Gas Discharge Display

[MotoDan]

I recently picked up a piece of avionic equipment that has a gas discharge display which is dead due to a small crack in the corner of the glass. The displays are no longer available and this one is much too complex to roll my own LED/LCD. The elements don’t appear to be damaged. Wondering if it might be possible to replace/repair the glass and re-evacuate?

[LaserSteve]

If it has a getter, and the getter has powdered, from exposure to H2O and O2,  NO.

If for some reason the getter has not powdered, if it is not depleted, and if it is one of the contained, somewhat reusable, reactive type,  maybe, but designing the RF inductive coil  system for reheating the getter is a problem.   Placing a getter in the fill stem is a possibility, but usually there is no room in the equipment for an enlarged fill stem.

The second problem is the COE of the glass, coefficient of expansion.   Without knowing the COE, it is very difficult to select a new glass for the fill stem.  It the COE doesn't match within 7 to 10 parts per million, it will likely crack, especially with soft glass.  Usually the fill stem is sealed so close to the body that getting a seal onto it will do further damage.  Soft glass (soda lime, or Lead Glass )  of   C0E 110  and COE 80 are both  out there in displays and common.  COE 52 is rare, its a Borosilicate or more likely Aluminosilicate,  and it can be out there.

The third problem is resealing the crack.  In many years of playing with glass, I can assure you, with soft glass, no matter how gentle I am with heating, ie pre-annealing,  the crack will propagate forwards.  So next folks ask what happens if  I seal it with a vacuum grade epoxy.  If your lucky you get a few minutes to a few  months of operation.

The fourth problem is re-activating all the electrode surfaces to degas any water vapor.   Will it survive bakeout?  If the seal is epoxy, likely not.  I have a ceramic loaded epoxy I use for this sort of thing, and it is hit or miss.  Worse yet it if has a phosphor on the anodes like a VFD, is cleaning the phosphor.
 
Its easy to process a new build tube.  Its not so easy to refill a used tube.

If I had many of them, eventually I'd get it right.  I could switch to a sealing frit that fires at a lower temperature then the body glass melting point.   That firing point is usually 100-150'c lower then the melting point of the body glass.   There are two types of sealing frit, one devitrifies and thus it's melting temperature goes through the roof.   If the display uses frit, and it is of the devitrifying type, maybe. If its of the non-devitrifying type, it gets soft and the old seal can move.   Is there a stripe of black or dark green, maybe grey, all around the edges of the tube, usually inside the clear  glass?  If so, that is Frit. 

In the rare case that it uses Dumet seals to reach the  electrodes, has no frit,  and the crack is not near a glass to metal seal, its blackened to indicate its COE, and has a hugely long fill stem.. That might be worth a try.

I started with  COE80 leaded soft glass (Neon Tubing) and soda lime, back in 1986 or so.  I quickly moved on to borosilicate  for a reason.

Your fill will be a Penning Mixture of  Neon 99.5%, Argon balance, most likely.  There can be traces of mercury or xenon for various reasons.  Fill pressure is likely 30-60 Torr, again, likely, not for certain.
 
For a one off, probably no.   Returning to flying status is a NO.   If it were easy, rebuilt Nixies would be a thing.

Got a picture?

Sorry for the bad news  in advance...   

Steve

[Andy Chee]

The displays are no longer available and this one is much too complex to roll my own LED/LCD.

What do you mean by complex?  I would try and find suitable fitting OLED graphical displays, then code up a microcontroller to interface between it and the original gas discharge segment drivers.

[LaserSteve]

.

Motodan, got your PM...

Steve

[CaptDon]

If this is one of the older DME or LORAN-C style dash mounted avionic units chances are you can find a suitable replacement display, perhaps even new in the box. Fixing the old display has a 99.999% chance of failure from the start and a probable short life expectancy or unsatisfactory 'look' to the rebuilt display. The biggest problem we had (besides physical breakage) was the segment would retreat from the intended line to a 'dot' and the reason was visible. It looked like a tiny dot of lower resistance and the segment glow would accumulate in the area of that dot. The gas fill will also be a problem to obtain. All of the plasma display avionics I ever worked on used simple off-the-shelf neon plasma displays, anywhere from 2 to 8 digits per panel and some with 'signs'. Perhaps the avionic rated glow discharge displays may have had extra supports to survive extreme G forces. These displays tend to come up surplus new in the box at vendors like Sphere and even some of the vacuum tube hucksters. We went through a lot of misery with the gas glow discharge tubes in pinball machines when I worked in that industry. They had a very short life and I think were the main reason most of those old machines ended up crushed, burned or in landfills. The pinball repair parts houses charged about $100.00 per panel and generally you need 4.

[jonpaul]

pix unit and display and crack?

j

[LaserSteve]

it's a DME panaplex made by Dale. One large chip in the face on large chip in the edge connector portion of the glass.

OP had problems uploading,  I did it for him.⁹

Steve

[LaserSteve]

I see arcing / burning of the contacts. and ITO film.

I have no idea what COE the black glass would be on the fill stem.

Steve

[MotoDan]

Thanks for everyone’s comments. It seems obvious that repairing this display is out of the question. It’s amazing to me that one of the largest US manufacturers of avionic equipment chose to use the already proven problematic gas discharge display instead of LED, LCD, OLED, TFT, etc. They did switch to LCD for this unit mid-way of their production cycle.These GDD’s are fragile at best - especially in a high vibration application like an aircraft.

[CaptDon]

They really looked 'cool'. They had a certain 'wow' factor when it was dark in the cockpit and the orange glow (sometimes with a red filter) did not diminish night vision. Looking at that display it is very easy to tell it has a lot of hours on it and was nearing the end of its life cycle. I guess all gas discharge displays suffer from ion bombardment sputtering damage. Metal displacement in Nixies is so bad they eventually develop shorts between the numerical elements. The clear glass gets a mirrored 'getter' look. Ever see one of the old 'flip phones' with the plasma display? They were retro cool also. About the size of a pack of cigarettes with a little pull up antenna. Like something right out of the Jetson's cartoon.

[MotoDan]

This display is for a Nav/Com, not a DME or Loran. It has a 1998 date code. It was a custom display with no apparent NOS available.

[CaptDon]

Imagine how pissed off you would be if you paid $8000.00 for that NAV/COM and had to chuck it in the bin because the display took a shit.

[CaptDon]

I bet you could buy a similar working unit with the same display from an aircraft salvage company. Sadly, that display will be headed to the dumpster in another 1000 hours or so probably. I bought a fully functional RCA monochrome radar AVQ-46 for $200.00 It needed a new T.R. tube from Richardson Electronics for around $98.00 and I replaced the mixer diodes with some lower noise hot carrier devices and adjusted the L.O. injection accordingly. Sold it for around $1000.00

[tooki]

Thanks for everyone’s comments. It seems obvious that repairing this display is out of the question. It’s amazing to me that one of the largest US manufacturers of avionic equipment chose to use the already proven problematic gas discharge display instead of LED, LCD, OLED, TFT, etc. They did switch to LCD for this unit mid-way of their production cycle.These GDD’s are fragile at best - especially in a high vibration application like an aircraft.

Were they problematic, and if so, was that actually known at the time it was designed? You said its date code is 1998, so it was likely designed many years before that, due to the time to get certifications. 1980s and early 90s LCDs weren’t that great, so for an application like avionics where it had to be legible no matter what, it’s unlikely a contemporary LCD would have been quite good enough. People forget that it wasn’t until the mid-2000s that flat panel LCDs started to actually be good. Until then, even a low-end CRT handily outperformed the best LCDs. The best monochrome active-matrix LCDs of the early 90s still struggled with many lighting conditions. (Backlit models struggled in sunlight; reflective models were impossible to read in low light. So neither works for a cockpit.) And of course LCDs can cause problems with the polarized sunglasses widely used by pilots.

OLED was not commercially available at the time, still being years away.

1980s-early 90s LEDs likely did not have the brightness needed for direct sunlight, which is an absolute need in aviation.

Sounds to me like you wish they had simply used their time machine to purchase displays from the future, rather than limiting themselves to what was available at the time they designed it.

[SeanB]

Yes a lot of the current glass cockpits use an actual CRT, because you can make it non reflective, and daylight readable. You also still find some manufacturers using numitron displays, because they are easy to interface, they are dimmable easily, and are actually pretty long life items, especially if the designer included the keep alive resistors to keep the filaments at around 400C when off, so they are already pretty high resistance and faster to respond. Also an easy item to retrofit to LED 7 segment displays as well, often running the LED at the same current as well.

[tooki]

Yes a lot of the current glass cockpits use an actual CRT, because you can make it non reflective, and daylight readable. You also still find some manufacturers using numitron displays, because they are easy to interface, they are dimmable easily, and are actually pretty long life items, especially if the designer included the keep alive resistors to keep the filaments at around 400C when off, so they are already pretty high resistance and faster to respond. Also an easy item to retrofit to LED 7 segment displays as well, often running the LED at the same current as well.

In what way can a CRT be made nonreflective and daylight readable that a modern LCD cannot? CRTs have glossy glass faces until an antireflective film is applied, and LCD brightness can be increased to make them daylight-readable. (Outdoor signage LCDs do this to great effect, and simply need active cooling to keep the panels from melting in the sun.)