Michael:
Somehow I lost track of this thread, and it slipped ignominiously into Archives. After reading your response, I did the same as you--- searching for parts which might be pressed into service. Unfortunately, I, too, have come to the conclusion that NO CURRENTLY AVAILABLE PART will do what I hoped could be accomplished.
I feel like Emily Lattela (sp?) from the old Saturday Night Live: "Oh. Never Mind..."
Not having a 55 or similar calc at hand, I had to dig for info about what display it had. It was, indeed, unusual. The characters were .11" high. This alone is unique: I might have expected, though, to follow the lead of finding the supplier (prob chinese) of the displays for the Retro LED watch made by Fossil just last year. (Fossil "RoadTrip" JR-7749 is one model). Digit size, color and brightness and encapsuation in "bubbles" made it seem close.
But HP Classic displays were unique in quite another way, too: unlike most "seven-segment" displays which actually have 7 segments to make the digit, then also a decimal point located either to the left (LH) or right (RH) of the digit elements, the displays used by HP held the decimal point INSIDE the lower digit "square". This implies, firstly, that NO OTHER DISPLAY WILL LOOK RIGHT, and also, that HP displays took what normally would have been a digit position to display the decimal alone. Very unique.
So, assuming that no display will work but for the originals, we can see WHAT and WHY they are what they are: a substrate on which the LED structure is built, with 5 digit position areas (the 5 common cathode plates), and layered or printed on top is the set of anode segments; these are tied together with clever circuit tracing so that all top horizontal segments are common, all top left vertical segments are common, and so on. I would expect that building all this in sets of fives would make yield and brightness-matching fairly easy. So digit selection was by activating the cathode, and segment selection chose the corresponding anodes on all digits. You could run through all the positions pretty quickly. All the same, this "multiplexing" would be a constant thing: the 40 segments (including the decimals) of each 5-digit display would be run through, lit or not, with each given a bit less than 1/40th of the "display cycle". I don't know how the classic calcs were wired, but it is MOST likely that the 3 5-digit displays were operated simultaneously, rather than in sequence, so that, at any given moment, three segments of the overall display were being tended to.
Now. Could a display be custom built as a "Classic" LED replacement? Sure. If yield were unimportant, you could chance it as a garage operation; this I do NOT recommend as the MINIMUM number of wire-bonding operations (to run a little wire from anode to connecting pads) would be 240 for the 15-digit display... and so your success rate is drastically increased if you start with a Clean-room environment and closely controlled bonds.
You'd create a multi-layer "thick-film" substrate, with connected cathode pads for each segment of each digit, and pads beside each segment which run to each segment position across five adjacent digits. At this point, I think you see that it could be done as single digits, five at a time, or all fifteen at once. The difference is entirely in how you want to make the "wiring together" of these things. The description of each digit itself is a "common cathode, 8-segment" device. If *I* were doing it, the "five at a time" or fifteen at a time would be too complex-- I'd go single digit and use a circuit board outside of them to tie them in fives...
Then, of course, you'd buy "LED die", in hi-efficiency red... (One major supplier of these is Marubeni). Die are square, and not particularly sized right, at 350 micro millimeters a side. But, gluing them down with conductive epoxy and wire-bonding to the top anode side with a tiny gold wire, they would be bright enough so that one per segment is all you'd need.
Dark epoxy poured between die (50 micro-millimeters high) would secure the wires and provide some optical isolation between them. Over that, a laser-cut "mask" would provide the segment "slots" that the LEDs would shine through. If the mask is somewhat thick, and "filled" with clear epoxy, you effectively have created a diffuser/light-pipe to the outside, which will make the individual segments bars instead of points. If this clear epoxy is not just syringed in to make a light-pipe, but overflows the mask face and is shaped into an over-digit "bubble", then you have done as close a job as you can probably do to creating an HP-equivalent part without going into the Ga-As business yourself.
Weell, that's a WAY to do it, maybe there are others. If you want to do this, it would be best to use a Class what? 10000? cleanroom environment, automation for bond-welding and encapsulation, and each individual die pre-screened for brightness characteristic. Wow.
Unless the market for new displays for Classics is bigger than I'm thinking, this is not how I plan to spend MY summer. Maybe I'll just scavenge a while and try to find an undiscovered stash of HP 5082-7405's lurking out there.
Sorry now that I "off-the-cuff" figured single-digit displays existed that could be used to make a replacement display. My! The display business is certainly chock-full of suppliers, all making the exact same BIG displays. Wonder how they all stay in business?