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NotTaR of Television Sets : Wiring transmitted interference
Copyright ©
1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.
I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.
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The power that comes from the wall outlet is supposed to be a nice sinusoid
at 60 Hz (in the U.S.) and it probably is coming out of the power plant.
However, equipment using electric motors (e.g., vacuum cleaners), fluorescent
lamps, lamp dimmers or motor speed controls (shop tools), and other high power
devices, may result in a variety of effects.
While TVs normally include some line filtering, the noise immunity varies.
Therefore, if the waveform is distorted enough, some effects may show up even
on a high quality TV.
Symptoms will usually be one or two areas of noise moving slowly up the screen.
The source is probably local - in your house and probably on the same branch
circuit - but could also be several miles away.
- The rate will be the difference between the power line frequency (60 Hz in
the U.S.) and the scan rate (59.94 Hz for NTSC). This results in a drift of
about 16 seconds for a complete cycle (8 seconds if the inteference is at
120 Hz).
- A single bar would indicate interference at the power line frequency.
- A pair of bars would indicate interference at twice the power line
frequency.
Either of these are possible.
- Try to locate the problem device by turning off all suspect equipment to see
if the problem disappears.
- The best solution is to replace or repair the offending device. In the
case of a light dimmer, for example, models are available that do a better
job of suppressing interference than the typical $3 home center special.
Appliances are supposed to include adequate noise suppression but this is
not always the case.
If the source is in the next county, this option presents some significant
difficulties :-).
- Plugging the TV into another outlet may isolate it from the offending device
enough to eliminate or greatly reduce the interference.
- The use of a line filter may help. A surge suppressor is NOT a line filter.
- Similar symptoms could also be produced by a defective power supply in the
TV or other fault. The surest way of eliminating this possibility is to try
the TV at another location.
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