How to disassemble Tek 2236 ? ... for A1 mainboard HV section


I have an old Tektronix 2236 with a focus problems.
Here, at link below, as you can see, the focus potentiometer is fully counterclockwise and the trace is blur:

Can you give me any aid?
And how to disassemble the circuit parts to have access in focus-circuit (R892, C893, Q885, etc.) in A1 main board ???

Thanks everyone!

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