First things first:
If it exists check the mains power selection is correct.
Check the AC value on the DC voltages of the power supply, should be 10's of mV or less. Don't mess with HV power supply, from your video, it didn't look bad.
Are there any inputs on the back? is there anything plugged in them?
Unplug and replug all internal conectors. Don't do this for the crt gun connector, just gently push straight in if it's slanted. don't touch the suction cup, if it's not sparking it's fine as it is.
I googled and got a similar (but not the same!)scope from
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Gould%20Advance/OS250.pdfUse the settings from page 5, section 3.2, with nothing plugged in you should get a trace, if not toggle the "bright trace" button repeatedly.
If you don't get a trace go to page 6, section 3.7, you need a function generator, feed the same signal to both inputs, but set the volts/cm to be one step different from X to Y. You should get a lissajoux figure. If not toggle all switches back and forth a few times.
Post results.