First off that set is running very good, about how I remember
one back in the 70's. Well done !
The vert collapse issue is bizarre. Both the vert & horz osc should
be free running at about the right freq without a signal. Only thing
I can imagine is the vert sync pulses from your source arnt right
& the vert is trying to lock way off freq. I do remember a few sets
that "blinked". It was always a bad cap in the vert feedback
( parts running from vert out plate to vert osc grid).
Now that this is wrapped up hows about finding another set for us
to work on
73 Zeno
LFOD !