27 Oct
2005
27 Oct
'05
6:29 p.m.
I'll wade in with my comments now.......
Board swapping is a useful method for high reliability systems (I work in
I hate to say this, but _random_ boardswapping, which is what I am really
complaining about -- that is, when you easter-egg parts until the machine
seems to work again -- is the last thing I hope would be used on critical
systems.
Unless you know what/where the fault is, you can't know you've fixed it
IMHO (I've explained the problems many times before).
[...]
Now back to our systems! With the above in mind:
1. we rarely know the state of any parts we have - a board that has been
un-used for 20 years is quite likely to be faulty!
Even worse when you're trying to make one good machine out of 2 defective
ones and you haven't a clue as to which boards are good or not.
Note that I have no problem at all with 'test rigs', which often are
working examples of the machine in question that you can plug defective
boards into for testing. A good example of this is for the HP41
calculator. Due to the mechanical construction of that machine (the
inter-board connectors are clamped by the case screws), it's impossible
to have the machine in a working state where you can probe pins of the
chips. HP serice centres had a 'test calculator' which, from the picture
in the serviec manual was a mechanically modified HP41. An afternoon in
the workshope with a battered 41 produced a workable substitute.
2. We are self trained on these systems. If we can narrow a fault down to=
a
board, we can often take it down to the component.
Actually, this is a great advantage. A lot of 'training' is based on
known or common faults, and if you're not careful the serviceman will get
totally lost when faced with something that's not been considered. Since
we don't get the manugacturer's traing, we (or at least I) approach such
problems from the viewpoint of standard electronic principles and are
less likely to get totally confused (well.... that's another story :-))
3. We are lucky to have any spares!
Looking at a less than vintage system, for the most part, it is difficult=
if
not impossible to go below board level ( I can sympathise with Tony here)=
,
and nigh on impossible to get information on how that board is working, o=
r
even get test equipment that can capture the signals involved (I have a
Hence my comment that I don't have a modern PC because I can't afford the
test gear I'd need to maintain it.
100MHz (valve) scope with a 4 trace plug-in, or I can
borrow a very old
logic analyser from work). At this point, we are reduced to board swappin=
g
as the only option, though often with little measurement to back it up - =
the
only thing we have over the teenager in PC World, is that at least we hav=
e a
multimeter and can check the power supply voltages........
Hmmm... I'll go a bit further than that. There is a suprising amount you
can do on a modern machine with limited test gear intellegently used...
We are also at the point where PC components are so cheap they are not wo=
rth
repairing - I can buy a LAN card for less than =A310. It costs me more th=
an
that to pay an engineer to replace it, let alone try and repair it. Even =
the
kid in PC world expects =A35 an hour - if you try and charge some one =A3=
50 for
changing a LAN card, you'll soon be out of business
Hmmm.. I have a fairly well-stocked junk box, but it contains components,
not boards. Suppose a serial port fails. OK, it's going to take me a
little longer to find the defective 1488 (or whatever) rather than just
swap the board _but_ it takes less time than me going over to PC-world or
whererver to get that replacement board.
Of course the fact the PC-world doesn't sell boards for any of the
machine I own is a secondary issue :-)
-tony