Resistor? or caps? connections?
http://www.arcade-museum.com/manuals-monitors/Atari Monitor TM-210 1st Printing Disco 19in.pdf
Based on the voltages you mention of 100vs
on all 3 pins means the transistor is not turned on..
Pg 13 schematic
R411 6.8 ohm would be a suspect in some case's
Pg 20 pictoral of parts and there location and transistor basing..
or pin configuration....
Q402 is 2sd478? is the part you were testing all pins had 100v's
Q403 is 2cs2073 Emitter is ground 0vs, Base ? collector ?
without the drive pulse from ic 401 the output q402 and q403 wont turn on.
from my experience both transistor should be replace as a pair
with new heat sink compound on the transistors.
Pg9 circuit description of vertical circuit
Check all vertical pots for
physical damage they are easy to break when using the wrong tool
to adjust bad connections to any of the pins on any pot could cause
loss of vertical deflection or vertical collapse vertical roll or open traces..
and it needs a cap kit if the electrolytics have never been replaced before.
Even bad connections on the fly could cause vertical issues because the supply
voltages come from it.