On 9/27/2019 2:26 PM, Conrad PA5Y wrote:
What did make a difference was moving the radio away from sources of magnetic
fields such as monitors.
First, an important clarification -- is it HUM (pure 50 Hz) or BUZZ
(harmonics of 50 Hz)? You description sounds like hum. The important
thing is that the causes are different. Hum is, indeed, magnetic
coupling of the 50 Hz field to some loop in the audio wiring.
Magnetic coupling is proportional to the area of the loop. Suggestions.
First, get power for all interconnected equipment from outlets that
share the same protective earth. Second, bond chassis-chassis of every
piece of equipment in the audio interconnection path. Third, try to
figure out what the loop is that is doing the coupling and reduce its
area. In some recording studios where a strong magnetic field exists,
the solution is to run shielded twisted pair audio wiring in close
proximity to power wiring.
Also try to find or eliminate the source of the magnetic field. In North
America, neutral must be bonded to earth at one, and ONLY one point,
usually where it enters a building. A second bond between neutral and
the same protective earth can create a magnetic field. Is it possible
that there is a second bond somewhere in your system, perhaps associated
with power to your EME setup?
The I/O board for the original K3 uses un-shielded audio transformers
that are a sitting duck for magnetic fields. My neighbor K6XX is an
engineer at Elecraft. When he was testing an early K3 prototype, he set
it up almost on top of his 1.5 kW power amp. When he tried to run RTTY,
The AF circuitry was set into regeneration by the 60 Hz field! The
solution I suggested was a steep high pass filter for AFSK-A, and that
solved it.
Many years ago, a Chicago FM station whose engineers I knew well moved
their studios from a location downtown to another location that is
common to a TV studio with whom they had a relationship. When they first
moved in, a strong magnetic field was coupling into the mic input
transformers of their music mixing desks. The cause was an error in
mains wiring -- the neutral for the distribution panel for the FM studio
was exchanged with the neutral for a distribution panel feeding lighting
in a different part of the building.
73, Jim K9YC
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