Tektronix 2465A only uses a tiny part of the display area

[sprok]

.

[lukier]

It looks like the beam finder button is stuck. Normally pressing beam finder produces this kind of image.

Of course power supply problems and horizontal & vertical deflection settings might also squeeze the image like that so it is always worth to check voltages & ripple on the main board (I forgot the designator for this test connector, it is in the service manual and somewhere in the big thread on 2467B).

[lukier]

Now I just need to figure out why turning any knob or pushing any button causes every single light on the scope to briefly flash. Also hear a faint buzz sound when it does it. Maybe power supply? (it's always the power supply )

Now I would be interested in the answer to that problem. Some time ago I bought 2467 (2465A with MCP CRT), replaced all the electrolytic caps on the PSU (except big input HVDC ones) board (A2/A3), most of the electrolytics on the main board (A1) and I think two electrolytics on the CPU board (A5, not the SMD version with bad tantalums as in 2467B). The voltages are now ok. I've also replaced the fan (dating from 1987) and Keeper lithium battery.

The scope roughly works, traces are fairly sharp considering the age of the MCP CRT, but I have the same issue with LEDs flashing with every button press or when I turn some knobs (for example turning readout intensity on/off). Sometimes even the scope reboots itself. I have no idea what might cause that strange behavior, some undervoltage watchdog?

The only information I found on this very problem is here: https://forum.tek.com/viewtopic.php?t=138353
where it says:

You have seen what happens to people when they change multiple capacitors at the same time? Change one at a time.

Which is of not much help as I've already replaced > 90% of the caps 

[Skimask]

Problem is I can't get the stupid knobs off. The ones with set-screws are no problem but the push-on knobs are stuck like glue.

They may very well have been glued on.  I just rebuilt a 2445 where more than one of the 'outer knobs' were glued on...since I broke them taking them off.  The knobs that don't have set screws have 2 pieces, the outer piece that you grab attaches to an inner piece which goes on the shaft of the pot/encoder/whatever.  You can buy sets of replacement knobs on ebay easily enough.  I tried 3D printing a few.  They came out 'OK' but not good enough.