HP-97 Printer Repair Adventure


Well, I'm needing to repair the paper feeder wheels and have two thoughts so far:

1- Mask all other areas and try to "resurface" the gripping areas with some kind of spray-on rubber, or

2-Find some very thin heat-shrink tubing and try to re-cap the tires (being very careful about the heat of course).

I have removed the axle itself (very ingenius) and can see the original rubber was probably over-molded and that the axle itself has some extrusions to lock the rubber in place. I suppose those could be removed and some odd rubber wheel could be modified and glued in place. Anyway, I'm looking for any further thoughts.

I did try searching for this topic but found only one dead link. I didn't see it mentioned specifically in the repairs area either. Forgive me if I have missed the obvious!

I love this calc. It's a 97S I found in a pawn shop 30 years ago or so. I found some supplies at the time and still have enough paper, cards, etc. to make it useful if I can fix the paper-feeding problem. I did carefully remove the umbilical but still have it and can replace it if the need ever arises.




Here's the thread you need. Follow the link to Ignazio's very detailed slide show.


What an active forum! I didn't even think to check the archives. Thanks so much for opening my eyes!

That's a good tutorial and probably a pretty good solution. The modifications make me slightly nervous as I have no spare parts in case something doesn't go quite as planned. This will be my plan if I can't find a less invasive solution. I'm going to Lowe's tomorrow. If I can find some rubber spray (like the ad on TV)I may try that first as it's the least likely to do any permanent damage.

If I do happen to find a workable alternate solution I will post it here.



Curiosity got the best of me and I looked through my life-long collection of heat-shrink tubing. I found some that had rubber-like characteristics and was only slightly smaller than the remaining friction wheels. I used a ball-point pen tip to stretch the tubing enough to slip over the wheels. Then I used a small SMT hot air nozzle to carefully shrink the tubing on each side of the wheels. I started with a very low temperature and gradully increased it until the tubing reacted sufficiently. I worked slowly and only did a small area at a time, cooling it quickly with my fingers between each pass so the original wheel material wouldn't melt. Finally I trimmed the excess with a razor blade. The resulting "recap" actually looks great and the printer is working like a charm again. Only time and use will tell if this is a good long-term solution but one good thing is that nothing was modified with this process so I can always try other methods later if this doesn't hold up. I did take some quick photos of the finished product and will provide a link as soon as I hear back from the administrator about the space I requested.

There are two possible problems I have noticed at this point:

1- The tubing was about 0.010" thick so the wheel diameter is probably slightly larger than it was originally. This will only affect the space between lines so I don't expect it to be too concerning.

2- I noticed the carriage doesn't quite reach the right side when returning to the rest position. It's very, very close and it doesn't appear there are any adverse side-effects but I might move the magnetic sensor if I notice anything unusual.

Other than that the printer is working better than ever (possibly also due to the new NiMH battery), printing clearly and feeding the paper perfectly.

Once I find all my old supplies I'll try the card feeder. That may be the next adventure!


Edited: 14 May 2012, 4:42 a.m.


I'm getting envious. ;-) I still have to disassemble my printer...


This is still very preliminary news but after sitting all night I tried it agin this morning and it worked perfectly. One concern is the softness of this heat-shrink tubing which may become indented more easily. But the old wheels also had a few indentations which seem smoother now.

Yes, disassembly is tedious and must be done with great care. Knowing what I know now I would start by removing the print head and all electronics from the printer assembly first so the mechanical assembly can be handled and studied without undue stress on the head's flexible cable. I would leave a short piece of paper in place while removing (and re-installing) the head to reduce any chance of head damage. Then it's just a matter of removing the various shafting, lead screw and carriage so the feeder shaft can be removed. Be careful not to loose any of the "C" clips! Study everything carefully to see how it works (quite clever), take a few photos or make some sketches so you'll remember what goes where and you should be fine.

On my unit I also noticed that the motor gear was only contacting about 1/2 of the idler gear. While I had it apart I made a 0.050" shim out of some heavy cardboard to put between the motor and the housing. The gears are nearly in line now and working fine.

One other thing I did was add a tiny amount of lubricant to the lead screw, gears (model train gear grease) and the two tiny rollers behind the paper (thin oil).



I do wish I had taken more photos during the process but I suppose it's actually pretty self-explanatory. Here is an overview of the tools I used:

This photo also shows a small area I removed from the center part of the axle near the non-ratchet end. I did this to allow room for the heat-shrink tubing but it may not have been necessary and/or I could have removed less. No harm done though.

Here are a few photos of the completed wheels with their new treads:

You do want to be sure the ratchet still operates effortlessly and the heat-shrink causes no binding.

Well, there it is. I hope I'll be writing back in a year or two describing long-lasting results. I'm so excited to have both my original HP-35 and the HP-97S back in action I ordered the Volume 7 DVD and even found a new 97 Users Manual on eBay! Should be fun!

Special "Thanks" to the HP Museum for being here and also for providing the space for the photos!


Edited: 15 May 2012, 12:42 a.m.


A very good job, congratulation!




Thanks! It's been a day or two since the repair and the printer is still working perfectly. Paper feeding is also flawless. Maybe this info will be helpful to others one of these days.

A little more minuitiae......

I did make a few measurements along the way and have now guessed that the original friction wheels were probably a nominal 0.318" in diameter which would result in a paper feed rate of about 1" per revolution. I believe there are 12 teeth in the ratchet and two steps are used during each line feed. This would produce 6 lines/inch which may be some common text size and line spacing. The "re-treads" produce a feed rate of about 1.038" per revolution or about 0.006" of additional feed per line. Other than a slight waste of paper I don't think this is a cause for concern.

Along these lines I would also like to comment about the amazing design and construction of these products. It's really quite something that this product is so old but still works essentially like new. Even the paper itself still works perfectly! Today I design modern mixed-signal audio/musical products and all of my test equipment is of a similar vintage as this calculator (or older) but it was all designed and built by the same mentality it took to design these calculators. All this equipment stills works about as well as it did the day it was made and, with just a little TLC and occassional minor repair, will probably continue to work almost indefinately. All these things really are "works of art" and deserve to be preserved not only for their continued usefulness but also for the statement they make about the dedicated and talented designers and engineers who made them.



I guess this could be considered "beating a dead horse" but I've had one more idea: plastic, silicone or rubber tubing. I'm sure one could eventually find some tubing of the proper dimensions/consistency and use a razor knife to slice off some new "tires". Installing them may require some adhesive and modification of the axle to some extent but the job should be simple and easy. Here is one possible source for materials:


Seeing an ad for this company brought the idea to mind. Seems like my life has been taken oven by this enjoyable but somewhat mundane project! I MUST MOVE ON! I MUST MOVE ON! I MUST MOVE ON! I MUST MOVE ON! I MUST MOVE ON! I MUST MOVE ON! I MUST MOVE ON! I MUST MOVE ON!I MUST MOVE............................. :-)


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