In fact, it's becoming much less common to find parts in commercial
equipment with only the OEM user's part number rather than the
manufacturer's PN on them. Partly this is being driven by contract
manufacturing of the boards, but it was a trend even before CM became
so common, in my experience.
Roy's comments about military techs are slightly idealistic, if I
relate things to my personal experience some years ago. My shop
supervisor at a remote site quickly learned to trust my judgement in
finding ways to get things repaired and functioning reliably when
parts simply weren't available. When it comes to a decision between
completing a critical mission using a part that gets you through the
mission, and waiting six months for a part, guess which wins. You can
find info about some of that sort of thing on the NASA web site.
Astronauts don't wait for delivery of XPQ1453762 if their lives (or
even an expensive experiment) can be saved by using something on hand.
Clearly, you'd prefer to use the exact part specified, or a
documented replacement, but there are lots of times when waiting for
that would be very poor judgement. (Darwin award candidates??)
OTOH, do NOT expect modern parts to be fully labeled. Don't expect
0.1% SMT resistors to have anything on them telling you that they are
0.1%. Don't expect 0603 and smaller resistors and capacitors to have
ANY marking, though some do. Don't expect SOT-23 and SC-70 parts to
have anything but a simple 2 or 3 character code on them. Some
(most?) SMT tantalum cap manufacturers don't seem to put a marking on
them that tells you which series they are (std ESR, low ESR, extra-low
ESR, fused, mil-spec...).
And count your blessings that you're dealing with electronic parts,
where labelling with the value is pretty common. That's not the case
with most mechanical parts, for example. Would you expect Honda
pistons to fit any Toyota engines? Have you ever seen ANY springs
labelled with their spring constant, let alone any of their other
parameters? Most aren't even labelled with a part number.
Finally, there IS a reason that schematics don't always have values or
manufacturer's part numbers on them, or even house part numbers, but
only a reference designator. That's because values (and therefore the
part numbers) may well change over time. It's a lot easier and less
error-prone to maintain only one changing document: the material list
used to build the board. Not everyone agrees with that, and in fact I
do prefer to have values and mfgr's part numbers on my schematics.
Cheers,
Tom
Jim Adney wrote in message . ..
On Tue, 09 Mar 2004 19:59:19 -0800 Roy Lewallen
wrote:
Jim Adney wrote:
. . .
So I still don't think it's too much to ask that the most important
data be printed out separately.
Obviously, I don't tend to think like the military....
While you've got a legitimate point, you've got to realize the function
of the part number. If you're a technician servicing a piece of military
gear, you replace a 20035942 *ONLY* with a 20035942, not *ANY* other
capacitor, regardless of its value. To order a replacement, you put in a
requisition for a 20035942. Now, it's vital to you that the capacitors
in the stock bin or coming in from the supply system have 20035942
printed on them, but it's not important that the value is. The
technician can find the value in the parts list in the manual and likely
on the schematic.
If I'm that tech, how do I correlate that p/n with this cap on the
schematic? Does the schematic have both the value AND the p/n on it?
I understand your point about using the exact replacement, but I don't
see why a part should not have BOTH sets of data. To me, that seems
like it adds a lot of value to the part.
-
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Jim Adney
Madison, WI 53711 USA
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