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Interesting that the screen un-blanks and lights up when you set the sweep speed to X-Y. This mode causes the Z-Azis logic to unblank. See if you can find out why it's (almost) always blanked. I suspect you've got an undiscovered shorted tantalum gumdrop.

The 465 series scopes use tantalum capacitors. They are known to short out and bring the scope down.
Typical symptoms will be no trace, or powering on and going off in a few seconds due to built-in fold-back protection. The shorted capacitor in the power rails will bring down one (or more) of them causing the entire scope to be non-functional. The quick way to troubleshoot is, after powering off - and ensuring all capacitors are discharged - check resistance to ground on each power supply rail and identify the shorted power rail. Then trace the offending tantalum capacitor in the shorted rail with the help of a schematic. This could be in any board/sub-assembly, not necessary in the power supply or A9 board. You can replace tantalum caps with general electrolytic. Remember to ensure you install them in the correct polarity, specially on the negative rails.

Have a look at this thread:
/g/TekScopes/message/161306?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3ACreated%2C%2C465+No+Horizontal+Sweep%2C20%2C2%2C0%2C40966357