Try these three steps:

1. Check the TV's horizontal and vertical hold.
2. If the TV is connected to a VCR, try these options:
   
1. Make sure the VCR is on channel 3.
2. Verify that the VCR is "fine-tuned" to channel
3. Locate and adjust the fine-tuning mechanism until the picture comes in clearly, and Confirm that the <3/4> switch is in the correct position. Locate the switch on the back of the VCR and set it to <3>.