MicroVAX II

From: Rick Murphy <rmurphy_at_itm-inst.com>
Date: Sat Nov 23 22:12:49 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
Received on Sat Nov 23 2002 - 22:12:49 GMT