> Well, the Johnson 2000 PS is an example of old construction. The
> neutral is connect to the chassis and the 120V circuits are wired
> between one hot phase and the chassis so you have to make sure you have
> a good ground connection.
That's a problem and needs correction. It wasn't a safe design practice in
1955 and it's just as much a safety concern in 2009. Here's why: If the
current-carrying neutral+ground lead faults with a lift between the amp and
servicing load center (i.e., breaker panel), then the amp's 120VAC circuit
will find a conduction path through anything that's grounded, including RCA,
BNC, SO-239 connectors, a grounded mic -- and even a grounded operator.
Steps toward the correct solution require the following:
(1) Lift the neutral connection in the amp/PS and elevate it above chassis
ground;
(2) Dispense with the old 3-wire power cord and replace it with a 4-wire
cord of suitable AWG size. Connect the L1, L2, N, and G wires to the new
power cable;
(3) Terminate the power cable with a new 4-wire plug of suitable
current-carrying capacity; and
(4) This is the hard part. Run new 4-wire service of appropriate AWG size
between the servicing load center to the shack.
One benefit of running 4-wire service to the shack is that it allows the op
to safely run any of the older amps with appropriate modification, including
the Alpha 70/77 series, Henry, and as you've discovered, Johnson as well as
others.
The usual safety disclaimers apply. If you're unsure about performing these
changes yourself, consult with someone who understands the needed changes in
the amp/PS, and also consult with a licensed electrician to perform the load
center to shack portion of the change.
Paul, W9AC
.
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