31 May
2005
31 May
'05
5 p.m.
Provided the
screws have not been lost, the bit of paper is still
around/readable, etc. The 'safest' place for those screws is in the
original holes.
What about all the notes you've made of the modifications you've effected on various machines you own? Are they with the
machine? Are they secured
to it so that it won't get separated? Will the ink remain readable
forever? Will the paper remain intact forever? If not then wouldn't it
have been safer for you to have never made those modifications in the
first place?
Yes it would. But you're forgetting something. I _use_ my classic
computers, and therefore make modifications to make them more useable
while preserving the basic design of the machine (what I mean my that is
that I will hack a memory board to use larger-capacity ICs, I will make a
mod to use a more modern replacement chip [1], etc. What I won't do is
totally change the architecture of the machine, e.g. by replacing the
guts with a PC running an emulator).
Yes, if I was only interested in preserving the machines, I'd never run
them, I'd never change anything. But alas I am interested in using them too.
However, as I keep on saying, I still want to do as few modifications as
possible. Yes, I will happilly do the 640K mod to a PC/XT, including a
5155 portable. In fact I did this mod to my 5155 (expansion slots are
tight, not much software will run in the standard 256K). That's a useful
mod. But there is no good reason to change those damn screws!
[1] for example, an HP9810 repair. I did eventually get the right IC (and
put it in place of this mod), but this got the machine working again:
Replacing the 74H52 IC on the memory timing PCB (09810-66522)
-------------------------------------------------------------
Well, have you ever tried to get one :-) However, _on this PCB_ it can be
replaced by a 74LS51, suitably wired.
Remove old IC carefully, in case it's still good.
Bend out all pins of a 74LS51 apart from 1, 7, 14. Put this chip in place
of the old one. solder 1, 7, 14 to the PCB. Solder wires (wire-wrap wire
is ideal) as follows :
PCB 2 -> IC 13, IC 12
PCB 3 -> IC 11
PCB 4 -> IC 10
PCB 5 -> IC 9
IC 8 -> IC 2, IC 3, IC 4, IC 5
PCB 8 -> IC 6
mu(26)/ -------|\
| \ +---|\
ALU(0) ----+---| >-----+ | | >----+
| | / | +---|/ |
+---|/ | | +---)\
+---)\ | ) >o---- TRegSin
) >o---+---|\ +---)/
mu(25+24/)/----|\ +---)/ | | >----+
| \ | +---|/
T(0)-----------| >------+
| /
mu(26)---------|/
-tony