National Oscilloscope VP-5220A 20MHz analog oscilloscope repair - no picture

[NeoRecasata]

Hello! I have this scope that was stored in a hot area of my house. It was working well before storage but after some years, it seems to have no picture/ display on the CRT screen but the power led and screen lighting is working.

I tried googling the service manual for this scope but I couldn't find it.

I've tried to work on repairing this scope before, here's what I have done that time:
1. Removed the main power connector from the AC mains step down transformer and measured all voltages.
    it has 3 windings measuring 46v AC, 89v AC, and 18v AC (center tapped).
2. Isolated the power board (removing all 5 jumpers to other boards).
3. Measured voltages silkscreened where J1 is connected (J1 connects the H amp board)
    it measures +15v DC on the +12 volt rail, other silkscreened voltages measured fine, +B1 is 115v DC, +B2 is 250v DC.
    -- I have seen a small trace jump on the CRT screen of the scope when I switched the power on (while disconnecting 5 jumpers to other boards), in other words, a dot goes down from the center and then disappears.

Those were the things I have done before I gave up 

I have no knowledge about repairing analog devices ie. oscilloscopes, but I have tinkered with CRT displays before, knowing the risks of working with CRT screens.

I think heat caused this, probably the electrolytic caps must have dried.

Here are the photos of the scope with descriptions:
It includes some pictures of the boards and where I measured voltages.
https://imgur.com/a/AIW3MDK