Edward Greeley etgreeley at WORLDNET.ATT.NET
Thu Feb 19 01:13:32 EST 1998

Hello Rod & list

Your line cord scheme will work, of course, but will also violate the
NEC (Natl Elec Code) in that the Equipment Ground (NEC term), or "safety
ground", or green wire, or whatever you know it as, is NOT supposed to
be a current-carrying conductor.  I can't address the possible insurance
ramifications of the scheme.

99.9% of the old "AC-DC" sets had what was called a "floating ground";
the chassis was NOT connected directly to one side of the AC line.
Rather, there was an isolated "bus" for one side of the AC line and
minus DC.  This bus was connected to the chassis, usually, through a
parallel combination of a 470K resistor and .05 (or .047) mfd capacitor
(values varied somewhat from model to model of set).  This put the
chassis at RF ground potential, but prevented one's contact with an
otherwise hot chassis while one was grounded from being an exciting, if
not lethal, experience.  One could get enough of a zap through the .05
mfd cap to get his attention, though.  The rub comes in with the old BA
stuff in which the caps have become leaky, if not shorted.  A shorted or
very leaky cap connecting the floating ground bus to chassis could,
indeed, be a dangerous situation if the line plug was inserted such that
the hot side of the AC line was connected to the floating ground.

There were a few AC-DC sets made which had the chassis connected
directly to one side of the AC line.  In this case, great care was taken
in the design to assure that no metal parts which were in contact with
the chassis appeared on the outside of the case where the user could
touch them.

Bottom line:  I don't have a schematic of the S-38 in front of me, but
since the set has a metal cabinet, I'm quite sure that the floating
ground approach must have been used in its design.  The thing to do is
locate that cap which connects the floating ground to the chassis and
replace it with a modern, mylar cap rated at 630 VDC, regardless of
whether it seems to be leaky or not.  Also check the value of the
parallel resistor to make sure it hasn't decreased greatly in value
(rare, but not unheard of).  I would NOT use the Equipment Ground line
as a power return.


Ed Greeley
Mobile, AL

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