HP 67 broken springs



#8

I've bought an old HP 67 calculator (SN 1702) some weeks ago.
After fixing the battery charger (thanks to Tony Duell to make me look at the fuse), I would like to make the keyboard work better : some keys have a bad feeling (0...9, +, .), some keys register multiple copies of the typed character(2,5), some keys don't even work(C,D,E).

After opening the case (without peeling the label : just with a little cross shaped cut along 2 diameters upon both holes, then carefully lifting the 4 angles of each cut, unscrewing ... and the same in reverse order to close back. The label looks about as before, but I'm not a purist... ;-) ) it appears that the metal strip is broken under the 12 key : 10 numerics, "." and "+". The broken "2" spring is almost flat.

To fix the C,D,E problem (key springs mechanically like never used) I follow the way described by Steve Loboyko ("Bad keys and What to do About Them") only replacing the sandpaper by a paper strip soaked in alcohol : it works.

First question :

- is there a quicker way, with a voltmeter, to test the result of cleaning without having to close the calc and using normally the keyboard : for instance putting the first continuity probe on the ENTER metal strip, the second one on the golden hole immediately beneath the strip, and then pressing the key, we get sound. The same method can be applied to some couples of (key / hole), but I couldn't find the complete mapping : which hole (or which electric trail) for which spring ? Is there any plan available ?

Second question :

- is there any way to fix broken springs (for instance by putting a new spring under the broken one) ?

- can I avoid the double or triple typing with the "2" key ?

Thanks in advance for any help.


#9

I have succesfully repaird several broken key domes on classic calculators. It requires you to have a scrapped machine for parts.

I first find a good feeling key on the scrap machine. Then, using a square flat Xacto knife blade, I cut the key strip containing the donor key off of the donor machine by carefully sliding it under the spot welds.

Next I cut the key I want to use from the donor strip leaving about 1/16-3/32 inch tab of matal each side of the key dome.

Next cut the bad key dome off of the machine you are repairing. Just slice the key dome off where it begins to rise up from the rest of the key strip using the Xacto knife blade. The cause of totally mushy keys is almost always a split in the key dome here.

Apply a thin bead of solder to the tabs of the donor key and the machine you are going to repair.

Place the donor key onto the repair machine and heat the solder tabs. I use the Xacto knife blade to hold the key tabs down and in place while soldering.

I have also replaced whole rows of keys in a similar manner. Work from the inside of the key strip outward as you resolder it. I had an HP55 with four rows of totally split keys that was restored to like new feel this way.

Occasionally the donor key will be hyper clicky. It can be toned down by applying few (1-3) layers of Kapton tape over the key dome.


#10

hello David,
thx for your details, it remains to find a "scrapped" machine and, when founded, to decide which is the donor and which is the receiver :-)
If I can't find any, do you know some alternative stuff I could use to replace the metal strip ?


#11

All of the HP classic calculators have similarly constructed
keyboards, so you might be able to find an inexpensive
HP-35 and scavenge it for parts. This is what I did. I
strongly recommend replacing an entire strip at a time. It
seems to work best.

#12

I have been using the key strips from an old HP45. The key strips in the '35 and '45 machines are gold plated and generally are not as crisp as the '65 and '67 machines, but work just fine. A lot of machines have one or more mushy keys that should be avoided (although I have found that they can lose their mush once transplanted). Always check for signs of cracks and splitting where the key dome starts to rise above the key strips.

I had to buy about six "bad" machines before I found one that was bad enough to scrap. They kept cleaning up nicely and working just fine. And the only problem with the "bad" ones was the cases were heavily engraved and smudged up.

I don't know of any easy way of making them from scratch. I have had good luck just replacing a single key. I have found it more difficult to replace a whole row and get a good consistent feel across them all. This may have to do with the fact that the donor machines were fairly junky to start with.

#13

Hi Pierre,

Just a suggestion about your bouncy "2" key, try cleaning it with sand paper and alcohol like the others. This work for me once, so it's worth a try.

Cheers,
John


#14

You're right John : after cleaning ALL the contacts (very boring !) I don't get any bounce any more from any key (the "5" was sometimes bouncy too)

Thank you very much.


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