475 Vertical Position control problem


My 475 just developed this problem:
On either ch1 or ch2, I cannot properly control the vertical position of the trace,
in any mode - single channel, alt, chop, add.

The control does move the trace, but not smoothly, and never below about the
middle of the screen. The kicker is that either vertical position control moves
the trace for "it's own" channel and for the other channel.

Any pointers before I start? Fortunately, I just finished repairing the
Z axis amp on my other 475, so I'm not scope-less. Thanks in
advance for any help you can offer!

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