Sylvania Halo model 21C508MP Fringe?

[noah300g]

With Predicta problems solved, onto the Sylvania Halo set that has some of what I'll refer to as 'video overload' problems. There is no local/distance switch on the Sylvania set like there is on the Predicta. I'm feeding the RF on Channel 3 from the cable TV set top box, and it seems to be overloading the front end of the set. If I tune off of the signal a bit with the fine tuning knob, I can stop the overload, but not get a very good, crisp picture. The overload is bad enough to cause sync circuits to lose their ability to cope. There is something called a 'fringe' control/potentiometer that is in the sync separator path, but I don't really know what it does? The AF 3AV6 tube also has an AGC clamper function, but that tube is good. I haven't finished all the Sam's voltage and resistance measurements yet, though. Video detector 5AM8 and video output 6BA8 are also good, as is the 3CS6 Sync Separator. That 'Fringe' control is between the AGC clamper 3AV6 and the 3CS6 Sync Separator. It also ties back to the output of the video detector and input of the Video output tube. Seems like this is the area where my problem may lie, or am I just plain overpowering the tuner? Haven't had this overload problem with any of my other sets that connect to the cable box.
Thanks for any ideas from experienced Halo set repair folks.