Okay, today morning I had some time to take off top and bottom covers. I put scope on its side so I could see both sides. I turned it on and everything was fine (power-led on, both traces visible, no strange noises...) until... After about one or two minutes I heard first snap. Snapping interval got shorter and shorter until it sounds like intermittent buzzer. And then I saw where it is sparking. It is sparking between CRT and two springs (see attached pic, red circle). What does this mean?
When sparking, power led turns off and screen goes blank.
As i mentioned in 1st post, manufacturer is Leader Electronics Corp. Japan, model LBO-315, serial 6010129. I don't know manufacturing date but it is last serviced by FinnMetric Oy in 30.6.1989.