Why do you waste your time with useless and not interesting videos?
I asked you:
You could try to tweak the VR501 trimpot and see if you have something on the screen.
Why have you not done this ?
Want you really to fix this oscilloscope?
We are trying to eliminate one by one all possible causes for which you have nothing on the screen:
- The supplies have been checked and are OK
- The high voltage seems to be OK (no way to measure it)
- Heating of the CRT looks OK
- The spot is at the center (D1 = D2 and D3 = D4)
Other voltages you measured have no interest because it can not help to have something on the screen.
It is almost impossible that the CRT suddenly become defective.
The only possibility would have been a defective open filament, but it glows so it's OK
Then, only the cathode voltage / Whenelt may still be responsible for the problem.
You measured this voltage and it is - 20V at max brightness
This is correct but without the blanking pulses, it is too much negative voltage for having a spot on the screen.
That's why I asked you to vary the trimpot VR501 with brightness potenciometer at maximum intensity so to reduce the cathode voltage / Whenelt and see if you can see something on the screen.
If it's ok, that's mean that you have a blanking pulses failure.
Why do you not do what you are asked to do ?