I "repaired" one of those a couple months ago, found a set of batteries (intended as spare for an old model cordless phone) and used a tiny DC-DC converter to u

I "repaired" one of those a couple months ago, found a set of batteries (intended as spare for an old model cordless phone) and used a tiny DC-DC converter to use a USB charger, the tricky part was to re-tin in a few places the PCB and re-solder one of the power cables that came off due to oxidation. It came out as new, the only issue is one of the display segmemts on fifth number (from left) not lighting up, luckily it is the right bottom one, so it is still "readable". My HP 28-C (circa 1987) is still going strong, and it is 34 years old by now.