[Test-Equipment] Tektronix T935A Scope - seized knobs

[RJ Mattson]

Tektronix T935A Scope.
The center red "VAR" knobs on both Chan A & B Volts/Div Amp. knobs have 
seized.
In other words, either red knob won't turn at all.
What is the cure?
Do I have to open the cabinet to work in a penetrating oil on the shafts 
from the inside?
Or,
Just remove the knobs and try to un- seize the shaft from the outside?
Or ?
bob...w2ami