27 Oct
2005
27 Oct
'05
3:43 p.m.
You know, that always surprises me - I'd expect
board swapping to rarely
cause problems for reasonably modular systems (I can believe it with
such as DEC hardware though, where you so much as cough near it and
something breaks ;)
OK, one that bit me (it's not a board-swap, it's a module swap, but...).
The fan in this PC's PSU decided to fail (bad bearings). I didn't have a
fan in stock, so foolishly, I grabbed a 'spare' IBM-brnaded PC/AT PSU
from the shelf amd swapped it in. It would be quicker, I thought, than
extracting the fan from that PSU and popping it into the existing PSU.
Result : the machine didn't power up at all. It turned out that the 2
PSUs were of different makes (although both IBM labelled), and both met
the IBM spec _which specifies a minimum load on some of the outputs). The
'old' supply had been happy with less load than that, then 'new' one
wasn't. And of course my modern hard drive didn't put much load on the
12V line. It would have been quicker, and a lot less hassle, to just swap
out the fan.
-tony The PS swap you did was not a total loss, atleast you
learned about the
minimum load needed in the system you have.
Well, technically (!) I already had that information. The IBM PC/AT
Techref gives the PSU spec (although no schematics). Thing is, the PSU
was supplied by various vendors, all _met_ the official spec, some
exceeded it rather. My system did not have enough load to be withing spec
(I should have added one of those IBM load resistors :-)), but it worked
with the original PSU (which exceeded the ofifical spec), not with the
one I swapped in.
OK, in sesne it was my fault in that my machine was out of spec. But
it's the sort of thing that can easily happen when you replace a module
with one that's not quite identical (but appears to be).
I don't like blind board swapping (where you just keep replacing parts until
it sort of works fine again), but don't have a problem with tracing the
That, agreed, is what board-swapping usually means, and what I really
object to. If you're going to swap boards, you _have_ to do enough tests
to be sure the board in quesiton is the defective part (and the fact that
the machine appears to work when that board is replaced is not enough, as
I've explained many times in the past). And to be honest, if you've done
enough tests to _know_ which board is faulty, you don't need to do much
more to find the faulty component on that board.
-tony