The 2N3904 should do in a pinch, the 2N3565 is only rated at 25V Vce. Only concern might be frequency response if in the vertical or horizontal amplifier circuits.

I am parting out a number of SC503, SC502, and SC504 scopes, if there are some parts you need from the boards, let me know and I will check if I have them. I have bags of transistors that were socketed so I would need the number from one or it's designator on the schematic so I can cross reference to the TEK number.

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