[Hammarlund] Hammarlund power ground

[rkerr]

Mike,
The ground associated with the power cord is for safety, not for RF
ground.  It may not be a good RF ground at all, but providing a power
ground whenever a power cord is plugged into the wall provides an
automatic margin of safety against electrical shock that isn't there
with a separate chassis ground connection - you might forget the latter
for a long time and the radio would still work until something inside
the radio fails with a short circuit to the chassis and you get a nasty
shock.

The two power slots in the wall electrical connector are do not have
symmetrical voltages to ground.  One is "neutral" and is actually
connected to ground at the house's power panel.  The other is "hot" or
"phase" or whatever your term, but has nominally 125 VAC relative to
ground these days.

In many old radios the impedances at 60Hz from the two sides of the
power line to the chassis ground are asymmetrical.  This can be due to
L/C power line noise filters or capacitances within the power supply to
chassis ground.  In any case, there is usually some very small AC 60Hz
circuit flowing directly from the hot side of the power line to the
chassis ground.   This current causes very small AC voltage gradients
across the chassis itself, which can be picked up and amplified by the
audio amplifiers in a radio.  Because the voltages relative to ground on
the two sides of the power cord are also asymmetrical, you can usually
make a significant change in the amplitude (either increasing or
decreasing) of these currents simply by flipping a two-wire plug over.

This is one of the reasons that one of the slots in a post 1950s US wall
outlet (neutral) is bigger than the other.  Even the two-wire plugs are
supposed to be polarized, although prior to about 20 years ago most were
not.

Bob
K4ERR

Mike Taylor wrote:
> Hammarlund manuals say that if there is objectional audio hum, reverse the two wire plug at the outlet. I've never noticed any difference one way or the other. Does this have anything akin to the use of a two wire plug that's polarized? Also, what is the difference between using a three wire plug with the green wire connected to chassis, or connecting the "G" terminal on the antenna input strip to a good earth ground (copper rod driven into the earth or water pipe connection)? Either way, your connected to earth, so what's the difference?
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