> I would
have thought the 5150 upgrade involved removing the
existing RAMs
> nad rpelacing them with larger-capacity chips, more like what I did to
> my HP9816.
I mentioned in passing that this was similar to what I did to my 9816..
That machine came to me with 32 4164s on the logic board, for a total of
256K RAM.
After figuring out the circuity, it was clear that pin 1 (the extra
address input on the 41256) was already bussed between the ICs. Moreover,
there was a plasce o nthe PCB for an 'extra' 16 pin chip, whcih turned
out to be for the multiplexer for this address line, and a place for the
series terminating resistor.
There were also some traces to cut and jumper, but in all cases, the cuts
were little traces between jumper pads (i.e. one position of the jumper
was already in the etch), the juper wires had to be soldered between
adjacent pads. These mods changed which address line selected between the
2 backs of the ICs, and changed how many addres lines were used in the
overall RAM select decoder.
Anyway, that time I did have to desolder all 32 RAMs, and there wasn't
height for sockets, so I had to solder in the 41256s. Added a 74F158 mux
(a 74F157 would do as well, but the former was in my junk box), did the
cuts and jumpers, and now have a 9816 with 1M on the logic board.
-tony
