On 9/3/2012 8:25 PM, Mike Hyder -N4NT- wrote:
1ST CHECK YOUR AC OUTLET GROUND IT MUST HAVE GOOD CONNECTION
2ND. MEASURE FROM THE -13.8V TERMINAL TO CHASSIS, IF NO DIRECT
CONNECTION,
RUN A PIECE OF WIRE FROM -13.8V TERMINAL TO CHASSIS.
Damn! This advice is fixing a manufacturing problem that I found in all
three of the Astrons I own -- the green wire in the power cord that IS
REQUIRED BY LAW TO GO TO THE CHASSIS for safety, does NOT make that
vital connection. The reason is simple, and a common manufacturing
defect -- the green wire is soldered to the chassis lug of a terminal
strip that is INSULATED from the chassis by PAINT! In one of the three
units I own, the two pieces of the chassis were also insulated from each
other by paint.
Bonding -12VDC to the chassis causes a problem when powering a SSB rig
that SOUNDS like RFI, but is NOT RFI. W8JI pointed out this problem to
me several years ago. Ohm's Law causes IR drop in both V+ and V-
conductors that varies with SSB modulation. Nothing new. But when V- is
bonded to the chassis, and when audio accessories are running from that
same supply, the modulation of V- is added to the audio. That modulation
SOUNDS like SSB in an AM detector -- it SOUNDS like RFI. But it's not.
You can hear this modulation if you connect headphones between one end
and the other of the V- conductor of your power supply wiring.
I have long advocated bonding chassis to chassis for all equipment in
our ham shacks to minimize problems with hum, buzz, and RFI. This
practice also reduces the above problem to the point where it is not
audible. .
73, Jim K9YC
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