The following excellent article has been contributed by Mike Davis, and is reproduced here with permission.

The photos below may be of use to those of you who wish to repair the gummy wheel on HP-97 card readers. The focus of this page is to show how and what to replace. It is intentional that it does not provide suggestions for dealing with the clutch. I do not have a solution for this, that I want to commit to, at this time. It is also not a tutorial on how to treat electronic devices. You should consider your 97 as static sensitive and treat it accordingly.

If you have signs of gummy substances on your cards, after running them through the reader, the drive wheel may need to be replaced. Other signs are erratic card movement, no movement, slow movement or errors.
The first thing you must do is to remove the case back. This is not difficult but there are some precautions you must be aware of. Just remove the 6 screws shown and the case back can be removed. You may want to remove the battery and battery cover first.
To remove the interior support frame, remove the screws shown. The card reader is mounted under this frame, in the upper right.
After these 7 screws are removed, you can flip the main frame over to the left but first you should understand how the main PCB connects to the keyboard. Failure to understand this can lead to bent pins when reassembling.
Simply lift the main PCB and it will come free of the keyboard/display connector. When you reassemble this, be careful to observe proper mating, or you may wind up with bent pins.
You will notice that there is a slot in the frame. This slot is to allow the keyboard/display contacts to pass through the frame. When assembling, be careful to see that these contacts pass through this slot. If you don't... you get it... more bent pins.
Now that the frame if free of the case and keyboard, simply flip it out to the left. Be careful not to put too much tension on the printer switch connections. These can break off easily. You may want to make note of the colors and positions of wires. Leave a short strip of paper in the reader. I have found this makes testing easier.
The card reader is mounted to the frame by 3 silver colored screws. There is also a ribbon cable that connects to the PCB as shown in the photo. Two of the screws are beneath the ribbon cable.
Carefully pull the ribbon straight out to the right, to disconnect it from the reader. Use your hand and NOT a screwdriver that is shown in the photo. That is simply there to hold the ribbon back for the photo. Then remove the three silver colored screws. This will free the card reader from the frame.
You may want to make note of the colors of all wires and their connection points. These wires tend to break off when opening up the reader. Remove the 3 silver colored screws shown in the photo.
Flip the card reader over and take a look at the drive wheel. You may see goo on the wheel. You may want to perform this step before removing screws but it can be done in any order. This photo does not show the goo that is typical but will be replaced anyway because of age.
After you remove the 3 silver colored screws, you can move the PCB to the side of the reader. A couple of cautions here. Remove the tape to free the red and black wires, if they are taped to motor. Also, be careful to avoid damaging the switch contacts. Then, remove the two screws shown, in the photo.
This part is tricky. Flip the card reader over and grasp in the left hand as shown. Then remove the last screw at the back of the motor. With the right hand, grasp the motor half of the reader and pull it off of the switch half. You must observe this orientation (feet up), or you will lose then nylon balls.
If you have done this correctly, you should see 4 nylon balls and one pinch roller on the left half and two copper spring clips on the right half. Remove these for safekeeping, until they need to be installed. You should clean these parts before reinstalling. Remove the slotted shaft, indicated in the photo, to remove the gummy drive wheel.
Gummy wheels typically look like this. Clean all the gum off of the drive wheel. Be sure to check and clean the interior of the wheel where the shaft is inserted. This can sometimes have goo and will slow the motor.
I use pink silicone tubing that I get from a local model shop. The outside diameter is almost perfect and the inside diameter is tight enough that I don't have to use any glue. Push the tubing over the shaft and make sure it is fits evenly all the way around.
Trim excess tubing flush to the metal. Make sure the tubing does not extend beyond the metal end. If it does, it may drag on the frame. I trim it a little smaller to avoid rubbing.
Clean the card reader surfaces and the read/write head before putting the reader back together. Install the nylon balls, pinch roller and spring clips. Make sure they are clean before installing.
Reverse the process that you used to disassemble. The left hand grasps the half that has the nylon balls (feet up) and the right hand places the other half on top. Replace the screw shown in the photo. Tip: Practice this without the loose parts first. Finally, install the reader PCB and replace the 3 silver color screws.
To test the card reader, assemble the frame and case as shown. Don't forget to connect the keyboard and display connector. Then, pull the ribbon cable through the frame and attach the card reader. This makes it easy to test without assembly and disassembly. Adjust slotted shaft until the card reads and writes properly.
When you have the card reader working properly, remove the ribbon cable. Thread it back through the slot and reinstall the card reader. Reconnect the ribbon cable. Check for broken wires before closing case. Note: There are slots in the reader feet. Adjust position of reader so that cards to not touch case, as card exits case slot. This takes practice.
Install the 7 screws that hold the frame in place. Check that the keyboard/display connection is mated properly. Check that you have not disconnected the printer. NOTE: Be careful with the top center screw. The screw post is fragile and breaks easy. Just make it snug.
Replace the final 6 screws in the case and your done. That wasn't hard, was it?

Remember above when I said "don't touch these contacts"? Well, I have found on many units (don't fully understand this) that I can correct "write errors" by simply wiping these contacts with steel wool. Be very careful to avoid bending these contacts. They are very hard to adjust.

I'm sure there are many other ways of making these repairs. This is just the method that works well for me.

Copyright (2001) Mike Davis
Send comments, suggestions or corrections to Mike Davis

I only have a few minor suggestions to add: first, when I have difficulty adjusting the reader so that it pulls the card through smoothly, I sometimes disconnect the motor from the control circuit (some soldering required!) and apply a 1.5V power source directly to the motor terminals to test. Second, when the switch contacts do not seem to operate correctly, I attach a DMM (in short circuit test 'beep' mode) to the appropriate terminals at the end of the keyboard PCB, pull a card through manually, and test for a good contact as the individual switches are reached. These methods are useful because they let you perform essential tests without having to partially or completely reassemble the calculator. Lastly, I personally prefer not to remove the ribbon cable from the card reader; I just flip it over in place to access its guts. That's because I've had some bad experiences when trying to reattach the cable (old, worn, or corroded contacts that didn't work as expected.)