If it were arcing the focus pin of the CRT socket was common
on these especially if it had open S-caps or divider.
Test by pulling all ground straps & check them for HV.
But this looks external. Try using an AM radio to track it.
It can be almost anything. I live out in the woods & the QRM
kills AM radio. During blackouts it comes alive with Philly
at high noon ( apx 300mi).
We used to have an FCC that worried about interference but now
they are just a rubber stamp for bad ideas.
73 Zeno
