I'm sure that there's service documentation or
schematics online somewhere.
ftp://ftp.pdp8.net/misc/4010/
Tiff and PDF are identical so you only need to download one.
In a quick look I didn't see an adjustment in section 4 stated for the
problem. 6-48 says how it's supposed to work.
I did find out with mine that if you have some faint ghost image from
previous drawing that leaving it at the bright screen you get on power
on for a minute seems to get rid of it.