Ok someone explain this gummy wheel problem


with card readers on HP67s.

Do these wheels just disolve or what?

What makes them gummy?

Where does one get replacement wheels?

If one had an unopened package card reader can it go bad just sitting there, without ever having been used?


1) yes (disolve) -- well sort of. The rubber used is not particularly stable (did HP expect them to have to last 20+ years???)

2) see above

3) See article by Dave (The lost rabbit) at...


4) yes



My understanding is that when HP developed their card reader back in the mid 1970s, the best material for use in the drive wheels of the card reader motor was a rubber derivative. Well, that rubber will oxidize over time and turn to a gummy mess. So, it's quite possible (and probably very likely) that an unused card reader would have it's drive wheels deteriorate. Oxidation is a process that doesn't discriminate between used and unused card readers!

Materials technology has progressed since then. As mentioned, Dave and a few others on this forum have described replacing those wheels with silicone-based materials that are much less prone to oxidation. Replacement wheels per se cannot be purchased anymore, but several ingenious collectors have found that certain O-rings, tubings, etc., match the required size quite well. Replacing the wheels is apparently a fairly straightforward process, although I've never done it myself.

Best of luck,

Todd Garabedian,
Glastonbury, CT


I've done about 10 card reader repairs trying various materials (o-rings, rubber feet, tubing). I've made all of these work, but by far the best one and the one probably closest to the original that I've found is silicone rubber tubing 1/4" OD x 1/8" ID. It fits on the gear shaft perfectly without the need to glue it on and provides plenty of contact area with the card (unlike the o-rings).

This is readily available from Small Parts Inc. (www.smallparts.com) (800) 220-4242. They sell 10 feet of this (enough to repair 1000 card readers!) for $24 (part # 210005). BTW, they also sell the 1/8" Delrin balls (4 of them) used in the card reader in case you loose them while doing the repair (10 for $2, part # BDLR-2) -- I've never seen these disintegrate, however.

Also, they have .005" beryllium copper sheet (part # BCS-005) that can be used to make card contact springs used inside the reader. (These springs do corrode over time.) And this same material (or the ticker stock BCS-006 or BCS-010)) can also be used to make replacement battery contact springs for the Spice series calculators. (Just cut to size, shape it like the original contact, drill a 1/16" hole in it, drill out the rivet holding the broken contact and secure the new contact with an 0-80 screw and nut. Takes about 5 minutes and looks and works great!)


Is that tubing from small parts the same as "Tygon" hose. I do have their catalog and will look up that hose you specified. Too bad they don't sell the correct spurr gear for HP-97 printers! I looked into that once.
Even though the Tygon hose is 1/4"OD x 1/8 ID, it always seems to have a little bit larger OD when slipped on the gear. My remedy has been to chuck the hose in a lathe with drill on the ID and machine off a little of the OD.



The silicone rubber tubing is much more flexible and softer than Tygon. It's about the consistency of silicone bathtub caulk and can stretch up to three times its original length. (Not that you'd want to do that in this case!). It doesn't need to be turned down at all probably because it's soft enough.

I checked with Small Parts on the 97 printer spur gear too. They don't carry any 24 pitch gears but said that they might be able to special order some. I'm going to get a price quote from them and see if it makes sense (i.e., it's cheaper than buying old 97's on ebay just for the gear). I'll assume that if I have to buy 10 or so, to get a decent price that I'll have no problem finding people on this list to share the order with. (Otherwise, I'll have to go into business repairing them, I suppose!)




I'll gladly "pitch in" and purchase a few spare gears if that helps to keep the price down.



Hey Katie,
How about posting this info to the articles forum? It would be a good place for this info. Randy



I was planning on writing up an article on several repairs that I have some experience doing. I've just got to find some time to work on it. I'd like to put some pictures in with the write ups. It's very hard to understand what's being talked about until you have the calculator in pieces and many people are reluctant to go that far without a really good description of what to expect.


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