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

[Jon Schlegel]

Hi All,

I'm still working on the problem.  Thanks to all 
for your interest.  My plan is to isolate the IF 
module from the radio as much as possible.  To 
that end I will hose in B+ and filament from a 
separate supplies and do it in such a way that 
heater ground currents will not flow through the 
radio and/or the extender cable when used.  If 
I'm still seeing 60 Hz, then I'll call in Dave's 
big guns with a rectifier brick and capacitus gigundus on the filaments.

As luck would have it, the module has developed a 
new problem.  The strip has started to 
oscillate.  It will randomly burst into 
oscillation for a brief period and then 
stop.  The recovered audio has somewhat the 
appearance of "motorboating" where there isn't 
enough decoupling on the B+ or AGC lines, etc. 
yet I can't see any action like that on B+, AGC 
or Cathode Bias Lines.  That's when I decided it was time to quite last night.

Tonight I "took the evening off" to fix a nagging 
problem with the scope.  My bench area is rather 
small and I'll probably take another evening off 
to straighten up as I'm starting to trip over stuff at this point.

My plan is to publish what I learn.  It's just 
right now things seem to be breaking faster than 
I can fix them! <G>.  Also I used to work on high 
dynamic range wideband FM receivers where 
minimizing discrete responses in the recovered 
audio was a really big deal.  When I saw the 60 
Hz coming through on the audio I just thought, "Man, that's gotta go."

Regards,
Jon


At 05:54 PM 5/7/2009 -0400, Flowertime01 at wmconnect.com wrote:
>Not to encourage you away from Roger's recommended environment,
>but an extender _would_ give you entrée to substitute DC on
>the heaters etc.  That would tell you Right Now whether it's
>heater, B+, or whatever other weird thing it might be.
>That wouldn't get you all the way home, but it would at
>least partition the problem.
>
>Right On, Dave.
>
>Just a bridge rectifier and a couple big caps in the filament lines of the
>extended cable could go a long ways and be useful.
>
>Roger AI4NI
>
>
>
>
>
> >
> >
> > Dave Wise
> >
>
>
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