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[RFI] HF SSB RFI on CATV Sound Only!

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Subject: [RFI] HF SSB RFI on CATV Sound Only!
From: EDWARDS, EDDIE J" <eedwards@oppd.com (EDWARDS, EDDIE J)
Date: Tue, 31 Oct 2000 10:23:10 -0600
> -----Original Message-----
> From: David Rich [SMTP:kg0us@swbell.net]
> My neighbor came by for a visit during the CQWW phone contest.  My
> transmitter was interfering with the sound on his RCA big screen TV.  He
> said that he gets interference on two other small portable TVs in the
> house.  He said he generally does not mind because I am not on for long
> periods of time but I was on for a long time during the weekend and he
> wanted to watch the Sunday football game.
        He could still "watch" football with the sound turned down! 
        Couldn't he? ;^)

> He is on CATV and the
> interference only effects the sound and not the picture.  I inspected
> his TV while my licensed wife transmitted on around 28.400 MHz and sure
> enough the picture was not effected but you could hear the garbled sound
> of SSB in the speaker.  I reduced the transmitter power from 150 watts
> to 75 watts and the sound level was reduced but not eliminated.  My
> television connected to a outside antenna works fine while I transmit.
> Do you think a common mode choke on the cable where it connects to the
> TV might help to reduce the interference?
        I doubt that will help.  It's probably due to audio rectification in
the audio amplifier of the TV.  The audio amp's transistors are acting like
a radio receiver (P-N juntions are detectors) and then then audio amp's
feedback circuit reintroduces the detected audio to the amp's input for
amplification, and then on to the speakers for that Charlie Brown teacher
sound!  A good clue if this is the case is if the detected audio goes up and
down in volume with the TV's volume control.  If so, it's audio detection.

        The only way to eliminate audio rectification completely is to place
ferrite chokes on the speaker wires right at the TV's audio board.  This is
probably an internal modification (for internal speakers and should only be
performed by qualified TV techs.  I had to do this to my old Sony XBR.
After almost a hundred dollars (well, it seemed like $100 ) worth of
ferrites on the Sony's cables going into and out of the TV including the AC
power line they didn't even make a dent.  Then I placed one of the little
Radio Shack ferrite squares on each internal audio cable (right & left
channels) and it was completely gone forever.  The speaker cable had plenty
of excess wire in the chassis (that was part of the problem) and nice plug
in connectors on the audio boeard making it easy to disconnect and reconnect
them.  I now have a nice supply of ferrites if neighbors ever come calling.

        I do not recommend doing the installation yourself unless the
neighbor is a close relative or life-long friend because the first time the
TV breaks down it'll be your fault for putting those damn filters in there.
So then who is gonna pay the TV repair shop bill for doing the installation?
If it were me and the neighbor's a nice guy, I'd pay to have it done so I
can enjoy contest weekends!  I'd tell him the other TVs are his to get fixed
though.  What a pain it'll be to take a big screen in to the shop though.
Uugh!  See if there's a shop who will do the installation, per your
instructions (or Radio Shacks that come with the ferrites), in the guys
home.  It'll only take 5 minutes once the cover is off.  They'll only need a
screw driver to remove the cover (hopefully).

> I do hear a carrier on 145.25 and sound on 149.75 with my HT so perhaps
> there is a cable leak somewhere.
        Then he'd be getting video interference as well, and he's not.  So
the leaking cable is not the root cause here.  

        145.25 Mhz is the video carrier for CATV channel 18.  There will
always be some leakage, but it may or may not exceed allowable specs.  A
call to the cable company might get that signal reduced a little, but it may
never go away completely.

        So, how'd you do in the contest?  Conditions went from OK on Sat. to
bad on Sunday for me.  Had some fun though.

        de ed -K0iL

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