NotTaR of Television Sets : Problems with ground loops and video hum..  
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Problems with ground loops and video hum bars

"I am having a problem isolating where my ground loop problem is coming from. The symptoms I see are Bars on my TV which scroll up the screen. The problem is these bars come and go, and when they are present they vary in intensity. I have verified that the cable ground is connected to the earth ground on the outside of the house, but the problem still remains. This problem is also screwing up any attempts to do video electronics experimentation. I am really tired of seeing these bars and any help you could give would be appreciated."

(From: Paul Grohe (grohe@galaxy.nsc.com).)

  1. Do these bars show up on other TV's connected to the same cable?

  2. Is your TV connected to anything else? A/V receiver? VCR?

    If so, unplug ALL the equipment and plug it in one-at-a-time until the hum appears. If you have an AV receiver in the system, try running a jumper wire from the incoming CATV ground at the TV to the receivers chassis ground (usually the "phono ground screw").

    If you have any devices with un-polarized plugs, unplug them and rotate them 180 degrees, and plug them back in.

  3. If you connect a temporary antenna and view "off-the-air" signals, are the bars still there?

If you still cannot eliminate the hum, try building a simple "ground isolator" out of two 75-300 ohm baluns, as described in the link below:

Place it as close to the TV as possible.

(From: Charles Godard (cgodard@iamerica.net).)

This seems like a cable company problem, but you need to prove it to the cable guy before he will start climbing pole's and changing amps and couplers looking for an intermittent amp. (And I don't blame him.)

At the main cable line to the house and remove all couplers and put a single line from the cable direct to the rf input on a single TV, then watch it for a few of days. If the problem re-occurs call the cable guy and show him what you have done and explain the problem again.

Put yourself in the shoes of the cable guy. He comes into a house with VCR's and all the gadgets we all have hooked up to our TV's with lines running all over the house, and can't get to the back of the TV to see what's there, and he's not a TV repair guy anyway and nobody else in the neighborhood is complaining and this problem may happen when it rains but it may not. mmmhhh

If it does not show up on the single TV, then the problem is probably yours. Add one device at a time until you find the trouble maker. Start with the your Cable AMPLIFIER.

(From: 4real (alan69@iname.com).)

You eliminate all of the other junk attached between your main cable input to your house and your TV to be sure it isn't the cause.

You will definitely want to suspect a problem with the amplifier you have installed. Especially if it is one of those cheap ones. Usually when the filter capacitor in an amplifier goes bad it will cause the hum bars you are describing, and they can be intermittent. Another problem may be that you have too much signal going into the amplifier. Amps are rated to handle a certain amount of input signal (measured in db) depending on the number of channels you wish to amplify, and the gain provided by the amp. If you try to feed an amplifier with too much signal it will overdrive it and cause a venetian blind, or herringbone effect. It could also be possible that the cable company is supplying a signal with reverse tilt. That means more signal on the lower channels and less on the higher ones. The lower channels might be the ones overdriving the amplifier. The only way to tell for sure is to measure it with a signal level meter. (very expensive unless you happen to be in the business and have one handy) If this is the case (too much signal going in) you probably don't need the amplifier to feed only 2 TVs.

The last thing I can think of and the cable guy should have checked this: They use 60 VAC on their main trunk lines to power their line amplifiers. The taps which feed the individual houses are supposed to prevent this ac from going to the individual lines. Occasionally one of these devices fails or a line guy forgets to pull a fuse and hence the ac gets sent to your TV. It won't necessarily fry your TV but can cause problems. It may even damage the TV tuners that are connected to that feed. In most cases if you touch the center conductor of the cable and a good ground you can feel the ac. It isn't enough to hurt you but you will definitely know it's there. To be on the safe side you should test it with a volt meter.

(From: Cliff R. (craeihl1@nycap.rr.com).)

My guess would be your cable amplifier. The fact that you see TWO bars on your screen tells me that it's 120 Hz interference - the frequency caused by ailing full wave power supplies used in these amps. Take the amp out of the line for a few days. If you don't have "snow" in the picture with it out, s...can it! If you find it was bad and can't live without it, you might try making sure all your internal cable, splitters, and connections are good quality & in good shape. Radio Shack stuff......well, it stinks! You can purchase primo splitters & cable from your cable company and its not that expensive. Certainly cheaper than an amp (which you might not need if the cabling were up to snuff).

You could also cry to the cable company for more signal into your house. This may or may not work but it's worth a shot.

I would put an amp in line only as a LAST resort. Most of the inexpensive amps sold are......cheap. They can easily cause more trouble than they cure. If you must, go with a primo unit from Blonder-Tongue or Jerrold.

(From: Charles Hope (charles.hope@argonet.co.uk).)

It sounds very like a problem that I had and solved.

Cause: Modern TV sets antenna connector does not have true earth on the screen but is at a potential of half mains supply. It is possible to draw about 30 micro-amps from this.

Hum bars are induced in the amplifier because there is a small resistance in the earth path between output and input giving about 1 volt drop of this stray mains signal. Worse when raining because the cable ground is better then.

Solution: Either ground the antenna screen or fit a "braid breaker" in the screen.

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