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Restoring a classic

Bringing a HP-67 back to life

Scritto da Silicon_Simon.
Mercoledì 12 Novembre 2025, 1:38 pm. Tag usati: calculator, hp, retrocomputing, rpn

I have always been fascinated by RPN, finding it surprisingly fast, intuitive, and more direct compared to the algebraic input methods of common calculators. The Reverse Polish Notation eliminates the need for parentheses and operator precedence considerations, allowing calculations to be entered sequentially and efficiently. Many users appreciate how RPN mirrors the manual process of solving problems, making the workflow smoother and less error-prone, especially for multi-step computations in technical and scientific contexts.

Coming back to the HP-67, this amazing old-school device provided a complete set of scientific, statistical and engineering operations, including trigonometric, logarithmic and exponential functions, coordinate conversions, average/standard deviation etc.

Time has not been kind to this device. When I first powered it on, nothing happened. After cleaning up some internal contacts and inspecting the circuitry, I traced the main issue to the battery pack. Much information about the original battery, model HP 82001A, can still be found online today, for example here. 

The Classic Series battery pack HP 82001A was designed for use in several vintage Hewlett-Packard calculators, including the HP-35, HP-45, HP-65, HP-67, HP-70, and HP-80 models. It consists of three AA-size NiCd rechargeable cells connected in series, delivering a nominal voltage of approximately 3.6 to 3.75 volts. The battery cells are housed inside a plastic casing that often forms part of the battery compartment door. The battery pack connects to the calculator through specific terminals marked for positive and negative polarity, with a spring contact mechanism that ensures a secure and correct connection.

Original charging was originally done externally using a dedicated charger that provided both operating power and charging current. This charger featured a unique connector that was difficult to replicate, making battery replacement and power supply challenging for restorers. Due to aging, original NiCd cells commonly leak and degrade, so modern restorations often replace these packs with custom battery holders using contemporary NiMH or LiPo cells while maintaining the original dimensions and connector layout for compatibility. This battery pack remains a key consideration for anyone restoring or maintaining a Classic Series HP calculator, due to its unique form factor and electrical requirements.

My original pack was well beyond recovery: fully discharged, the terminals corroded, and the cells leaking residue. Since I didn’t own the original AC charger - which uses a rather uncommon connector - I decided to power the calculator in another way. My first attempt was to use a single 3.7 V 600 mAh LiPo battery, the kind often found in cordless phones or small RC models. This improvised setup allowed me to verify that the handheld calculator still worked.

However, recharging a LiPo pack externally turned out to be cumbersome. I needed a more elegant and practical solution. So I decided to reconstruct a completely new, removable battery pack, faithful to the original design but easier to maintain.

Using calipers, I took precise measurements of the original battery casing with its exhausted nickel-cadmium cells (or what remained of them). Then, I created a 3D model in CAD, sculpting the geometry until I achieved a printable replica suitable for resin printing on my Elegoo Mars 2 printer. The final design houses three standard AAA NiMH rechargeable cells, arranged exactly as in the original pack. These can easily slide out for charging in any common NiMH charger.

To secure the two halves of the printed battery shell, I embedded four small magnets (10 × 5 × 1 mm). For the electrical contacts, I cut, shaped and brushed thin aluminum pieces from a soda can, fitted into the model's designed slots to link the three cells in series. This configuration outputs about 3.6 V, matching the original HP specifications perfectly. By contrast, the temporary LiPo solution delivered 3.7 V when fully charged, slightly higher but acceptable for testing.

After soldering the connection wires and recreating the characteristic V-shaped contact tabs, I made sure the alignment matched the calculator’s housing grooves, ensuring a secure and polarity-safe fit. The printable design accounts for these physical constraints.

The result is a robust and elegant battery pack that preserves the calculator’s vintage feel while offering modern convenience. I’m sharing the printable STL file here, so fellow HP enthusiasts can use or adapt it for their own restorations. You can see the final outcome in the photographs attached to this post.  I’m genuinely pleased with how this small reproduction project turned out.

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