BK Precision 2120 rotation control

[Sparks]

I recently scored a nonfunctional BK 2120 oscilloscope from a thrift store and brought it back to life. The only problem I'm having is that the trace rotation pot is completely missing. After a quick search on the web I came across a schematic (didn't save it ) which had it listed as a 5k?. Once replaced it worked, mostly. The trace is all straight, except the lower right quadrant drops down. Any advice would be appreciated. 

[tautech]

Got another scope?
Might be a non-linear sweep.

You'll need that schematic, better save it this time.
Better still a Service manual.

Edit
You'll need another scope to confirm sweep.

[Sparks]

Even drops if I lower the ground to the lower half of the screen

[tautech]

Even drops if I lower the ground to the lower half of the screen

Happy with my faulty sweep initial diagnosis but post a photo to be sure.