I have a couple of Seventeener III sets that both have this problem. When the brightness is set about right I get white retrace (scan) lines at the top of 1/5th of the screen. I can make them disappear if I back off the brightness but this makes the picture too dark.

Seems to be a horizontal blanking issue.

I assumed that the issue was related to some bad networks. So I ordered a set from Crist. But sure would like to know where to poke around and discover the actual problem instead of just replacing parts. I have the schematics.

I did a complete recap of both sets. This fixed a multitude of issues. but...the retrace lines remain.

Are they constant, or do they come and go? Retrace lines that come and go are due to Copyguard pulses on the source video.

They are constant. They are playing on all HDTV OTA tv stations. I'm using blonder tongue modulators to broadcast on different channels..