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Well, the seemingly impossible seems to have happened. I did not think 11 series calcs could break, but this one did, one day, for no apparent reason.

I have a 15C with two non functioning columns of keys, the fourth and seventh columns. That is, the column that includes the cosine key, and the column that includes the numbers 7,4,1,and zero. None of the keys in these two columns work.

Can this be repaired? It's obviously some hardware problem. Anybody care to take a crack at fixing it?

Any suggestions would be greatly appreciated.

Ion Abraham
Albuquerque, New Mexico

I believe the problem is a bad connection between the flexible PCB and main keypad PCB. It is in the far upper right is you take the cover off and are looking at the calc. from the back. There is one of those spongy silicon type connectors that HP used that has imbedded wires. You might try cleaning the connector first. The back part of the case applies pressure to this contact when it is screwd to the front case. The flexible connector is aligned on two plastic pins.
I once fixed this problem by taking the spongy connector from a bad HP-41C display or CPU (can't remember which) and cut it to the right size. Good luck!

Although the earlier 11C and 12C machines had the separate logic/display module with the zebreastrip connector to the keyboard PCB, I thought all 15Cs and 16Cs were on one PCB (keyboard, display, chips). Certainly all the ones I've worked on have been
The keybaords on all these machines are a matrix of switches, so most problems apart from a single dead switch will result in an entire row or column malfunctioning. The keyboard interface on these machines is part of the Nut CPU chip. I've had problems in other Voyagers due to dry joints (bad soldered connections), so if you're happy soldering surface-mount devices, and have a temperature-controlled soldering iron, try resoldering the nut CPU. IIRC, this is the rightmost chip, looking at the machine from the back (it is certainly one of the smaller chips). If that doesn't fix it, you might be looking for a 'parts machine' to take the CPU out of.

Tony, you might be right about the connector not being omn HP-15C's. I had a similar problem with a 10C but was able to fix it by tackling the zebra connector.

Dear Tony and Erik,

Thanks a lot for your suggestions. I hate to sound like such a rank novice, but let's back up to, how do you take the back cover off, as in dissasemble the thing?
After that, I might have a chance to follow your advice.

Thanks again,


Remove the 4 screws located under the rubber pads on the bottom of the case. Remember where all the small parts such as plastic strips and small spring contacts that ground the back label go.

Peel off the 4 feet. To prevent the adhesive picking up too much dust, I normally stick them onto a piece of the backing paper you get from sticky labels or rub-down letters.
Under the feet are 4 screws. Most machines have crosshead screws here (I think pozidriv fits better than Phillips), but I've found the odd 12C with TX6 screws.
Take out the screws using the appropriate tool, and lift off the cover. On most 15Cs you're now looking at the component side of the main PCB. On some 11Cs and 12Cs, you have a anti-static plastic sheild to unstick and unfold, whereupon you can remove the logic/display assembly (there's a zebrastrip connector between the the keyboard and the logic in this version which can give trouble.
But most 15Cs are all on one board. Oh yes, there are 2 tiny compression springs in holes in the case to connect the metal plates on the front and back together. Remove these, or you _will_ lose them.

Dear Tony,

Thanks very much for your thorough advice. You mention a temperature controlled soldering iron. I have access to one, so do you have a recommendation for the setting?


I still use one of the old Weller Magnastat irons, and I use a #8 tip. That's 800F, or about 425C.
I find the #7 tip too cold for most work. Especially for surface-mount work, where I use silver-loaded solder (about 3% silver). Too cold an iron can do as much damage as too hot an iron -- it takes
so long to melt the solder that everything gets heated up.