When you turn it on, does it seem to come to life (makes the bong sound, disk activity happens, etc.)?

If not, you have bigger problems than a (possibly) bad vertical drive chip. Check supply voltages (if you haven't already done so). Check the CRT connector to make sure it hasn't worked loose, and to verify that no solder connections need refreshing there.

Did you check the yoke connector (by unmating the two halves and inspecting)?

Answers to these questions will go a long way toward narrowing down the list of suspects.