[R-390] A different GFCI option

[Gary Gitzen]

Hi R-390ites,

I'm surprised no one has suggested a different approach to
using a GFCI with an R-390X, one which both works safely
and is code legal.

First, let's rehash the basic "problem" with using a GFCI
in a "normal" 120 VAC circuit: the AC inpput filter has
caps going from each side of the line (hot and neutral)
to chasis. Under "normal" conditions, with a properly
grounded chasis and a neutral at 0v, we have unequal
current flow between the two supply lines.
The "hot" leg will show a load of a few (approx 5?) ma
to chasis, and the neutral will show a load of zero ma
to chasis. This approx 5ma differential current flow will
trip a properly functioning GFCI. That's why they exist.
Some folks, for whatever reason (typically National or
local electrical codes) must use a GFCI and experience
problems trying to power an R-390X.     End of rehash.

One possible "solution" to this issue is to not ground the
chasis of the R-390X. This will "work", but leaves the chasis
(and antenna lead in?) floating at roughly 60 VAC.
Not a good idea, especially if one of the "hot" caps in the
line filter should ever short.

Another suggested solution is the use of an isolation
transformer.  This will "work", but partly defeats the
use of the GFCI, especially during fault conditions. 

A third, as yet unmentioned, solution exists; one
gaining popularity with the professional audio crowd.
[Please note "Professional Audio" vs "Audiophools".]
This third option is to differentially feed the
120VAC line as two out of phase 60VAC legs, just like a
240VAC line is two out of phase (differentially fed)
120VAC lines.
Each 60VAC leg is derived from a center tapped
isolation transformer, with the center tap connected to
green wire ground. Each 60VAC leg goes through a breaker
panel with double breakers tied together, just like
for your electric dryer or other 240VAC circuits,
so that any dead short will pop both breakers and avoid
the "widowmaker" situation of a supposedly dead supply
line being hot.

Now for the good parts:
Each 60VAC leg can be connected to one side of a normal
120VAC GFCI. An R-390X can be plugged into this GFCI.
All leakage through the filter to a grounded chasis is now
virtually _balanced_, and thus will not trip the GFCI.
      The best news: it's code legal.
The bad news: it's not easy to set up, and the
appropriate center tapped 120VAC transformer may be hard
to find. One option is a surplus 120/240VAC  isolation
transformer, with each side having two 120VAC windings.
If these windings are seriesed for 240VAC, but one side is
fed with 120VAC, the other side is now a center tapped
120VAC source. Such transformers are sometimes found
in old mainframe computers. Mine has a rating
of 5KW, roughly 20A at 240VAC, but it will also carry
20A at 120VAC in the above configuration. I've seen
smaller such transformers available.
Agreed, load regulations suffers slightly, but that's
a small price to pay; and the R-390X _was_ designed
to operate from a wide voltage range.

Suggestion: if you do this, have a DPST toggle switch
(or relay) in the shack to open _both_ of the 60VAC legs
when you turn off power.

This same concept can be applied to a much smaller scale
"plug in" version which does not need electrical permits.
Obtain a similar 500W-1KW 120/240V isolation transformer,
an appropriate double breaker, another GFCI, and a metal box.
Build everything in the metal box, plug it into your
AC outlet (which may be already GFCI protected) and you
have the best of all worlds: an affordable, portable
GFCI trip-proof adaptor which will still trip if any
leakage fault occurs in your R-390X or other equipment
connected to it.


Hope this helps solve a nagging problem.

Gary