Dear Tom, Michael and Mark,
Thank you very much for your extensive answers.
I will attempt to diagnose / repair the TEK475 in the next few weeks and will surely report back on the diagnosis/results.
I'm also very grateful for the advise of Mark on the Changing Brightness due to an OFF SPEC R1378.
This scope initially had this problem very much, but somehow it became less ( I bought it 4 years ago).
I will have a look at this circuit as well.