[volt-nuts] VFD wearout (was "dark display for 3458)

[Chuck Harris]

Hi Didier,

That is most interesting!

The architecture of the displays is such that there are multiple
cathode wires, one over the top of each "row" of segments, usually.
The cathodes are run dark, so they are barely emitting, and cannot
really ever burn out.

I have found the ugly looking displays are the result of the cathodic
emission right under the wire being better than between the wires,
rendering a mottled appearance.

In some of the early clocks and alarm clocks, that used VFD's (since
the US clocks are always in 12 hour display mode) if they were changed
to 24 hour mode, after a long life, the "E" digits that make up the
most significant digit were always much brighter than the rest of the
digits on the display, rendering a "2" where the newly lit horizontal
segments were brighter than the rest.

In any case, I have some VFD's that have been in continuous use since
I graduated from college, and that was a long, long, time ago.  And
they are still going strong.

I have to wonder about the failures in the 3458 display.  My instinct
tells me that it isn't the VFD but rather the driver/power supply that
has failed.  Probably a bad electrolytic capacitor if things go the
way they usually do.

-Chuck Harris

Didier Juges wrote:
> After the thread about the 3458 display, I went back to one of those VFD
> that I mentioned in my earlier post. These had been used with only a few
> digits turned on on the first line, the rest of the display being normally
> turned off and only used occasionally. After several years of continuous
> operation, the digits that were used  had lost brightness and more
> interestingly, the other digits had also lost brightness but also looked
> botched (the brightness was very uneven).
>
> It turns out I have been using one of these old displays on a new project
> (it's a development job, so the unevenness does not bother me at the moment
> since this display will not ship). After about 2 weeks of continuous use
> where most of the digits are used, the display is now just about back to
> normal. somewhat dimmer than a new one, but the unevenness has completely
> disappeared.
>
> It looks like either the digits themselves, or the cathode wire, had become
> contaminated as a result of not being used, and a few days of operation
> restored their activity.
>
> Even more interesting, at the moment, I cannot detect a difference in
> brightness between the old digits that were on for several years and those
> that were not. The display is generally dimmer than a new one, but the new
> ones are a different part number, the original device having been
> discontinued, so it may also be that the new display is brighter because of
> process improvements by the manufacturer (something the manufacturer
> advertised when they introduced the new device).
>
> On this development job, most of the digits are used, and the display is
> constantly changing (scrolling messages).
>
> Anyhow, I thought it would be interesting to mention.
>
> Didier KO4BB