Lionel:
You cannot "fix" or "tweak" a weak CRT. In it's previous life, it was
probably left on 24/7 ..... this is most likely why you were able to pick
it up for "cheap" at a market.
A CRT rejuvenator like a B&K or Sencor MIGHT be able to eek some more life
and brightness out of it.

Best Regards,
Daniel Sofie
Electronics Supply & Repair
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snipped:
"Lionel" wrote in message
I just picked up one of these beautiful 21" monitors cheap at a market.
Except for this brightness problem, it works perfectly. I'm driving it

Turning up all the on-screen bias & gain controls to the
maximum values produces a usable, but still darkish image. I was
assuming that as the monitor is about 4 years old, it'd probably just
need a cleanup, & for me to tweak up the EHT, but having obtained a
schematic, I now know that there are *no* internal adjustments at all! I
Any tips or ideas?