[R-390] 60 Hz on Audio

[Barry]

> All,
> 
> Please check out these photos of the output of my R-390A IF Module.
> 
> This first image shows the audio output from the Diode Load signal 
> line.  Input to the IF module is 455 kHz modulated 30% with a 1 kHz 
> tone.  The 16 kHz mechanical filter is selected in this image.  The 
> audio looks pretty clean.  Any 60 Hz that might be there really isn't
> visible.
> http://www.vecnav.com/R-390A_IFmodule/DiodeLoadOut_16kHzMechFil
> ter.jpg
> 
> 
> This second image was made with the identical conditions as above 
> except that the selected mechanical filter is now 8 kHz.  A 60 Hz 
> component is now easily visible.  In fact it's about 25 dB or a 
> factor of about 18 times worse than with the 16 kHz setting.
> 
> http://www.vecnav.com/R-390A_IFmodule/DiodeLoadOut_8kHzMechFilt
> er.jpg
> 
> 
> Is this kind of behavior acceptable for these receivers?
> 
> If not acceptable, what could be causing the distinct difference of 
> 60 Hz level just by changing the BW setting?
> 
> 
> Thanks and regards,
> Jon Schlegel WA3MVM

Is it possible that since the filter is "mechanical", it's picking up a sympathetic 
vibration from the power transformer (or other source of 60kc vibration)?  Have you 
tried dampening (physically) the 8kc filter?

Just a wild guess...

Barry - N4BUQ