Messing with a Monroe 425

I’ve got a Monroe 425 desk calculator which I mostly bought for the power cable and to practice working on this type of device before doing too much to a Monroe Trader I already had.

Day 1 — 2022-01-14

The Monroe 425 arrived today. That’s exciting! Unfortunately most of actually working with the thing was dissapointment.

Visually, it was good: very similar to the Monroe Trader in terms of weight, plastic, apperance. It’s narrower than the Trader, which is unfortunate, but I expected. It powered on just fine, but none of the keys did anything (except sometimes “C/CE”). It still looks and feels like a sturdy artifact.

I suspected up front that the keyswitches were not the same as on the Monroe Trader: they felt “grainier”, less steady, with less even resistence and no satisfying “thock”. Opening it up revealed that the switches are… odd. Like the Trader, they’re MX-like but not quite compatible, but they’re also not the same incompatible switches as on the Trader, and… cheaper feeling. Part of this might be age and/or use, but it doesn’t feel like it. In addition (and perhaps a sign of the overall build quality), the switches aren’t even soldered onto the PCB. The bottom of each switch has a pair of thin flat contacts which get pressed up against the pads on the PCB. There’s a little visible corrosion on the (thick!) traces on the PCB, which might be contributing to the fact that the keys mostly don’t work. Also, less critically but still dissapointing: the fact that the stems are almost MX, but longer and thinner “arms” on the crosses, means I won’t be able to use them on anything else without some surgury.

I’d hoped to be able to use the original switches and just rewire the matrix, but now that doesn’t really seem worthwhile. I’ll have to decide between replacing the switches in the existing frame cutting into the top plate and dropping in a new keyboard (something like a 5x10 ortholinear, perhaps).

While I don’t intend to do much to the Trader (for now?), I did open it up just enough to do some comparisons. The internals are a lot more different than I’d expected. The first thing I noticed was that there’s far more IC chips in the Trader. I also confirmed my suspicions about the keyswitches: different construction, and in the Trader they’re soldered to the PCB. There’s also a sticker which looks to indicate a build date: “061780”. The biggest dissapointment of the day was that the Trader itself seems to be in worse shape than the 425: you can hear some transformer, but there’s no display or sign of activity at all. Dealing with that is outside the scope of this project, though.