Slow roll through video

[lajackson]

Okay all of you video experts. What causes a slow roll of light and dark that moves up through the picture during a satellite broadcast. It has no distinct edges, but is a set of darker areas that cover the entire width of the picture, similar to horizontal bars. They take about 20 seconds to move from bottom to top of picture.

Sometimes there is only one of them, other times there are four. I do not think they show up in the rack monitor, so there is something in the approx 250' run from the Library to the chapel. The chapel has an RCA video plug on the side of the pulpit and we run directly to the RCA plug on the projector at the pulpit.

Ideas?

[russellhltn]

AC hum. It beats against the 60Hz vertical rate in the video, but doesn't quite match it, so it moves though the picture. (Let me guess - you never had this problem until you got a new projector with a 3-pin power cable )

A quick and dirty test is to use a 2 to 3 prong AC adapter on the projector. (A ground loop between the equipment rack power ground, the chapel power ground, and tied together via the ground braid of the coax cable forms the loop.) But that's for a quick test. Since it removes a safety ground, that's not recommended for a permanent solution. The right solution is to install a "video ground loop isolator". A quick Google shows any number of them available fairly cheaply.

[lajackson]

AC hum. It beats against the 60Hz vertical rate in the video, but doesn't quite match it, so it moves though the picture. (Let me guess - you never had this problem until you got a new projector with a 3-pin power cable )

Same old projector. The problem showed up when we stopped going from coax through the VCR tuner to the coax input of the projector. Off to Google. Thanks.