HP 28S keypad issue-definitive repair solution

[speleo]

Hi. I'm new to the forum. I've taken my HP28S RH clamshell apart and believe I have a better solution to fix the keypad issue. I will be milling down some of the stakes, drilling holes and installing 3mm/1/8" bolts in place of those stakes. The bolts will apply necessary pressure to get good contact at the ribbon cable conns. I would think that everyone has moved on from trying to repair these calcs but I'm very fond of mine. I bought a prime Graphing calc but wish that I hadn't. Is anyone interested in me documenting this? Below are the links that got me started.Curt

https://www.hpmuseum.org/forum/showthread.php?tid=6468

https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=114283

https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=185173

https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=210284

https://www.teawea.com/open-a-hp-28s-calculator/

[rprosperi]

Yes, please do document it, and if possible, include links for buying the correct tiny bolts, etc.

Indeed, most folks have moved on past these, but as they age and continue to have problems, the need to make these repairs will continue.

It seems to me the hardest, or at most sensitive part is how to open the unit w/o causing too much damage so focus on this part is also important.

[vaklaff]

(11-08-2023 12:15 AM)rprosperi Wrote:  Yes, please do document it, and if possible, include links for buying the correct tiny bolts, etc.

I concur. And thanks in advance!

[Jeff_Birt]

For another idea of repairing heat staked joints see my video below. I scraped the dome off the heat stake and then used a drilling fixture I made to center drill each post that was left over. I used #0 or so screws to resecure it.

The mechanical repair of this KB was successful, the short circuit was taken care of, however the membrane developed a break later on and it quit working.

https://youtu.be/HJHGI7Wt4F8

[speleo]

Great and Thanks and I'll check the Youtube video and I appreciate the response. I'm waiting for Dremmel bits, etc. I don't think the stakes are important for my issue as I believe the issue is degraded foam pads under the flex PC board connectors (the little foam pads are to exert pressure under the flex conn to push it to the PC conn). There may be several failure mechanisms for the keypad but mine, like most apparently, has a column or row of keys that go bad. I suggest that someone that has this manifestation might press steadily and firmly on the top clamshell, immediately above the location of the connectors (see pic), while playing with the keys and see if that gets them to work. If this does seem to help, then I have a "quick and dirty" fix to suggest. Stay tuned. Curt

[speleo]

I attempted to post a reply and it complained about a pic that was too large so I said post it anyway but it apparently went byebye.
Thanks for the responses and I'll look at the Youtube video. I don't believe the stakes are to blame for my keypad failure (a column of keys) as I believe it's degraded foam pads under the flex connectors. If anyone has this same issue (rows or columns), then you might press firmly on the top clamshell, immediately above the connectors (see pic), while playing around with the keys, and see if that helps. If it does, then I have a "quick and dirty" fix to suggest.
Stay tuned and I hope the downsized pic makes it. Curt

[speleo]

Here are pics of my fix. It's not pretty but I don't see any other way to open the calc to replace the foam pads under the flex PCB connectors (which contact the upper clamshell PCB connectors) and then establish proper compression for reliable contact. Do you really want me to document this? I mentioned "a quick and dirty" solution which is to (1) carefully remove the keypad overlay (or do not remove and gamble on where to drill thru the overlay-based on my pics), (2) drill, with a drill press, thru both clamshells and the 6 additional layers of "stuff' at the "heat stake" located between the "log" and "plot" lettering on the upper clamshell (while constantly vacuuming out the debris) and (3) reinstall and drill overlay (if necessary) and (4) install a screw (3mm/0.5mm pitch @ 12mm) to compress the clamshells in the vicinity of the connectors and hopefully reestablish contact. Curt

[rprosperi]

I have to say this was not what I expected, you were right! But all the same, it's worth showing here as a functioning machine is sometimes more important than a pristine-looking one.

But I'd say your description and photos above are sufficient for this technique, no need to add more detail, it's pretty clear; the key here seems to be to know where it's safe to drill.

One advantage of a machine repaired like this is you'll never mistake it for the wrong 28S.

[speleo]

The naked truth on the HP28S-final pics:

[speleo]

The naked truth on the HP28S-final final pics:

[speleo]

I also bought a pristine 28S on Ebay for $100. I asked the seller to do a keypad test and he sent me a pic of the display results. But it's definitely "buyer-beware" on Ebay for this kind of stuff.

[speleo]

final cleanup pics. Obviously, it's awkward to get the battery compartment door back on. Some silicone grease on the slides and contact lube on the batt contacts and press the batts down as shown.

[vaklaff]

Thank you very much for sharing all this. If it works then it's a success. Not everybody is into cosmetics.

Is the keyboard OK? I mean are the keys reliable and with consistent action regardless their distance from the bolts?

I have a 28S which has been waiting for a repair for a couple of years already and I can't make myself to do it. About a year ago I bough a cheap and battered one which works fine so I keep kicking the work down the road but I keep an eye on how people do it.

And BTW thumps-up from my wife! I couldn't resist and showed her pictures with the "classic" treatment (styrene rods to replace the heat stakes) and your approach and you won her on the spot. She's a no-fussing-around person, fast beats meticulous anytime! We have a very apt (but somewhat vulgar) phase in Czech that describes it: "žádné velké sraní", sometimes shortened to ŽAVES. I am very sorry but my English isn't good enough to translate it :-) Maybe some of my fellow countrymen can?

Edit: grammar

[speleo]

Thanks. The bolts simply compress the clamshells together. The underside of the top clamshell has "standoffs" that surround each "heat-stake" to maintain proper clearance. I wish that someone would try the "quick and dirty" repair (I'll send them the M3 screw and nuts & washers) and all that is needed is a 1/8" brad-point drill bit. The intent is to compress the clamshells,and, as a result, the keyboard flex-printed circuit board to PC board contacts, just a bit to hopefully fix the problem. I'm working on a simple esthetic solution to the screws. Curt

[speleo]

Esthetics! Using "binding posts/Chicago screws", typically used for leather working and furniture. 3mmx6mm (x5) & 3mmx14mm (x1). see 3 pics. you can now fold the left side under. A couple of the holes had to be redrilled with the 1/8" brad point drill bit as maybe the alignment of the top and bottom clamshell holes were off a bit. here's the Ebay link to the posts. Curt
https://www.ebay.com/itm/145358645241?va...faf9913436

[vaklaff]

These special screws are a huge improvement, I think this is something I could easily live with, thank you very much for sharing!