IBM 5155 analogue display fault

[Tony Duell]

> 
> On Fri, 2005-05-27 at 01:56 +0100, Tony Duell wrote:
> > > Oh heck, yes. I went through every alan key and torx bit I had trying to
> > > find something that would fit the former. Ended up with a good ol' pair
> > > of long-nosed pliers, and those bristol spline screws are getting
> > > replaced with something a little more conventional on reassembly!
> > 
> > NO!! Those screws are part of the machine and should be kept. I feel very 
> > strongly about keeping odd fasteners, etc in machines, to replace them 
> > with anything else does change the character of the machine IMHO. Surely 
> > you can buy a set of Bristol Spline keys (I think Farnell do them...)
> 
> It's the sort of tool that not many people have though. Why make life

That is not an excuse. Anyone working on IBM 5155s (or for that matter 
Friden Flexowriters, which are stuffed with setscrews needing Bristol 
Spline drivers) should have a set. 

> difficult for the next person who comes along and has to fix the
> machine? The only reason I can think of for using those screws is that
> people were expected to go inside the case to swap cards every so often,
> but they weren't supposed to be poking around in the display section.

That was exactly the reason (and the reason for using tamperproof Torx in 
the PSU). Customers were only supposed to have normal tools, Failed 
Servoids hat the Bristol spline tools as well (but couldn't therefore 
open the PSU case). 

According to one manual I've got, if an IBM service engineer saw that the 
tamperproof torx screws on the PSU had been replaced with normal screws, 
or had been moodifed to allow a normal driver to get them out (e.g. 
removing the cerntal pin or cutting a slot across them), said engineer 
was supposed to replace the entire PSU (and charge the customer for it!).

> 
> That requirement's gone now; if someone owns one of these beasts these
> days then they're presumably just as likely to be inside the display
> section to fix stuff as they are digging around in the rest of the
> innards; doesn't it make sense to use the same type of screw throughout?

My view has alayws been to keep the machine as original as possible. This 
doesn't mean not repairing a machine, it doesn't mean not using modern 
components for replacements. But it does IMHO mean not replacing 
components unnecessarily. In particular, I always replace the minimum 
number of parts (e.g. a chip and not the complete PCB), and not replacing 
screws with different types.

I have a set of Bristol spline keys, so I don't really worry about this. 
I also have Torx, Allen hex, System Zero, and so on... I've found just 
about all of them necessary for classic computer repairs.

I would have hoped a museumwould also want to keep machines as original 
as possible, but then again my views on Bletchley Park are well known...

> 
> If the bristol-type screws were used externally or in a prominent
> position, or exclusively throughout the machine I think I'd agree with
> you, but the requirement to have a special tool to get to the display
> circuitry is long gone.

Well, I don't think a Bristol Spline key is any more special than, say, a 
Torx driver...

> 
> Having said all that, I tend to take the same view as you when it comes
> to my classic car, and try and use the right bolts / screws etc.

What have you got?

> everywhere even if it's somewhere that can't be seen - but then it
> doesn't use oddball fastenings that not many people have the right tool
> for :-)

Well, British classic cars tend to have Whitworth bolts all over them, 
and not that many people have a reasonably complete set of Whitworth 
spanners and sockets any more.

-tony