MicroVAX II - test-drb@ccmp.vtda.org

23 Nov 2002

At 04:20 PM 11/23/02 -0800, Antonio Carlini wrote:

...
  only the power
harness.  And assuming one couldn't find a replacement power harness for the
bad
 one, what would it take to modify/repair the bad
one to make it safe to use?  Is it just the matter >of replacing some of the
existing wire with
something of a heavier gauge?

I believe the problem is that the wires are of differing lengths
and hence resistance. Plus they were (or may have been) specced
a tad to near the limit. The end result was that the longest wire
(greatest resistance) heated up the most and eventually went phut,
leaving the other five (or so) to go pop shortly thereafter. 
My experience - having seen several badly melted connectors - was that 
the clips on the ends of the orange cable sets don't make adequate 
contact with the pins on the backplane and power supply. They heat up 
to the point where the connector shell begins to melt, which begins to 
oxidize the pins, making more heat due to the additional resistance. 
This is not due to different lengths - all the wires in the orange 
cable harness are the same lengths.

You can recognize the updated wire harness because it consists of two 
white blocks on either end of the assembly; the wires are different colors.

...
 So heavier gauge and same length would seem to be
required. 
Nope, it's not the wire that's the problem - it's the connector's 
bearing area.

...
 OTOH if the problem was that bad (none of mine in the
lab ever exhibited
any pyrotechnic tendencies) would it not have corrected itself by now
:-) 
That depends on how much load you put on the cables. Lightly loaded 
systems won't ever have a problem.
         -Rick