Jim Phillips writes:

> We have a 2934 printer that is located out in our plant.  We recently
>  moved a PC to the table beside the printer.  We immediately began
>  seeing strange, undulating patterns on the PC's monitor.  We just
>  determined that the printer is causing this interference (when we
>  turn the printer off, the distortion goes away).
>
>  Before I call the third-party vendor who sold us the printer, is
>  this a normal thing?  We've done this sort of thing before (that is,
>  2934 printers in close proximity to monitors) and never had a
>  problem.  Am I right in assuming that there is something wrong
>  with the printer?  Can I remedy this problem with some external
>  shielding?

Is it normal? Yes, always to some degree or another. The intensity of the
problem will depend greatly on the sensitivity of the particular brand of
monitor, proximity and angle of orientation.

Can it be corrected with shielding? Sometimes. The problem isn't likely to be
EMI per se, at least not for the electric field component in any appreciable
amount. What you're experiencing low-frequency changes in a variable magnetic
field produced by the printer.

If that's really the case, what you want to do is "short-circuit" the magnetic
field lines with a very high magnetic permeability metal such as mu-metal --
or move the monitor farther away. Magnetic lines of force travel just like
water, through the path of least resistance. If you were to put a bit of mu-
metal around the edge of the printer nearest the monitor, quite likely most of
the problem would go away. It doesn't take as much of this material as you
might guess, just a few inches (approaching a foot), shaped perhaps in a "C"
so that it would cover the printer's primary motor.

Your problem results from exactly the same effect as Lee Gunter's
recommendations concerning magnets on top of your CRTs, it's just a more minor
version of the same phenomenon. It's made all the more irritating because the
magnetic field is constantly varying in intensity, which makes it more
perceptible.

Wirt Atmar