120 Hz Ripple
A single horizontal line that usually travels up the screen slowly. Key is, here is there are 2 lines at a time on the screen.
This is typically caused by 120 cycle ripple from an A.C. line full wave power supply. This could be a low voltage power supply or the main power supply.
As with the 60 HZ scenario this offending ripple doesn't exactly match the phase of the 60 Hz vertical rate. We will see 2 horizontal bars or lines moving up or sometimes down the screen.
This symptom is not very common on most modern televisions because the main power supplies Chopper, or Switch mode power supplies. HOwever the main filter capacitor for the raw DC source can cause 120 hz ripple in the picture but far more intense, but will have the double effect moving up the screen.
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Dennis Viereck email@example.com