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

[Stan]

Thanks for the troubleshooting advice! I will check for trace continuity,
shorts, contamination, etc. and let you know what I find.
I'll be the first to admit that I'm still on the steep part of the learning
curve for VFD displays!
There's still a nagging question I have: What do you suppose might have
caused the display segment failure to happen spontaneously? The 58503A had
literally been running undisturbed on a shelf for years.

Stan


Message: 3
Date: Mon, 13 Dec 2021 22:41:19 -0800
From: ed breya <eb at telight.com>
Subject: [time-nuts] Re: HP 58503A Display Board (Option 001) Problem
	Update
To: time-nuts at lists.febo.com
Message-ID: <61B83C8F.6020805 at telight.com>
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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