Most likely problem: your flyback shorted out.
DEC used lousy flyback transformers, and they were always my
biggest problem with all the VT terminals. They would short
out (cracking the ceramic shell and oozing black goo that would
then harden), and often taking out a driver transister too.
You can repair this problem with a new flyback (probably
going to be hard to find), and a new transister. Been there,
done that (many times), got the T-shirt.
Thank you. Maybe it is easier to buy another VT220 for $5. I will
leave the faulty one alone for a while.
vax, 9000