My original HP-25, ailing

[Peter Klein]

Tonight I opened up my HP-25. I cleaned the battery terminals, which had quite a bit of corrosion on them. I also cleaned some oxidation on the power supply traces at the left of the main board (see picture). My keyboard is the type that has holes on the back of the board, so I flooded them with contact cleaner and brushed the "4" key out with a bit of stranded wire (it was barely working before), then flooded again and dried.

Results were dramatic. "It lives!" The RAM was already dead, so no registers and no programming. Everything else is working again. The "4" key has improved, but is still intermittent. So is the "8" key. So I'll have to clean them again. There is one quirk: The calculator shows low battery with a pair of Eneloop rechargeable NiMH AA batteries supplying 2.6v. Another, at 2.8 volts, works, but gets a low battery warning with 8 digits displayed. The calculator happily runs with a pair of standard AA alkaline batteries supplying about 3.05 volts, or a 3v regulated wall wart that supplies about 3.16 volts under load.

I suspect that the electrolytic capacitor in the internal switching power supply has degraded, even though it is not visibly leaking. See picture below, it's the silver cylinder near the bottom. It's 60 microfarad, 6 volts. I have a 22 microfarad capacitor that I may be able to fit in. A local electronics store has 100 uf 10v capacitors. Would those work? Failing that, does anyone know where I can get a replacement in the U.S. that's slender enough to fit between the main board and the keyboard?

I ran the tests on p. 12 of the Panmatik manual (http://panamatik.de/ACTManual.pdf).
Pin 1 varied between 5.75 and 6.1 v.
Pin 2: -12 v.
Pins 16 and 17: 176.8 khz, probably close enough to 180 spec.
Pin 20, Sync: approx 1.9 khz, with some variation. Some noise was evident, but I don't know if this was pickup from the test leads, or internal noise.

Pictures below. Further comments and advice welcome.