Saturday, July 31, 2010

Twice this year I have been transmitting at the full legal limit and my TV set has ‘popped off ...

Q Ron, N9RC, asks, “I have a 1994 RCA 35-inch console TV fed by cable. Twice this year I have been transmitting at the full legal limit and my TV set has ‘popped off.’ According to my TV repairman, the horizontal circuit has ‘shorted.’ (Apparently the horizontal transformer failed.) He says he has seen this happen in another case when a ham was transmitting at the house.

“This costs $400 per incident! I never had the problem until this year, but we have added a few VCR cables that I have now belatedly removed. I’ll be adding common mode chokes to these cables and elsewhere. I can’t afford for this to keep happening! Is there a fix?”

A Ouch! This sort of problem is rare, but it can happen. RF energy is usually picked up on any wiring connected to the TV (TV sets do not make good antennas, but wires do), then gets conducted onto the chassis of the TV, into its power supply and into its circuitry. In this case, it looks like the horizontal circuitry is particularly sensitive. There are a few things I’d suggest, although to find out if they work does entail some degree of risk.

First, the greater the separation between your transmit antenna and the TV and any wires connected to it (ie, ac, cable, antenna, speaker, etc.), the lower the RF energy will be. So, raising the height of your antenna or locating it farther away from the house could be helpful. In this case, do not use end-fed wires—only antennas fed with coaxial cable. If you have any problems with RF in the shack, correct the underlying transmit antenna problem.

Adding common-mode chokes to all of the wires connected to
the TV may also help keep the RF energy from getting into the set. You can make a common-mode choke with an F-240-43 ferrite core, wrapping about 10 turns of wire (coax, speaker, etc.) onto the core, just where it enters the TV set. For smaller wires, you can use an F-140-43 size core. You will have to treat the cable or antenna lead, the ac wiring and anything else connected to the TV. You may also want to add an ac line filter to the TV.

Some companies have developed service bulletins to deal with RFI problems, although this is generally the exception. Contact RCA and see if they have any info for you.

From QST October 2000