Hello I had the exact oppsite symptom: CH1, 2 30% nonlineariyt as traces enter lower 1/3 of CRT, but fine in center or top.
CH3, 4 OK.
I used a triangle wave to see the nonlinearity clearly.
Chuck mentioned that this symptom is indicating a failed Hybrid, Channel switch U400.
You can find the signals in and out of the TEK Hybrid on the A1 main bd schematic.
IF it is indeed U400, BEWARE: Used parts on epay or from China may also be defective, TEK 155-0236-00
Q service in Greece has them ~$100, or perhaps Sphere in BC.
Be careful in replacing, static sensitive and follow Chuck's procedures to remove and reattached.