Bringing an HP-42S Back to Life
Message #1 Posted by Juan J on 23 Mar 2007, 11:52 p.m.
Hello,
A couple weeks ago I found an HP-42S on that auction site, described as “non functional, for parts or repair.” The description went on, detailing scratches and blemishes, but ended stating that the calculator passed the self-test. This little detail was intriguing to me. So I decided to bid and ended up winning the auction, at roughly less than half the price of a fully functional 42S.
The day seller shipped the 42S I received a message: “I hope you enjoy tearing the calculator apart.” No problem, I wrote back. The 42 eventually came home and stayed untouched, as I was away. But I came back home and decided to get to work on it. A sick puppy, I thought.
This was a battered puppy, S/N 3046SXXXXX with initials engraved on the front plate, next to “42S,” and a few scuffs. I turned it on. It was functional and passed the self-test, as the description said. However, the keys of the rightmost column were dead: [XEQ], [TAN], [<-], [/], [x], [-] and [+]. Strangely, they felt fine and the tactile feedback was the same as the rest of the keyboard.
At first I thought it was a dirt problem and tried contact cleaner, applied through the slits between the front plate and the keys, to no avail. The puppy needed surgery.
I got Paul Brogger’s article on how to disassemble a Pioneer and followed the instructions. The disassembly was just as Paul describes it, and I pulled out the back case half, then the PCB. The keyboard assembly was just as the article described, complicated to tear apart. Since the keys were working, I thought it might be a faulty contact problem, and examined first the PCB and then the contact ribbon that links the keyboard to the PCB.
There are 15 contacts between the keyboard and the PCB, seven to the left and eight to the right. The last contact to the right had what looked like dirt over it. I carefully scraped it with a precision knife, put the calculator back together and turned it on. Nothing. So I took it apart again and this time checked the keyboard contacts ribbon.
Interestingly, the last contact, which went into the PCB contact I had scraped clean was worn out and there was little conductor material left on the plastic. Quite possibly this damaged contact was the cause behind the dead keys. Lacking a conductive marker, I cut a small (3/8 in. x 1/32 in.) copper strip from a piece of foil I had at hand and folded it in half, so that half of it would replace the missing conductor and make contact, and the other half would fit behind the plastic ribbon to hold the strip in place. Just to check this setup, I put the 42S together, installed the batteries and turned it on. It worked. It was time to check the keys.
All the keys worked perfectly, main and shifted functions. The strip was working. I disassembled the 42S, secured the strip in place with a little bit of glue carefully applied with a brush and put it together again.
The sick puppy is cured and now works like a charm. Although the front plate still needs some work, namely removing the engraved initials and the scuffs, now I have a fully functional 42S. It feels so good to bring back to life one of these little machines!
It might seem a contraption, but this keyboard fix may be applicable to Pioneers with the same problem. On the other hand, would this contacts wear be something to consider about long-term use (and abuse) of a Pioneer? Has anyone had this problem before?
The next step is removing the engraved initials. I am thinking about sanding them off and repainting the area, either by hand or using an airbrush. But this would need peeling the front plate from the calculator. Is it possible to do it without destroying it?
I have already thanked the seller. Now I want to thank Paul for his article, of which I keep a copy for future reference, and Dave, for this wonderful site.
The HP-35 page states: “it was built tough, and built to be repaired.” The same could be said about almost all the subsequent models; the Pioneers are a little harder to repair but it is possible. They may not be as tough as the original 35, but they do endure punishment. Which makes me wonder if we are not only into collecting and repairing, but also into restoration and adoration with our little machines.
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