Thank you for your suggestion.
I had checked the ripple content of power supply:
+30V: 20mV p-p; should be 3mV or less
-30V: 8mV p-p; should be 3mV or less
+200: 195, 1.5V p-p; should be 3V or less
+5: 8mV; should be 3mV or less
Plus 30V branch had significant larger ripple content, which may attributed to aging filter capacitor.
According the spec. the accelerating voltage was 4.5kV, pin 2 at CRT socket. My pocket DMM can measure voltage up to 1kV. A HV probe is needed.
Maybe I can order a new piece of 10kV/5mA diode first.
With Tek 485 probed at TP 12, I got an interesting picture of the vertical input signal when 5D10 display a standard graticule grid. It was digitally precise!