In article , frank
wrote:

Hi all,
has anyone ever seen a VFD (vacuum fluorescent display) failure in which half
the display is very dim and the other half looks good?
I have this 16 character (16 segments) 1 row display and the first 8-10
characters have a normal brightness, the rightmost ones are very dim, barely
readable.
I first suspected a problem with the filament supply, but swapping the
filament pins has no effect, so I'm a bit puzzled.
Waveforms look identical on all the 16 grids too.
Unfortunately I have no schematic of this instrument (digelec 824 eeprom
programmer), nobody else in the world seems to have this programmer also,
so I could not ask anyone for some quick waveform comparison.
I'm starting to suspect a bad VFD but I can't imagine how would it fail
like this.
Part of the filament supply is made with a NE555 that's getting quite hot
and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all
components I could test looks ok. I swapped the 555 and it doesn't make
any change. If I leave the 555 out, the display has no filament supply.
Any hint is really welcome.

If the filament were made up of two parts in parallel, and one of the
parts failed open, could that account for both the dimness and the
overheating 555?

Take the thing into a dark room; often you can see a dim glow from the
filament. Do you see it over all parts of the display, or only over the
"normal" part?

Isaac