Roderick M. Fitz-Randolph w5hvv at AENEAS.NET
Wed Feb 18 23:51:05 EST 1998

The below information, from Ed Greeley, is well taken!  Perhaps the
best way to do this would be to use the "floating ground" as outlined
by Ed (with new, quality components) and connect the green wire
Equipment Ground directly to the chassis????  Your thoughts on that Ed?

Rod, N5HV
w5hvv at aeneas.net
>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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