --------                --------

If you remember, the only pins we needed to re-route were those on
three "corners", the power supply pins 1, 8 and 9. Fortunately for us,
-5v is only used on the CPU board for the RAM, and +12v regulated is
only used for the RAM too. The +12v unregulated is used by the reset
circuit.

So, we need to change the +12v into +5v and the -5v does not need to
be connected at all. If we manage to do this we can plug 4164's
straight into the 4116 sockets without having to modify them.

The best way to do this which requires no modification to the CPU
board is to build a small adapter which plugs in between the CPU board
power conector and the molex connector supplying power to the CPU
board. All pins which are normally used should be wired straight
through except for:

Pin 9 - leave this unconnected (formerly -5v)
Pin 5 - Connect pin 5 at the PCB side to pin 7, which will provide pin 5 with +5v.

It'd also be a very good idea to cover the ends of the unused pins on
the male connector with heatshrink or some other insulating material -
partucularly pins 5 and 9 as we don't want +12v or -5v shorting out on
anything!

That's it! Just plug in the adapter if you're using a board with
4164's and remove it if you're using a board with 4116's or modified
4164's. If you don't use the adapter in the correct circumstance you
*will* fry something!

Thanks go to Clay who did the groundwork for this mod.