Weak or non-existent illumination of viewfinder information

the tube lamp is degraded an needs replaced . the high voltage to the lamp might be weak corroded connection too.
consider changing the lamp out for led strip
i ordered 1 ccfl lamp many times from china for 2.50 an they are always damaged . too long for the mail.
i can order the same ccfl lamp from us for 40.
so
im going to insert led strips in place of the lamp.
its not that hard if your a tinker

You may clean the display from dust from inside the receiver. Or replace the display if not suffering from solder cracks.

you should have LED replaced for the lcd screen

It sounds like you have manually adjusted the aperture on the lens, letting less light in and hence a dark image in the viewfinder.

If you hold a flashlight up to it can you see what is on the screen?
If so then the lcd inverter had gone out.
(That happened to my HP)

Sounds like your display's backlight is dying. Your scfreen is in need of replacement. The higher ther brightness setting the faster it quits working.

"stage lights" may still not be enough- there should be alow light indicator in the viewfinder (amoung the many things being displayed)- Also, the camera manual should tell you the minimum illumination that the camera will record in (usually listed as LUX or Footcandles, or sometimes in EV), but unfortunately you would want an old fashion light meter to check the light level of the stage ahead of time

no reset. if you think you can open it reseat all the ribbon cables going to the Lcd.

the odometer backlight bulb is replacable but the illimunation bulbs for the other gages are not.

try buying a *****