[R-390] 60 Hz Hum Output from R-390A IF Module

[2002tii]

Jon wrote:

>hum level drops by about 11 dB just by
>changing from 8 kHz to 16 kHz filters.  For this test, I had the IF
>module configured for normal operation, mounted firmly in the radio
>chassis with it's power plug (20 pin connector) plugged directly into
>the module (i.e. extender cable NOT used).

Do note that the insertion loss of the various filters can vary by 
that much.  I don't know how you are testing, but if you are below 
the AGC threshold (e.g., no antenna), any such differences would be 
apparent in the detected audio output.  The noise should go up and 
down similarly, but this can be confusing to the ear because unlike 
the hum, the bandwidth of the noise changes from one filter to another, too.

As others have suggested -- if you have not done so already, 
determining with certainty whether the hum is 60 or 120 cycles will 
potentially be very helpful.

Best regards,

Don