On Fri, 21 Oct 2005 01:26:14 +0100 (BST)
ard at p850ug1.demon.co.uk (Tony Duell) wrote:
Somebody earlier claimed that IBM didn't
bother with termination on the
disk drives in the PC family.
I've just looked in the 'Hardware Maintenance and Service' manual for the
PC/AT. This is the only HMS manual I have (I don't normally bother with
I know you disagree with a 'boardswapping' culture deeply and do so
with great conviction, but it's important to understand that there has
I haev many reasons for objecting to it, which basically fall into two
The first is the obvious one. It's wasteful. OK, I'll sometimes replace a
module rather htan a part (e.g. I'll replace a trasnformer, even though I
am quite capable of rewinding one, and would do so if a suitable
replaceemnt was unavailable). But I find it totally ridiculous to replace
a complete PCB because one cheap IC has failed.
Ah, but you're going to say to me that finding out which IC has failed
takes time, knowledge, and experience. I fully agree with this, which
brings me to my second point.
Board swapping, unless done after a lot of measurement and thought, is
not necessarily going to solve the problem. There are 3 sub-problems.
Firstly, a fault elsewhere in the machine could damage the new board. An
obvious exaple of this is a PSU problem that blows up chips. Secondly,
the new module mioght not be totally compatible with the old, you may
have to make changes elsewhere (I've been bitten by this..). And thirdly,
and most seriously, even if you find that replaceing a particular board
seems to cure the problem, you may not have really cured it. A classic
example of this is that you have 2 modules, A and B, with a signal going
between them. The timing of A drifts and the system fails. You replace B,
and by chance the new one has a looser spec. The machine works, but A
continues to drift, and it fails again a few days/weeks later. OK, a new
A now gets it going, but that would have worked with the original B,
which in fact was never faulty. Seen it happen.
So if you're going to swap boards, you have (IMHO) to make plenty of
measurements anyway. And then you pretty much know which component has
failed. It's probably less effort to just change that one component.
always been a hierarchy in computer servicing.
Customer Engineers have
often only been trained to the level of identifying pulling and swapping
A second rant of mine is about people who are trained to do a particlar
job. Such people generally have no real knowledge of the device (or the
subject in general) and are incapable of logical though. I remember one
such idiot who wanted to replace every PCB in an expensive graphics
display system, when 10 seconds with a voltmeter showed that the 5V PSU
was sitting at 4V. He was escorted off the site by our system manager and
told never to return, BTW. But I digress.
boards, which then are shipped to a facility where
troubleshooters do the component level testing and repair.
I personally have worked in such a component-level board reworking
facility. There are often fabulously skillful troubleshooters working
at those benches. Often this repair function is carried out in the same
facility where production rework is accomplished. And it would be a
severe waste of expertise and skill for these people to be out driving
around to customer sites performing mundane tasks.
Hmmm.. I know pletty of such knowedgable people who can't get a job in
the electronics industry, either design or repair. Is this somehow less
wasteful than having them doing field service work?
Boardswapping has a place and is a legitimate part of the history of
computers going way, way back. It is how things were and are done, and
I have never found 'that's the way things are done' to be a sufficient
reason for carrying on doing them that way.
it's not something to look down on. Of course,
when working on vintage
hardware where there is no longer a repair depot to ship the boards to,
it is obviously a different matter. Restoring and operating old
equipment is an artisan-type task.
Indeed, and then it's a pity that proper serice docs don't exist. I have
tried to remedy this for the HP desktop calculators, but it's a long job...
It's important not to pretend, however, that the published 'board
swapping' service manuals are an abomination before nature. In many
Oh don't get me started. The worst is the Torch XXX service manual. It
contains nothing of any use at all. It contains a block diagram that's
fundamentally incorrect (it's the video memory, not the main memory,
that's shared between the main processor and the service processor), it
contains a 'glossary' that would insult a 2-year-old.
HPO desktop calculator service manuals are curious. They have some
useless stuff in them (a typical repair procedure for the HP98x0 machines
is to swap out the 4 processor boards, if the machine then works to put
the original ones back one at a time until it stops working). But they
also have some odd bits of information that would appear to be useless to
a boardswapper, but are useful to us now. For example, the 9825 service
manual, at least the version I've seen, contains almost no schematics
(expected). You do get the PSU schematic, and you get the schematic of
the printhead test fixture. That's the one part, of course, that's not
part of the machine, so it couldn't be reverse-engineered from a real
cases the detailed manuals and docs never leave the
repair depot, which
is a shame and a real loss for us today, but not a reflection of