Replacement ROM revives ailing HP-48S calculator

Broken gear is a respected hacker tradition for the time being bought cheaply, and although we may not always succeed in reviving it, we rarely leave empty-handed. There are always parts to be rescued and you can’t afford the value of the knowledge gained while wandering around in an interesting piece of hardware. So we are not surprised at all [Tomas Pavlovic] For a few bucks he jumped at the chance to grab this faulty HP-48S calculator.

Fortunately for us, the story doesn’t end at the bottom of his parts bin. When he returned home with the HP-48S, he immediately went out to see if it could be repaired. After changing a few preferred components and seeing no results in the behavior of the device, he suspected that the problem might be with the firmware; Specifically, the solder-on chip that holds it.

Original rum dumping.

After carefully removing the NEC uPD23C2000GC from its resting place for the past 30 years, he attached an adapter that allowed him to attach the chip to his programmer so that its contents could be dumped. Instead of trying to find another ROM chip, he decided to wire it into a socket and find a rewritable SST39SF040 that could stand as a replacement. Flashing the recovered firmware on the new socket chip re-enabled the calculator and with the added bonus of permission. [Tomas] If he wants to pull the chip and flash a different firmware version a little test.

Now, we know what you’re thinking. Where exactly was it? What revived this piece of gear from the 1990s? That part, unfortunately, is not very clear. Would you mind if the original ROM chip was somehow defective, [Tomas] Will not be able to pull a valid firmware image from it so easily. This leaves us with some pretty mundane possibilities, like a bad solder joint on a chip pin. If so, this fix could be as simple as running a hot iron on a pin … but of course, where’s the fun in that?

Update: We heard back [Tomas], And it turns out that compared to a familiar good copy, the dumped firmware had a few interchangeable bits. His theory is that the NEC chip is in some weird failure mode where the calculator won’t run, but it was still effective enough to shut down most of its contents. What do you think? Let us know in the comments.

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