From 15 V to 28 V seems a rather small change compared to the data in the calibration section. Maybe you set the time base for a slow speed that activates the Intensity Limit?
With Q1107 (a slow 2N3904???) removed on my 7603, the output at point
B is 14.8 volts which closely agrees with our current calculation so I
wonder if Marian's <public@... mailto:public@...> measurement of 20 volts at the
output of the z-axis amplifier is significant and indicates a problem.
I get the same 14.8 volt output with Q1107 installed, the readout off,
and the intensity control turned to minimum.
I also measured the output at point B and as the intensity control is
turned fully clockwise, the output increases to about 28 volts