Swapped ROM revives struggling HP-48S calculator

Buying broken gear on the cheap is a centuries-old hacking tradition, and while we don’t always manage to revive it, we rarely come away empty-handed. There are always pieces to scavenge, and you can’t put a price on the knowledge to be acquired by snooping inside interesting material. So we’re not at all surprised to hear that. [Tomas Pavlovic] jumped at the chance to grab this faulty HP-48S calculator for a few dollars.

Luckily for us, the story doesn’t end at the bottom of its coin tray. When he brought the HP-48S home, he immediately investigated whether it could be repaired. After changing a few choice components and seeing no results in the behavior of the device, he began to suspect that the problem might be in the firmware; more specifically, the soldered chip that contains it.

Original ROM dump.

After carefully lifting the NEC uPD23C2000GC from its resting place for the past 30 or so years, he wired up an adapter that allowed him to connect the chip to his programmer so its contents could be dumped. Rather than try to find another ROM chip, he decided to wire a socket and found a rewritable SST39SF040 that could replace it. Flashing the salvaged firmware to the newly inserted chip allowed the calculator to work again, with the added benefit of allowing [Tomas] to pull the chip and flash a different firmware version if he wants to experiment a bit.

Now we know what you’re thinking. Where was the fix? What exactly brought this 1990s equipment back to life? This part, unfortunately, is not very clear. You would think that if the original ROM chip was somehow faulty, [Tomas] wouldn’t have been able to extract a valid firmware image from it so easily. That leaves us with some pretty mundane possibilities, like a bad solder joint on the chip pins. If that were indeed the case, this solution could have been as simple as running a hot iron over the pins… but of course, where’s the fun in that?

Update: We have heard from [Tomas], and it turns out that compared to a known good copy, the dumped firmware had a few bits swapped. His theory is that the NEC chip is in a weird failure mode where the calculator wouldn’t work, but it was still functional enough to extract most of the content from it. What do you think? Let us know in the comments.

Eleanor C. William