Panaplex display history

[Brent Hilpert]

On 2015-May-30, at 8:14 AM, tony duell wrote:
>> calcs that used the smaller versoions. In calcs, they were largely superseded by vacuum-flourescent displays 
>> which were easier to drive, had a longer life, and could also be made with bright, large digits.
> 
> I am surprised about the lifetime claim here. The VF display is a hot cathode device, which tends to imply a 
> shorter life than a cold cathode part. VF displays were commonly used in consumer electronics (VCRs, etc)
> in the 1980s and 1990s over here, and uneven segment illumination due to low emission from the filament 
> was a common fault. Conversely, my HP9815 and HP59304 are, AFAIK, still using the original panaplex
> display unit with no problems.

Well, I have some number of functioning calculators using both Panaplex and VF displays. So yes, they both have the potential for longevity, but then one has to know the actual runtime of the considered devices for an accurate comparison. In my experience, failure over the typical use lifetime of the application device or for long runtimes is more likely for Panaplex/7-seg GD than VF displays. I've seen more faulty Panaplex displays than VF displays, even though the Panaplex are less prevalent.

Both Panaplex II and the Beckman displays seem to fail with a sort of 'burning'  around the segments and/or the anode coating on the glass. I'm not sure if it's ion deposition or deterioration of the element. The segment/s then fail to ignite uniformly. I'd say they are less reliable even than the similar-principle NIXIE tubes.

The filaments of VF displays are run at a low enough intensity that they have a very long life. I haven't seen problems with emission, I didn't think they were prone to such as it's simple thermionic emission, not the 'chemically assisted' emission of coated, indirectly-heated cathodes. I think some of the large complex VCR VF displays had uneven illumination just from border-line design of the physical relation of filament to segments.

I have a 6-digit clock I built in 1978, from individual VF tubes scavenged from a 1972 calculator. The calculator was used in an accounting office so it likely saw a fair degree of daily use in that first phase of life. As a clock, they've been running continuously 24/7 since 1978 (with the exception of the occasional power outage, repair, and one ~ 5 month period. It does have variable pulse-width dimming on the 60 Hz filament supply which would improve the VF tube longevity. I kept the two additional tubes from the 8-digit calculator as replacements but have yet to need them - all the clock digits are fine and bright, while the LSI clock IC failed once. Compare that to the lifetime of neon bulbs running continuously as power indicators on things like power-bars and freezers, which commonly fail or start blinking after some number of years.