[R-390] Added the IEC line filter

[Tim Shoppa]

Background: a couple of weeks ago I added GFCI outlets in the
basement, and discovered that indeed the stock R-390A line filter
is guaranteed to trip them (leakage current of a properly functioning
stock filter is >5mA so it has to trip the GFCI.)

After unsuccesfully looking for a "low-leakage" line filter with
roughly the same terminal configuration as the stock unit,
I took the common wisdom and added an IEC-jack-style
line filter.

I got a Qualtek 3 amp filter from Mouser for circa $6.  A couple
minutes with a nibbler and a file turned the 1" circular hole on the
back (where the screw terminals for the original filter came
out) into a rectangular hole for mounting the new filter in.

The filter I got had a metal kind-of-gasket on the back of the
black plastic face that seats nicely against the remnants of the
1" circular hole.  I'm not sure that all brands/models of IEC
line filters have the nice metal gasket (I saw it on none of the
engineering drawing PDF's the manufacturers have.)

Others here report putting the IEC filter inside (and not through)
the back panel so they don't have to enlarge the circular hole
but that seemed awkward to me.

The result is that it works just fine (leakage current under 1mA
so no GFCI trips!) and looks great too.  Will
repeat the process for my other rigs in the near future.