HP Forum Archive 03

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HP-28S Malfunction Message #1 Posted by R. Miller on 13 June 2000, 2:16 p.m. A friend of mine recently gave me a HP-28S calculator. Since I have been using an HP-41CX for years, I guess he thought I would know how to use it. Well its a new breed cat to me. (That dates me!) I purchased "HP-museums" CD with all of the manuals and went about trying to educate myself. So far I haven't been too successful. I'm having a problem getting the "statistical" function to work. Then I realized none of the keys in the first two rows on the left-hand side of the calculator half are working. I guessed this could be a connector problem, then I wondered how the two halves of the calculator communicate. Is there hard wire connectors or commutators across the hinge, or is data send some other means. If anybody has any ideas or suggestions and where to go from here, I would appreciate the adice (even if its to junk it). Thanks, rmiller@i2k.com

Re: HP-28S Malfunction Message #2 Posted by Bill Duncan on 13 June 2000, 2:30 p.m., in response to message #1 by R. Miller There are notes around (I think on www.hpcalc.org and a site specifically for the hp-28 listed in the links section here) for opening up an hp-28. However, I had the same problem as you, where a friend gave me an hp-28S with 4 keys not working. I looked at the notes about opening it up and chickened out. I took it to HP and they gave me a deal on a new HP-48G. If you go that route, try to talk them up to an HP-48G+ or an HP-48GX. The extra memory is worth it. (I now own one of those too.)

Re: HP-28S Malfunction Message #3 Posted by R.Miller on 21 June 2000, 9:29 a.m., in response to message #2 by Bill Duncan Bill, I was unable to find any info on the disassembly of the 28S. I checked hpcalc.com without any luck. Do you have any further sources for disassembly?

Re: HP-28S Malfunction Message #4 Posted by Kevin Schoedel on 21 June 2000, 4:47 p.m., in response to message #3 by R.Miller Try this: http://www.hpcalc.org/docs/misc/hp28io.txt

Re: HP-28S Malfunction Message #5 Posted by R. Miller on 21 June 2000, 9:50 p.m., in response to message #4 by Kevin Schoedel Yea gads!!!! I can see why a person wouldn't want to attempt this. I would screw it up for sure. Thanks for the info.

Re: HP-28S Malfunction Message #6 Posted by Kevin Schoedel on 13 June 2000, 4:04 p.m., in response to message #1 by R. Miller Somewhere on the CDs, I believe either in the Journal or Digest section, there is an article on the construction of the HP28C and/or HP18, that includes description and pictures of the keyboard and case assembly. Both keyboards have as their common base a single sheet of plastic printed with conductive ink, which extends in a sort of "UU" shape inside the spine. (You may be able to see it if you look at the lower hinge joint from the front of the open calculator.) Presumably there is a break somehere. I'm not sure whether anything can be done about the problem even if you open it; I don't image you can solder onto the plastic backing.... If mine were to fail like this (which it probably won't, since I hardly dare use it), I think I'd go for building a "desktop 28S" using a matrix of conventional computer keyboard switches attached to the 28S electronics (which are all in the display/battery area).

Re: HP-28S Malfunction Message #7 Posted by Andrés C. Rodríguez on 13 June 2000, 5:44 p.m., in response to message #1 by R. Miller Just some suggestiosn to think about, no warranties: Both halves are connected by a flexible plastic "printed circuit" or"cable", that goes from the main (right) side, exits at the the lower hinge, goes halfway up thru the spine, again goes down, and enters the left side via its lower hinge. It is placed that way to reduce (or spread) torsion stress when the calculator is opened/closed. As far as I remember, the cable only has 4 traces (wires), if I am right, that is not enough for the full keyboard matrix to be connected, and so it suggests some electronics (perhaps one single and very simple chip) are located on the left side. If so, the problem with the rows of keys should be localized inside the left half, and not related to the flexible cabling. A failure on the flexible cable should affect all the left keyboard. You can look for a HP Journal article on the HP 28 (around 1987/1988) that may add some extra information. Again, this is only a unconfirmed idea. Be careful!

Re: HP-28S Malfunction Message #8 Posted by Kevin Schoedel on 14 June 2000, 9:51 a.m., in response to message #7 by Andrés C. Rodríguez The HP Journal article (which I forgot to look up last night, sorry -- maybe someone has it on hand) says that it's a multi-layer connection with (I think) four layers providing 12 (?) lines, enough for the left-hand key matrix.

Re: HP-28S Malfunction Message #9 Posted by Andrés C. Rodríguez on 14 June 2000, 11:15 a.m., in response to message #8 by Kevin Schoedel For a 36 key (6 x 6) keyboard, 12 lines are needed; if you are right about the multilayered cable, then it is enough ! And if a flexible, multilayered cable is broken, a repair will be very hard to do; but if the cases are "openable", then a couple of wire-wrap cable runs may go the same way of the original cable (sharing the small space available)and substitute for the broken lines... Just not to abandon the idea of fixing it! Now, if some of the original cable is broken, you may expect a good chance for the other lines to fail soon :-(

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