Oscilloscope HP 1703 under repair

[fant]

I have this oscilloscope that has an odd problem:
the luminosity of the screen in normal operation is very low, at the limit to be seen.
Instead if I use the memory function the luminosity is normal and the trace remains till one week even if switched off.
All the other functions are fine.
The luminosity in normal operations dropped from normal level in about one month (I use it not very often, say once a week).

What could be?

Thank you in advance

[m k]

Maybe a contact problem.