One small correction/clarification: VFDs didn't use more power in the sense that they drained batteries faster, in practice they used less in that sense, but they did require a step up converter to supply a 30v rail for the cathode. That was expensive and complicated the assembly process (especially when the circuitry had otherwise been consolidated to a single chip), so TI tended to avoid VFDs most of the time in their battery operated calculators. Eventually LCDs made the whole point moot, as they were cheaper to make and sipped on batteries (at the expense of backlighting.) TI did produce a handful of calculators with VFDs, mostly slim pocket models, but also the TI-1265 which was essentially a TI-1250 with a VFD. It retailed for a couple dollars more than the 1250. I have a 1265 and the display is much, much more readable than their standard bubble LED displays IMO.
As a nerdy child of the 90's bought these from yard sales and did this routinely after a salesman at Radio Shack sold me the parts. Even had a soldering bench set up in the mall where you could still smoke cigarettes.
Great find! Same form factor and display as the TI-55. If I recall it was a less expensive model that didn’t have all the “bells & whistles” but still was enough for high school and university students.
If you're ever inclined to try to restore the calculator back to a more accurate version of its original self, you can almost certainly still get basically exactly the same NiCad cells fairly easily. The rechargeable nature of this calculator was actually a really cool feature at the time, so it seems a shame to just throw that part of its design away completely...
You raise an interesting and valid point: Whether to restore back to a more "original" version or alter to a more "useful" version. If I were restoring this calculator for a museum and authenticity & faithfulness to posterity were paramount, I would most certainly have kept the battery pack as it was and acquired & installed a pair of NiCad batteries. But in this instance, I thought the modification to accept a 9v battery would be simpler and more useful even if it meant sacrificing the option to recharge the batteries. Also in my mind was my goal to eliminate NiCad batteries in all items I have. Battery leakage (particularly from NiCads and Lithium batteries) can be enormously destructive to electronics and I want to take steps to mitigate future losses.
To me black is negative and red is positive no matter whether it’s PMOS or NMOS. Whether the colour code was nefarious or not is debatable but I’m sure many chips were damaged by unwittingly connecting power in reverse.
