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Author Topic: Tough TVI  (Read 883 times)

Posts: 628

« on: May 20, 2002, 09:11:25 PM »

I know we have had some TVI discussions, so I want to add my mine the mix and see if I can
get some help.  I’ve been working on a TVI problem for awhile.  I can use 80, 40, 17, 15, and
12 meters with no problems.  I have a minor interference problem on 10 meters and a major
problem on 20 meters.  TVI consists audio interference, but the picture screen is fine.  I have
these TVI problems running just 100 watts.  Even when the TV is muted, my wife says she can
still hear my voice.  My antennas consist of 2 dipoles, one over the family room (where TV is
located) at 30 feet height, and the second one at 30 feet height approximately 45 feet away
from the TV. The TV is a 1983 Magnavox.  So far I’ve applied the following filters.
1) The cable from outside to cable TV control box.  Applied at cable TV control box entry
point is a radio shack high pass filter and one clip on ferrite on the cable.
2) I put 2 Radio Shack RF chokes (273-104) on the RCA cable line from the cable box to the
VCR, four turns each of the cable on the RF chokes.
3) From the VCR to the TV, I have a Vectronic  high pass filter and 2 clip on ferrites on the
cable at the base of the TV.
4) All power cords have 2 Radio Shack RF chokes (273-104) each at the base of the unit .
Three to four turns each.  I am still having problems.  So, how do I solve the 20 meter
interference problem?   Thanks, Merrill

Posts: 13


« Reply #1 on: May 21, 2002, 01:20:27 AM »

Try grounding the TV chassis, but do this using a .01 ufd 1 KV ceramic capacitor in series with the ground wire.  Do not connect the ground wired directly to the chassis without the capacitor.  Chances are the sound interference is a result of RF pickup on the wiring inside the TV set itself relating to the audio output circuit.  A likely culprit could be the leads going to the speaker.  Another source could be the leads going from the chassis to the volume control.  Try wrapping the speaker leads around a ferrite core kept as close to the chassis as possible.  Try to get 8 to 10 turns around the core.  You may have to extend the length of the leads to do this.  A ferrite core around the bundle heading to the volume control would be another try if the treatment of the speaker leads doesn't do the trick.  If neither of these work, you may be in for more exotic treatment.  This could include bypass capacitors for the audio transistors or IC, as the case may be, or even shielding material for sensitive audio circuits.

Posts: 79

« Reply #2 on: May 21, 2002, 02:13:03 AM »

Sounds like straight rectification in the audio circuit.The key here is that the picture is unaffected.Filtering is not going to help ,as what you have is on-frequency RF getting in your audio detector.Other than circuit mods inside the TV , The two options I see are (1) reduce the RF field strength at the TV by improved shielding or relocating the antenna ; or (2) see if another TV in the same location exhibits the same symptom. $200 at Wal-Mart and you get stereo,remote,and a nice new bright screen,plus you eliminate the whine in the XYL .Chances are you aren't bothering the neighbors ,even at higher power levels, as this problem goes away as a function of the cube of the distance .Come to think of it...this is a wonderful excuse to RAISE the antenna , as a little more separation may make a world of difference.

Good luck,

Posts: 79

« Reply #3 on: May 21, 2002, 02:31:24 AM »

After re-reading your post ,and noting the problem exists even with the audio muted, it may be that K0KP was more on-point than I realized.If the RF is being picked up AFTER the detector ,in the output circuits , I'd try the simple fix of putting a .01 mF ceramic disc directly across the speaker terminals.Regardless of where the RF is entering the audio circuits, the solutions I listed previously will still apply .

Posts: 2080

« Reply #4 on: May 21, 2002, 09:44:43 AM »

I agree with the speaker theory.  I get TVI (my transmitted audio) through my patio speakers even with the TV and stereo turned off.  The speaker leads are acting like antennas.  The capacitors should do the trick.

73, de Terry, WØFM

Posts: 2193

« Reply #5 on: May 21, 2002, 09:52:22 AM »

Is it possible that the TV audio IF section is somewhere around 14 Mhz? If so, then it's doubtful that anything simple that you do will fix the problem since it would be due to inadequate shielding of the audio IF in the TV set. Consumer electronics are so cheaply built anymore that they hardly ever put shields over the IF sections.

It's possible that some portions of 20m will not affect the TV as much as others, maybe you can find a clean spot? Just a thought if you try everything and can't get it to work.


Posts: 628

« Reply #6 on: May 21, 2002, 08:54:12 PM »

Well, thanks for the information guys. Maybe its just time to go out and get a new Sony 31 inch TV and hope it stands up to RF better than the Magnavox did.

73, Merrill    
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