Hum Bars Hampered the TV Screens

DN Staff

March 7, 2014

3 Min Read
Hum Bars Hampered the TV Screens

Here at XTI Vancouver, we recently completed an audio-video system for a local restaurant/bar that underwent a major renovation. The video portion of the system has 4 LG 60-inch TVs behind the bar, with a Sharp 80-inch and two Sharp 70-inch screens mounted on a faux brick wall perpendicular to the bar.

The LG sets all have three wire U-ground power cords. The Sharps all have two wire cords with no ground. The customer's electrician ran two Cat5e cables to each set, and we crimped 8P8C plugs on each end.

All sets are fed from a Crestron 16x8 component video matrix switcher. This particular switcher has Cat5 balanced outputs as well as standard 75 ohm outputs, so we used the balanced outs and installed passive Baluns at each set to convert the balanced 110 ohm signal back to the unbalanced 75 ohm coaxial inputs on the sets.

About a month later, the 80-inch Sharp developed a 1 pixel high dark line across the width of the screen. It refused to go away. Evidently a driver had gone bad. No big deal because that's what warranties are for, right?

We didn't supply the sets, so I had the client call his supplier and get him to bring us out a new one. A brand new 80-inch Sharp (of the same model) arrived the next day, and we proceeded to install it.

As we were lifting the 130 pound set into position, I noticed that we had inadvertently put the mounting plate on the wall upside down. This particular mount has an expandable backplate with a double flange on one side, but only a single flange on the other. The double flange is supposed to be on the top.

Technically, it doesn't really matter which side is up, but the arms fit better into the side with the double flange up, so I removed all 12 lag bolts, flipped it over, put the lags back in the same holes, and put the set back up. We connected it up with the same cables and the same Balun, and turned it on.

We were rewarded with what appeared to be nasty hum bars all over the picture -- bad enough that it was unwatchable. How did that happen? After scratching our heads a bit, we decided to switch Baluns with one of the other sets that was working fine. Maybe a solder joint had failed while we were moving it around.

Nope. No difference.

Hmmm. Let's try the other Cat5 at the 80-inch to see if the original cable somehow got munched. No difference. Okay let's take the Cat5 cable feeding a "known to be good signal" to the set next to it, plug that signal into the Balun on the 80-inch, and see if that fixes anything.

Nope. Still nasty hum bars.

Next, we took the "bad" signal cable feeding the 80-inch over to the 70-inch next to it, and plugged it into the Balun on that set. It worked fine. So it couldn't be the feed from the rack or interference on the cable.

It was a long shot, but we tried a different AC outlet. Nope. We went back to the rack, disconnected all the cable vision feed cables at the receivers, since they have been known to cause grounding issues. Nope.

We disconnected everything from the switcher except an ungrounded DVD player and the one 80-inch set. Nope. Still the hum bars. Gad, I'm starting to lose my patience.

I then asked the client if any work had been done in the restaurant between the time we took down the original 80-inch (which worked fine, other than the line across it), and when we put up the new one. Apparently the electrician had run in a new line to a dishwasher in the kitchen, but it seemed pretty unrelated, so that was effectively a nope.

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