Need Help with 1978 Zenith Chomacolor II

[freakaftr8]

Hey there,

Well I cannot seem to figure this one out. 17" Chromacolor 2 with rotary tuners, circa late 1978.

The left side of the picture is dimmer than the right, I may be dumb and should know what this is, but any ideas? Picture focus, color and contrast and black level all operate as they should..

This is a module style chassis...