VFD anode contamination ?

[gregebert]

I just bought 3 NOS VFD tubes, still in their original Radio Shack packaging.

I'm testing one of them, and noticed that some anodes are not glowing fully, and in fact require a much higher voltage to get decent brightness.

I'm wondering if they have been sitting around for such a long time that internal contaminants have settled-in.

Has anyone else seen this before ?

I'm going to try running the dirty segments hot for a few hours; normal current is 0.5 to 1.5mA, but I'll try 2-3mA. The tube I'm testing now usually needs 25V for typical brightness, but I've got it over 100V now.

I have a few other DT1705 tubes manufactured by TungSol, but I dont recall seeing any with signs of contamination. It's also possible that Rat Shack got manufacturer seconds; who knows....

[Dekatron42]

I have several of different brands and models which show exactly the same weak anode.

I've successfully recovered displays for the HP34401a and other HP instruments in the same series by running the filaments just a little dark red hot - there is a lot of information on this, just Google it.

However with other VFDs where the phosphorous are has dimmed below the filaments I haven't had any success in getting them brighter with the same process as I guess that is something different that has happened to them.

/Martin

[gregebert]

I'm certain that dim phosphor near the filament is an unfixable wearout mechanism caused by electrons (from the filament) bombarding the surface. The electric field-strength is at it's highest closest to the filament.