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

[Jon Schlegel]

Hi All,

The saga continues in my attempt to characterize the 60 Hz component 
present on the audio of my R-390A IF module.  The bursting 
oscillation I encountered previously has subsided for the time 
being.  I've refined my test setup and that may be a factor in helping.

Without going into a bunch of detail at this point, it's starting to 
look look like that the 60 Hz heater supply for the IF module does 
contribute to hum on the module's output.  This is based on comparing 
hum output when alternately using the normal internal AC heater 
supply and an external Lambda linear DC supply for module 
heaters.  No real surprise here I guess.

Of more significance is a marked change in hum output for certain 
mechanical filter selections.  This hints that the hum could be 
entering the signal path both before AND after the mech filters such 
that the delay variation from filter to filter could either cause 
constructive or destructive interference when the hum components 
combine.  As an example, 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).

More to come.

Jon


At 08:29 PM 5/10/2009 -0400, Jon Schlegel wrote:
>Roger,
>
>Something tells me it will take more than the addition of some
>parts...  When I get the IF module behaving again (bursting
>oscillation) I'll try DC filaments straight away.  Dave's remark
>about the bench supply reminded me of the semi-boatanchor Lambda I
>have squirreled away that will do the job nicely.
>
>Regards,
>Jon
>
>
>At 06:13 PM 5/10/2009 -0400, Flowertime01 at wmconnect.com wrote:
> >Jon,
> >
> >I see your concerns. Stay with it and see what you can do. If you can find
> >a point to stick in a couple extra filter caps, or change a resistor to
> >reduce the hum let us know. A few good changes can always be in
> >order. Most of
> >us do not really get down into the subassemblies as we should. You could
> >provide us some insight from your experience.
> >
> >Roger AI4NI
>
>
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