How to open the HP28


Anyone has opened the HP28 calculator ? Does it have hidden screws or is it assembled by presure/click only ?


Take a look at this older thread:

I got a nice 28S recently. Too bad there are two columns in the alpha keyboard that are not working, those starting at 'B' and 'F'. Do you want to open yours because of a similar problem?




Yes, the method suggested in the link in Gersons' post above do work. I've used it for several 28S with broken keyboards. It's somewhat difficult to glue it together again and one should not expect the calculator to feel as solid as before ( one could of course argue that this calculator didn't have a solid feel in the first place... :) )



Hello Bo,

Before I attempt to open it up, how may I be sure the failure is due to a bad contact or a broken trail in the flex printed circuit? Given the columns that are not working, what keyboard overlays should I peel off, both?




Hi Gerson,

First of all, you may want to practice the disassembly procedure with throw-away unit ( I did ). Remember the Clamshells are often stated as "not repairable" - for a VERY good reason. I agree with Howards' post below, but with a great deal of patience and a little luck it might be worth a try.

With the 3 28S I've disassembled, I haven't experienced the type of column failure you describe. In all cases, individual keys have failed due to bad contact so a simple cleaning with isopropyl alcohol was sufficient. However, in your case a broken trail seems more probable. I don't know how to possibly diagnose and/or repair a broken trail, but perhaps some other on this forum knows?

And about the overlays: it's always a good advice to take the calculator apart as little as possible ( even though the overlay removal is the easy part ). Try first the side with the failing columns and see if it helps. The left overlay is most easily peeled off starting from the top ( the A row ), while the right overlay can be peeled off from either side.

Hope this is of any help! Let us know.



But the main advice in the referenced thread is still good: given the difficulty of surgery on the clamshell models, and the low value they command on eBay, buying a used one there is almost a no-brainer.


Good evening Gerson, Inaki et al

Opening an Hp-28 can be done with some chances to restore the calculator in working order but very few chances to restore the original cosmetics.

I did it to add an interface in a friend's 28 using the following directions:

For english only speakers:

Dismantling the Hp-28. [source Joris Ballet]

For fellow french lovers :-) :

The excellent book from Sébastien Lalande. [source: courbis/Lalande]

The first link provides a tested procedure to disassemble/reassemble the calc.

The second link provides a much more detailed procedure and...all the rest.

I would advise to check them before X-actoing savagely your calc.

Best regards from France.


Edited: 6 Dec 2005, 4:57 p.m.


Thanks Etienne, Bo and Howard,

Since I feel I am not exactly skillful in removing the overlays and putting them back in place again, I think I won't try it on that 28S because it looks so nice and I might ruin it. Besides, the menus in the defective columns can be accessed via MENU, which in turn can be accessed via CATALOG. Perhaps I can find out workarounds for the missing keys (using CHR, ->STR, etc.) If it only had a two-way infrared interface... Anyway, I'll try to introduce a complete program despite the missing keys.

Howard's suggestion is good, the problem is most 28S calculators I have seen at eBay would not be shipped abroad.

For fellow french lovers :-) :

The excellent book from Sébastien Lalande. [source: courbis/Lalande]

I'll take a look at it, je pense que je peux comprendre quelque chose :-)

Best regards,


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