You say there's continuity from each VFD pin to the driver IC, but what
about to everything else? Check the problem line resistance to ground to
see if something's shorted or overloaded. As I recall, the VFD segment
(plate) lines want to go up to some positive voltage like 20-30 V, while
the heater/emitter wire should be at some intermediate voltage, and the
digit scan/select grids should be near ground/logic levels - it's kind
of a vacuum tube.
Also as I recall, the segment driver IC outputs are typically not that
strong, having to supply maybe a few hundred uA maximum, so if a line
gets grounded or hooked to some low voltage, the segment won't be able
to get high enough to light. Anything that dumps enough of the drive
current to a low level point can blank it out, so look for solder
splash, bridging, dirt, corrosion, wire clippings, etc on that line over
its entire trip to the VFD - it doesn't have to be a dead short, just
low enough to dump the current somewhere. I doubt that the original IC
would be damaged from this.
Even though you don't have schematics, you can get a good idea of what
it should be doing by studying common VFD circuits used in other
applications.
Ed
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