https://groups.io/g/HP-Agilent-Keysight-equipment/topic/hp_8563a_spectrum_analyser/24815445

Hi, if the display characters You can see are not distorted, You only have to do one thing.

Place the analyzer on the bench, with outer chassie removed.

Unscrew 8 torx screws on the module-boards; 3 short, 2 extremely long, and 3 medium size. 

Lift up the left side of boards 1 inch, and disconnect the expansion module connector from the A2-card.

Flip all top modules to the right (A2, A3, A4 & A5).

Now You see the video-tube and it's control card on the left.

The second pot from the front on this card, I believe A17, is labeled  "bias" (the pot is placed deep down the board; 
You need a long insolated trim-adjuster.

With the softkey 'Display intensity" set to some 100, adjust this bias so the sweep and text will have the same display intensity.

Now You are set.

If the analyzer has not been used with 'over-intensity' under a long time, You can count on that the tube will last for some 
70.000 hours. (Accessed :recall: Elapsed time)

I have seen at least 20 of these lasting  this long with only slight degration in intensity.

If the text on the display is garbled, confirm the power supply unit (A6) is delivering the right voltages.

Good luck !
Bjoern.