[Elecraft] Audio Hum in TX Data Mode

[Jim Brown]

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