Wanted, 14 digit bubble LED display

[Specmaster]

Update on the displays, following the arrival of yet another calculator (SR4190R) I now require 2 of the 14 digit LED bubble display module please to fit into a Commodore SR4148R and a SR4190R which has a couple of segments missing. Unlike the 12 digit displays which are 8+1 & 2+1 digits, this one is a 12+1 & 2+1 digits.

Or of course, any suitable donor calculators, which might have a workable display module.

[Zoli]

Did you thought about building replacement from scratch? 0201(0.6X0.3mm) LED's are available, and you can order ready built boards, so you don't have to deal with µscopic parts.
If you need help, I'm pretty sure you will find it here, myself included.

[Specmaster]

I thank that could be a possible solution but would also need a custom PCB and then there is the problem with getting the bubble lens mounted to it, which also forms part of the fixing method to the red plastic cover which forms on the back of it a tray which the bubble lens is a pretty snug fit in it?

I can't find any of these ready build boards you refer to either, can you give me a link to them please?

[ledtester]

I can't find any of these ready build boards you refer to either, can you give me a link to them please?

I think what Zoli means is that you can design a custom PCB and also have the PCB manufacturer buy the parts and assemble the boards for you. For instance, PCBWay offers such an assembly service:

https://www.pcbway.com/pcb-assembly.html

[Zoli]

As ledtester said, I've meant to design the PCB from scratch, and give the design to a board house to build. Details about the lens mounting etc. is to be worked out; I suggest, that if you're interested to rebuild the display, open a tread under the "Projects, Designs, and Technical Stuff", so we can keep the present thread clean, and maybe attract some more interested eyes to the subject.

[Gyro]

You wouldn't get SMD leds under the lens assembly, those 7 segment LED dies were tiny - the bubble lenses provided considerable magnification. You could probably use them under an ordinary diffusing mask but it would ruin the authentic look compared to the original displays.

[Zoli]

I've presented the options: 0201(0.6X0.3mm)&up; are those too big?
IIRC, Specmaster has two "defective" 12 position displays, so maybe he can open up one, and start measuring around, if he choose to go this way.

[Gyro]

From memory, yes, I think they are too big, possibly too thick too although the 7-segment dies did have bond wires. Probably the main issue is that smd leds have square dies whereas the original 7 segment display dies have precise inclined line shaped light emmitting areas (plus a very tiny decimal point). Microscopic (photo developed?) transparencies would be needed to convert the individual LED lights to achieve some form of reasonable line segment display.

I'm not trying to knock the idea, just to point out some of the challenges.


EDIT: The bond wires give an indication of die size...

[Zoli]

Good point with the decimal points, that will make the whole project a no-go. Even if the segments can be rebuilt, no way to find proper decimal point.
Thanks for pointing it out.

[Specmaster]

There are 15 genuine correct displays in USA for £57.611 delivered (plus possible import charges) but frankly, that price just sucks, the whole calculator only cost £49.99 when it was new so no way am I paying that kind of money, something will turn up. I just purchased another none working calculator for £12.50 which may be repairable and or provide a suitable replacement functioning screen. More on that tomorrow when it is delivered.

https://www.ebay.co.uk/sch/i.html?_nkw=14+digit+led+HP+bubble+display&_sacat=181885&LH_TitleDesc=0&_odkw=14+digit+led+bubble+display&_osacat=181885&LH_PrefLoc=2

[Specmaster]

Update on the display issue and the incoming new calculator, which was very sick with battery corrosion attacking the PCB's and eroding various traces and via's etc, it is now working apart from the display see photo below, so now I'm looking for 2 of the 14 digit display modules. 

[james_s]

Maybe you can cut up the two defective ones and splice together 14 good digits? It's not an ideal solution but I don't see a lot of other options. Certainly making one from scratch isn't likely to work, the original segment dies are long and slender.

[Specmaster]

Hmm, somehow I really can't see that working out in practise, bubble lens is pegged and bonded onto the PCB and given that these almost 50 years old now, they are going to be rather frail as many will have been exposed to all kinds of attacks from leaking batteries and other stuff. Besides, I think that the dies are so close to each other that it renders it unlikely that a cut could precise enough to maintain the separation and not damage a good die while making the cut, then the traces would still have to be reconnected across the cut line without damaging either of the adjacent dies and still allowing the bubble lens to fit correctly, which is then also a snug fit into the red outer lens on the calculator.

I wonder if anybody has ever successfully carried out this procedure before?

[Gyro]

I played around with those displays when I was young, mainly to see what the digits would look like un-magnified (answer: tiny!). It is possible to get the bubble lens molding off by cutting the pegs flush but it is so easy to accidentally trash a bond wire! I doubt it would be possible to successfully cut the PCB between the dies - there's actually even less space than the digit pitch because the bond wires spread out like spiders legs. You would need some sort of razor blade thickness cutter.

[Specmaster]

Thanks for that, I've never seen a display without its bubble lens, but you describe it exactly how I thought it would be with the bond wires spreading beyond the die width. I doubt that it would be possible even to re-attach a bond wire by hand even.

[james_s]

I forgot about the bond wires. I played with some displays like that when I was a kid too and I remember marveling over how tiny they actually are. I wonder if it's possible to fix the dead segment? It might be a bond wire or trace failure rather than the die itself.

[Gyro]

I suppose a bond wire to gold plated PCB track might be possible, silver loaded paint or epoxy and a needle point? No chance at the other end.

[Specmaster]

I suppose a bond wire to gold plated PCB track might be possible, silver loaded paint or epoxy and a needle point? No chance at the other end.

If I feel brave, I could try that once I have some suitable conductive paint etc, I do have a couple other duff displays already removed from other calculators, albeit they are only 12 digit ones, if successful, it would be good to have some spare displays. I might consider this, after doing more research first.

[james_s]

There are machines that can attach bond wires to ICs, I don't know if it's the sort of thing that is ever done manually in one-off quantities but it seems like somebody somewhere would have a rig for that.

[Specmaster]

I'm not aware that anyone has ever done a manual repair to a display of this small size, it would certainly require a microscope, not sure if mine is going to be powerful enough but if I can find some suitable bonding method, I guess we will find out the answer.

[Specmaster]

Ok, then, as a matter of interest, I removed the lens from one of the already faulty displays removed from another Commodore calculator which had new displays fitted courtesy of austfox.

This display was then placed under my microscope and examined, and I made a short video of my findings, as you can see, there are clear signs of the bonding wires being attacked by possible battery leakage or other substances and I don't believe that there is anyway of repairing the damage, they are just too small to work on. Towards the end of the film I introduce a dental pick to show just how small these dies are in comparison, and you can see the substance has jelly like make up.

[Gyro]

Damn, it's a shame they failed at the die end, you might just have manage to get Gold bond wires soldered back down to the PCB. 

... I suppose it stands to reason that the Aluminium pads on the die failed under battery goo attack before the Gold plated PCB tracks and bond wires.

[james_s]

Here's somebody doing manual IC bonding.

[Specmaster]

Interesting, but I can't help but think that it would far more economical to toss the calculator in the trash bin and buy a new modern one 

[james_s]

Interesting, but I can't help but think that it would far more economical to toss the calculator in the trash bin and buy a new modern one 

I think you're missing the point entirely. It would be more economical to toss a classic car in the trash and buy a new modern one than to restore the classic, but people don't restore classic cars because they need a car to get around.