HP9816 RAM upgrade

[Tony Duell]

> >> 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