[time-nuts] Re: HP 58503A Display Board (Option 001) Problem Update

[ed breya]

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