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Author Topic: Another TVI question..  (Read 2416 times)
KD0PBO
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Posts: 67




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« on: February 27, 2014, 09:17:37 PM »

Recently, I discovered that our older tube type "big screen" TV picks up my TX audio anytime I transmit on 10m AM. It only does it when the TV is on. Our other two newer flat screen TVs are not effected by it. Also the signal comes through the speakers only. No picture interference, just the TX audio through the speakers. If the TV is off, the speakers no longer pick up the signal.


Some other details--- The TV is connected to ATT Uverse and a 6.1 surround sound system. None of the signal comes through the 6.1 SS system including the powered sub.


So what do you guys think is going on here and what can/should I do to fix it?

Thanks
73
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WX7G
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Posts: 6134




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« Reply #1 on: February 27, 2014, 11:50:44 PM »

It sounds like the RF is getting into the TV audio section (and being detected) via the cables connecting the TV to the surround sound system. Ferrite cores on the cables is the first thing I would try. Run all the cables together as a bundle through the ferrite cores.

DX Engineer stocks suitable cores.

http://www.dxengineering.com/search/part-type/rf-suppression-snap-on-ferrite-beads?tw=ferrite&sw=RF%20Suppression%20Snap-On%20Ferrite%20Beads
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KF7CG
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Posts: 840




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« Reply #2 on: February 28, 2014, 10:17:13 AM »

Another source of audio pick up is if the set is one of the older type with an approximately 21MHz if, since TV signals are almost 6MHz wide and if  I remember correctly the audio is on the high side, it could be getting into the IF.

The older set had quite a few ways of being interfered with and interfering with. Color burst at 3580, IF pickup, and image and harmonic generation and pickup in the tuner. A very nice can of worms.

KF7CG
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KD0REQ
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Posts: 971




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« Reply #3 on: February 28, 2014, 11:00:04 AM »

NTSC sound carrier is 4.5 MHz up from the video.  sound is FM, video is AM inverted, 100% modulation is black.  burst is at 3.58 MHz in round numbers.  almost nothing is shielded.

a tube projection set ought to have enough room and clearance to slip a Faraday shield around everything on the bottom, with some Mylar or fish paper over the shield so stray little poking ends of wires don't make sparks.  the problem with the chassis is if the three projection tubes are on the same chassis... moving it then can set you up for some bodacious convergence and focus fun.
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W8JI
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Posts: 9296


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« Reply #4 on: March 02, 2014, 10:28:32 PM »

99% of all this stuff is from differential excitation of the TV set by poor external bonding or cabling.

Go to WalMart or somewhere and buy a TV surge lighting suppressor outlet that has the coaxial surge on the same box. Make sure it bonds the coax shield to the safety ground with a capacitor or direct.

Run ALL of the wires to the TV set cluster from that single strip.

Do not run any audio lines or anything in or out of that barrier area.

This fixes almost all paths through the TV. Very few systems are not fixzed this way.
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