[amsat-bb] Re: Tone interference

David Palmer dave at zdap.com
Thu Jan 20 14:31:45 PST 2011

Hi Louis,

I have seen a few (rare) postings about the LM7806 voltage regulator
in the G-5500 not having enough bypass output filtration, and thus
becoming unstable.

Reportedly the '7806 regulator needs a 0.1 uF output capacitor for
stability, and the G-5500 uses a 0.01 uF cap instead.  An unstable
regulator can either generate RF noise (as you describe) or the
voltage output can become inaccurate as the system warms up, leading
to subtle (less than 10 degree) positioning errors.   For example:


I was noticing a slight drift in my display readings as my '5500
warmed up, so I checked the ouput of the 7806 in mine with a scope and
saw it was oscillating (about 200 mV at 5 MHz).  I simply soldered a
0.1 uF tantalum capacitor, in parallel, to the leads of the exisiting
(0.01 uF) cap ... and that removed the oscillation entirely.  Meters
then had nice stable readings after that too.  I never noticed RF
noise, but the fix should be the same for both.

Hope this helps,

73 de Dave KB5WIA / CM88
AMSAT #38466

> I have discovered a pulsating tone (about 600 HZ) in my 910H receiver when I am
> tuned to 145.900 MHz (working the Linear birds VO-52 etc).  The tone stops when
> I power off the G-5500/LVB tracker.  All equipment is grounded and ferrite core
> filters are on all of the cables to solve this problem.  Aside from moving all
> equipment across the room from each other, what might I be missing?
> Any help will be  appreciated
> Thanks,
> LOUIS, KD5GM in EL29kq

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