Jay W. hath wroth:

After several months in the field, contact resistance on the ATX
connector increases for some reason, and is causing a voltage drop as seen
on the motherboard (sometimes as much as .5VDC!)

Are you sure it's the connector contact resistance and not the wire
crimp on the ATX connector? I've seen something like this inside
wafer fab tunnels, where the rather caustic chemicals attack the
copper wire to solder plate junction on the connector pins. I've
never seen a 0.5VDC drop. More like 200-300mv at worst. Egads,
0.5VDC at 3A is 1.5 watts. The ATX connector should melt with that
much power being dissipated. Any discoloring of the nylon connector
shell?

On a system that is failing, unplugging the ATX connector and reseating it
will "fix" the problem and the system will continue to work for several
months until once again the resistance increases in the contacts and...
well, you get the picture.

Well, that certainly sounds like its the connector contact and not the
crimp. However, I'm suspicious and will pretend that it might still
be the crimp.

Have you tried to take an ohms-guesser or ESR meter and measure the
contact resistance (or crimp resistance) of the connections? Also,
measure the voltage drop when running between:
motherboard to ATX pin
ATX pin to copper wire (across crimp)
Case ground to motheboard ground (just for fun).

Instead of removing and re-inserting the connector, can you get it to
recover by merely wiggling the ATX connector from side to side, or
doing the same with individual pins by wiggling the wires sideways? If
yes, then you punching through a layer of surface crud, corrosion,
oxidation, or chemical buildup of some sorts. If no, then inspect the
crimp for insulation crimps, dissimilar metal issues, and corrosion.

Any evidence of copper wire embrittlement at the crimp? Heating of
the connection can do that.

We've tried using contact lube - no luck.

No. That's just solvent and light oil. Try Cramoline, De-Oxit or
some oxidation prevention chemical. Also try grease or silicon
grease. Anything to provide a seal.

Thinking that there might be a problem with dissimilar metals, we checked
that the contacts on the cable and ATX connector on the board are tin. The
contact specification claims that they are gas tight.

Gas tight over what area? Verify that you have a large mating surface
area and not a tapered loose fit. I've seen connector tolerances
create a loose connection that eventually fails. (Hint: I designed
marine radios in a previous life). Sacrifice one power supply
connector and remove the nylon shell with a pair of wire cutters. Do
NOT use an extraction tool for this. Take each connection and
*MEASURE* the insertion and extraction force. If the power supply
manufacturers crimping machine screwed up the connector fit, you have
a potential cause.

Also, tin oxide is almost transparent and rather difficult to see.
However, you can detect it with an ohms-guesser. Try passing the
probe tips over the ATX plug surface, without punching through, and
see if you get any insulated areas.

I think the best approach is to identify whatever chemical is
attacking the connectors. I'm not sure how to do this. Also, I'm not
convinced that the connectors pins are as great as you claim. Are
they tin, solder plate, or lead-free solder plate? If lead-free, what
substitute solder plating are they using?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558