HP1805A Vertical Amp Problem in HP180D scope
 

I have a problem with an HP1805A vertical amplifier plug in for an HP180D
scope.

The vertical amplifier does work to a point, but suffers from "dynamic
range" problems.

The maximum height of vertical scan is only about one major division above
the center line of the CRT,
and only about two major divisions below the center line. As long as the
signal is kept
within these bounds, the display is fine. However, anything that drives the
signal beyond
this range is clipped to the plus one major division, minus two major
division range.

The display deflection range is irrespective of input attentuator setting.

All mainframe power supply readings are spot on. The +35V supply in the
plug in is spot on.

All standing bias voltages in the plug in output vertical amplifier are
nominal.

This is a two channel plug in, and both channels act EXACTLY the same.
Thus, I believe it is in the common amplifier circuitry of the plug in.

Voltages along all the common vertical amplifier chain meet manual
standards.

Focus, intensity, etc., from the mainframe are all fine and respond
correctly to controls.
The display within the available display region is sharp, well defined, and
stable.

There are no vertical electronics in the mainframe-signals go directly from
the plug in
to the vertical deflection plates.

Any ideas for troubleshooting at this point? I have no other plug ins to
try.

John...