I recommend you check for a couple of failure possibilities:

First: DC restorer diodes might be damaged, as Jim says.
If ok, check for cold solder joints near insity pot (cracked due to
mechanical stess, cold solder, etc.9.
If ok: I suspect CRT (had this 3 times), but CRT seldom fails to my
experience in the 2213/2215 sereis scopes.


hth,
Andreas