Not that I'm any great expert on RF, but this looks familiar.  One of
those "This one bites everyone" things that is good for "Breadth of
experience" for newbie & old-timer alike.

  It's probably not a ground loop, I'd guess you've looked at that!

  Here's MY best guess;  Is the capacitance of that shielded cable,
what's killing the 54C14J's?  That's in common between all the failing
parts.  And I know I've almost been BIT by this one before, driving
MOSFET's gate capacitances would have killed my 4000 series CMOS
circuits dead if I hadn't thought it through in advance.  (Current
limiting resistance...  Could be FET oscillation, I know I'm not an
expert on that just now <G>

  My "quick dirty" solution:  Split the 1k resistor at the output of
that shielded cable, into 2 resistors - one, say 100R, between the
54C14J and the cable, to limit 54C14J max output current, and the other,
say 900R at the input to the FET.  (You want to prevent both FET
oscillation and drawing too much current out of that FET.)  There's
probably a better optimized resistor value splitting to be done there,
there are some past posts in the PICList archives that covered this
IIRC.  (http://www.iversoft.com/piclist/ - I'd go look there.)  I don't
usually memorize solutions, just "Gotchas", and look up the solutions
when needed.  And check datasheets <G>

  Mark

Kelly Schauf wrote:
>
> To the forum:
>
> The problem I have does not involve a PIC; however, there appears to be
> an abundance of knowledge in this forum regarding electronics based on
> some of the messages I've read so far.
>
> Is there anyone out there in the forum that has experience with Design of
> Experiments (Taguchi-type) in addition to their electronics
> troubleshooting skills.  I have been seeing repetitive problems with a
> MM54C14J Hex Inverter (an old ceramic-style part) where the Vcc is
> getting creamed.
>
> Because the explanation of the failure will be a bit too long to
> adequately put into a message, the best place to get further details of
> the failure type is to go to
> http://web.gmtcom.com/~k3jsch/sixsigma/access.htm
>
> The problem occurs regardless of the following:
>
> 1) Whether it has been in service for only a few hours or at least 8000
> hours.
> 2) Whether it is operating at a temperature of 55 degrees C or slightly
> past 100 degrees C
>
> 3) Whether it is operating in a 60Hz domestic unit or a 50Hz overseas unit.
>
> 4) date code of IC
>
> Any suggestions, etc., are welcome.  What I am trying to do is set up an
> experiment at our production facility to inject noise, etc. into the
> design as it is running.  Factors I am looking at are :
>
> 1) Levels of noise voltage to inject into the cable between the
> microprocessor unit that this IC resides on and the gate firing board of
> the SCR bridge that is directly mounted over the 3-phase, 6SCR bridge
> that ultimately runs either the left track or right track motor.
>
> 2) Brand name of chip.
>
> 3) Whether or not modification to the shielded cable and/or its placement
> in the circuit would make a difference.
>
> Regards,
>
> Kelly Schauf
> k3jsch@mail.gmtcom.com