hum from tv coax

bonsai171 · 2004-03-19 12:58 am

hello, my system has for some reason become sensitive to a hum which is coming from the coax connector which feeds the cable signal to my tv (which is right next to the stereo). I have tried crimping on a new connector, but that didn't make any difference in the amount of hum that was coming through the system. As it turns out, the actual hum is transmitted into the stereo system by a composite video cable running from the tv to dvd player, but if you disconnect the coax connector from the tv and play the dvd, there is absolutely no hum.. How can this situation be fixed??? tia

Dave

Da5id4Vz · 2004-03-19 1:54 am

I had OK results with this situation a few years ago. I used a pair of 300ohm twin lead to 75 ohm F connector baluns wired back to back.

The ground loop went away and the TV still looked OK.

I'm sure there is an isolation transformer made for the purpose, but for 3 bucks, this did the trick.

moamps · 2004-03-19 9:33 am

Hi,
Capacitor isolation network will work too.

regards

Frank Berry · 2004-03-19 9:41 am

I've tried both methods and found that using capacitors is superior. I got too much loss on the high channels when I used back-to-back transformers.

Da5id4Vz · 2004-03-19 3:08 pm

I'll have to try the cap method.

bonsai171 · 2004-03-19 3:27 pm

clarification

so that's a 10 microfarad and 100 microfarad 200V ceramic capacitor you're using?? How do you actually connect each capacitor? One more thing- what do you do if you lose some channels by doing this? Thanks a bunch.

Dave

moamps · 2004-03-19 4:04 pm

Hi,
values are 10 to 100nF (nano) ceramic.
With 75ohm load those caps make high pass filter witch attenuate ground loop current (50 or 60 Hz). I don't think that you will lose some TV channels (100 to 1000MHz band).

Regards

tiroth · 2004-03-19 4:22 pm

You are luckier than me. I have some 170VDC on my TV "ground" due to leakage in the switching supply. :dead:

Good reason to use HV rated caps.

bonsai171 · 2004-03-22 4:38 am

problem fixed!

moamps,

Thanks for the suggestion. I built one of these isolators yesterday, and it competely eliminated the hum, as well as improved the picture quality on the tv! Cheers!

Dave

Tensop · 2004-03-26 12:58 pm

hi guys, im having a sort of similar problem. im getting hum from my antenna, enough that it actually heats up the antenna cable. it runs from the antenna, to the VCR, through to my amplifier and to the VIVO port on my computer. when the ground connections from the vcr are removed(RCA shielding on both audio and video outputs) the noise goes away, however there is a disturbance in the picture... its unwatchable. is there any way to link it without getting the noise?

ta

SY · 2004-03-26 1:19 pm

Ummm... how about the two-cap isolator. It worked for bonsai.

This is a really common problem, so there are commercial isolators available, too, if the thought of adding a couple of caps intimidates you.

Da5id4Vz · 2004-03-26 2:01 pm

It worked for bonsai.

the journalsit?

moamps · 2004-03-26 4:57 pm

Tensop said:
hi guys, im having a sort of similar problem. im getting hum from my antenna, ......a disturbance in the picture... its unwatchable. is there any way to link it without getting the noise?

Hi,

You must cut ground loop somewhere. The best position is at antenna connector because information is on high RF spectrum (100 to 1000MHz). Use HF transformer (post 2) or caps isolators (post 7) for this purpose. Braking ground loop at video level (or audio) is tricky; video signal has high complexity (from low frequency 50-100Hz syncro pulses (you lose them obviously and result is disturbance in picture) to ca 10MHz).

Regards

Tensop · 2004-04-02 11:50 am

hi again,

I tried the modification( 10nf on the hot lead, 100nf on the ground) and it removed all the noise, however i've lost quite a bet of reception quality.

is it...... at all... possible to amplify the signal using an opamp with some gain?

or isnt an opamps bandwidth large enough for the purpose?

moamps · 2004-04-02 3:33 pm

Tensop said:
[I tried the modification( 10nf on the hot lead, 100nf on the ground) and it removed all the noise, however i've lost quite a bet of reception quality.

Hi,

did you insert this "mod" in antenna cable?

Regards

Tensop · 2004-04-03 11:44 am

bought an aluminium box, coax tv cable, 2 coax tv sockets, 10nf cap, 100nf cap, assembled.

Da5id4Vz · 2004-04-04 12:37 am

Have you isolated one of the F conectors? Othewise you may cary the ground througn the chassis.

Tensop · 2004-04-04 6:25 pm

Ofcourse, insulated the Antenna end ground and connected the mains ground through the VCR end antenna to the metal box.

hum from tv coax

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