hi, from op seems your vertical deflection is dead, trace the power supply for vertical board
so focus first on this vertical board i see in sm for OS-9020G page 60 the voltages are +-12VDC, so maybe this module or his input are the culprit (i didn't watched the video but the photo says V deflection is dead)
higher voltages means somebody doesn't operate and of course V goes higher
i'm not sure the schematics are correct for your model, but may be used as guidance for your case, usually they just 'tune' a little bit between models of same product series.
please refer to that manual if you can, it's simpler for us to trace this than to trace R801A or R428 in some old pdf, we can't search text in that pdf, at least not after 1 hour of ocr work on that pdf.
or draw on paper what you reverse engineer
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now i'm sure the schematic 134639836-LG-OS-5020G-Oscilloscope-Service-Manual.pdf is not the same, you have some picture with P210 connector and R428 comming from pin 6/P210 and in the schematic page 59 we have R428 2K tied to +12V rail. if any resistor is fried there should be R429, but in that V defl preamp i doubt resistors are hot.
i'll dig a little more see if i got something valuable