Buying broken gear for cheap is time honored hacker tradition, and while we might not always be successful in reviving it, rarely do we come away empty handed. There’s always parts to salvage, and you can’t put a price on the knowledge to be gained when poking around inside an interesting piece of hardware. So we’re not surprised at all to hear that [Tomas Pavlovic] jumped at the chance to grab this faulty HP-48S calculator for a couple bucks.
Luckily for us, the story doesn’t end at the bottom of his parts bin. When he got the HP-48S back home, he immediately set out to see if it could be repaired. After changing out a few choice components and not seeing any result in the device’s behavior, he became suspicious that the problem may be with the firmware; specifically, the soldered-on chip that holds it.
After carefully lifting the NEC uPD23C2000GC from its resting place for the last 30 years or so, he wired up an adapter that let him connect the chip to his programmer so its contents could be dumped. Rather than trying to find another ROM chip, he decided to wire in a socket and found a re-writable SST39SF040 that could stand in as a replacement. Flashing a fresh copy of the firmware to the new socketed chip got the calculator up and running again, with the added bonus of allowing [Tomas] to pull the chip and flash a different firmware version should he care to experiment a bit.
Now, we know what you’re thinking. Where was the fix? What exactly brought this piece of 1990s gear back to life? That part, unfortunately, isn’t very clear. You’d think if the original ROM chip was somehow faulty, [Tomas] wouldn’t have been able to so easily pull a valid firmware image from it. That leaves us with some pretty mundane possibilities, such as a bad solder joint on the chip’s pins. If that was indeed the case, this fix could have been as simple as running a hot iron over the pins…but of course, where’s the fun in that?
Update: We heard back from [Tomas], and it turns out that when compared to a known good copy, the dumped firmware did have a few swapped bits. His theory is that the NEC chip is in some weird failure mode where the calculator wouldn’t run, but it was still functional enough to get most of the content off of it. What do you think? Let us know in the comments.
One of the major factors for [Michael] was the great feel of the keys on these classic units. Wanting to experiment with different layouts without a lot of rewiring, the idea of keys with individual displays became attractive. Existing parts on the market were prohibitively expensive, however. Instead, [Michael] used a single touchscreen with a switch mounted underneath to provide tactile feedback with a nifty scissor-arm guide mechanism. Combined with individual see-through plastic overlays, the MP-29 has a fully reconfigurable pad of 30 keys with dynamically updatable labels.
It’s a creative choice, and one that looks highly satisfying to use. It has all the tactile benefits of individual keys, both in the keypresses and being able to navigate the keypad without looking. Combined with the benefit of reconfigurable keys thanks to the touch screen underneath, it’s a great way to build a user-interface.
The rest of the calculator design closely mimics the HP-29, though [Michael] is also experimenting with alternative layouts too. There are plenty of religious wars in the calculator community over usability, after all – mostly over which side of the pad has the arithmetic functions.
According to the author, the HP 50G is an awesome calculator. But as awesome as it is, it is powered off of not-so-awesome AAA batteries. These little batteries don’t last long under the load of the calculators awesomeness, so a mod needed to be done. The battery chosen for installation was a replacement battery for a Sony PSP. He was able to find one, including the external charge regulator board for $10. Dropping it into the case and wiring it to charge off of the existing mini USB port looks fairly simple, very few actual modifications are necessary to the body. Unfortunately, they did end up with an LED protruding from the bottom that makes it wobble a bit, but they’re guessing that they get about 2x the battery life now.