EMC trouble with a “Home Studio”

A neighbour bought a new “Receiver”, SONY STR – DN 1050 to his “home theater”. I was making a loud noise in his loudspeakers when I was transmitting. His old “Receiver” had no problem. The distance from the antennas is approximately 80 meters. Estimated field strength is well below 3 V / m which is, to my knowledge, the field strength a consumer equipment should withstand. The numbers are as usual not accessible to the man on the street. The disturbance was heard in ” surround speakers ” which had the longest lines.

In an attempt to cure the problem the speaker lines were wound with 5 turns in an FT 240-77 Ferrite core from Amidon . At the same time extension cord with an equal core with 5 turns was connected between the wall outlet and all units. The disturbances remained, but had changed character. If the antenna was disconnected from the TV digital box, the disturbance disappeared. If a galvanic isolator coupled into the antenna line a weak disturbance remained (Note 1).

Upon contact with the SONY support their only advice was to reset the Receiver to factory settings. How that should be able to cure the problem, they could not explain.

An EMC filter for common mode currents was made for the loudspeaker lines. It is made up of 7 FT-140-77 ferrite cores from Amidon. One for each loudspeaker. The Subwoofer is connected with a special line so it must be winded on a big FT-240-77 core.

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The attenuation on the different amateur band was measured.

The measurements were made according to picture 1 below, with my IC-735 as a signal source.

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Pic.1

 

Results:

Frequency in MHz         U1 Volts pp                U2 Volts pp                Attenuation dB

1.8                                           37                                0.2                               46

3.5                                           38                                0.25                             44

7.0                                           37                                0.23                             42

10.1                                         38                                0.28                             43

14                                            38                                1.1                               31

18                                            34                                1.1                               30

21                                            30                                1.2                               28

24.9                                         25                                1.1                               27

28                                            23                                0.37                             36

In an effort to try to find a point with the lowest attenuation, the frequency was varied and also the impedance at U2. The lowest value found was 20 dBs at 25 MHz.

 

Note 1:

At a later point, it was discovered that the attenuation on shortwave for that thing was only 5 dBs. Therefore a new extension with a ferrite toroid with several turns was made.

 

A line filter was made:
The power cord was winded 7 turns on a ferrite #33 core. 15 turns on line and neutral on a FT-140-43 core and 4700 pF ceramic capacitors.

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