HP 9810A Calculator
Card Reader Drive Wheel Repair

The drive wheel of the magnetic card reader of the HP 9810A calculator after 50 years undergoes complete decay, preventing the proper functioning of the reader.

However, there is a fairly straightforward way to repair it. A relatively simple solution to this problem has been described here.

On this website, I present my struggles with this issue.

To replace the drive wheel of the magnetic card reader, you need to partially disassemble the reader module.

In the first step, lay the assembly on a working surface with the drive belt facing downwards. Then, unscrew the 2 screws securing the read/write head (marked in the second picture) and remove the read/write head. This step is not crucial, but it facilitates work on assembly and protects the head from potential damage.

In the next step, rotate the card reader module with the drive belt facing upwards and unscrew all the securing screws located on that side. Now, you can disassemble the module, but before doing so, it’s important to pay attention to the placement of individual parts.

Next, slowly slide apart the assembly. I removed one light source, which was enough to slide it out of its socket. You just need to disassemble the reader enough to access the end of the shaft where the drive wheels are located.

Now, I carefully removed the remnants of the old drive wheels. All it took was using a flat screwdriver and some cleaning solution.

Next, I used three O-ring seals with a diameter of 5/8″ ID x 7/8″ OD x 1/8″.

I placed one around each drive wheel and one between the wheels. The third sealing ring is used to keep the other two at the right distance and is positioned low enough not to touch the card during its passage. The O-rings fit very snugly and do not require additional adhesive.

In the next step, I reassembled the card reader module. I didn’t encounter any additional difficulties with this. It was worth taking photos documenting each step of disassembling the reader module beforehand.

After mounting the assembled magnetic card reader and control board in my HP 9810A, we should obtain the result as shown in the pictures below.