IBM 5155 analogue display fault

[Tony Duell]

> > 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

They are, of course, in the appropriate service manuals, etc.

> 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