HP-25 calculator shows all zeros, or blank display

Something that old, certainly by now has some cold solder connections. I would reflow the whole board, especially if you say alkaline leaked out of the batteries onto the board and the cleaning had some positive effect. If there are any electrolytic capacitors on the board, change them too.

Have you tried the fansite, hpmuseum.org?

The battery compartment isn't sealed and the batteries tend to leak corrosive goo into the guts.

Argh. Use NiCad batteries instead. Alkalines have a higher terminal voltage (1.5V) than NiCads (1.35V). I don't know if you can damage anything with the higher voltage, but it doesn't hurt to be safe. If you don't have NiCads, use a bench power supply at 2.70VDC.

Contact cleaner leaves an oily residue. That's fine for the slide switches, but not a great idea for the rest of the circuitry. Remove with alcohol.

Did you inspect it for any corroded traces, especially near the battery compartment? The various IC's are quite close to the battery section:

Cleaning UNDER the IC's is tricky. If there's any corrosion under there, it can be scraped out with a stiff piece of wire shoved under the rows of IC leads. Also, brush between the leads.

Something might be loose or dirty around the clock oscillator area. However, my intuition suggests that it might have something to do with the over-voltage.

Ask on the HP Museum forums:

but first check the archives:

The old alkaline batteries I used measured 2.5 volts DC.

Yes, I inspected every solder joint and trace for problems with a high power magnifying glas. Been doing it for years.

I'll do that.

Thanks,

-Bill