HP-25 Power-On Issue


My HP-25 worked properly- up until yesterday, evening. One moment it was fine, then the next time I turned it on, all I got was a full display of zeros. Pressing the keys do not affect anything, it just has zeros all the way across the display, frozen. I've taken it apart and made sure that all of the connectors were making contact and that everything else looked visibly normal. No improvements. At this point, I'm thinking that it is terminal. Does anyone have any ideas on what might have happened? Are there chips/caps/etc that might have degraded with age and caused this problem? Thanks for reading.


Most likely (alas) it's ACT (processor) or ROM failure, but there are a few quick checks you can do.
Firstly, check the supply voltages. Vss (+6.2V) at pin 1 of the ACT (the 22 pin chip near the front edge of the board). And Vgg (-12V) at pin 2 of the ACT. The common reference is the battery -ve terminal.
Also check the 2.2uf tantalum capacitor near the ACT -- that's the reset timing capacitor and maybe the machine is not initialising properly.


I have found that a display of all 0's is caused by a defective ACT chip over 96% of the time. My pile of 0000000000 ACT chips is over 25 deep now. You can replace it with the ACT chip from any other Woodstock series machine (except for possibly the HP27)


I supsect you can also use an ACT from a Topcat or an HP67. I've done the reverse -- I repaired my 97 (which was dead when I got it) using the ACT from a broken (ROM failure) HP21. I was
quite suprised when it worked (I'd previously tried the ACT from a 67 in the broken 97, that worked too, so I knew the fault in the 97 was the original ACT).
The HP27's ACT is almost certainly not suitable though. It's NMOS, not PMOS, so the supply voltages (and logic levels?) are totally different.


David- Thank you for the comments. From reading your posting, you must have either stockpiled some ACT chips when production was discontinued, or you have a heck of a pile of calculator carcasses in your hobby room...


Hint, dead and ugly Woodstock machines make pretty good battery chargers for those oh-so-delicate 25C and 29C models... and the rest of the Woodstocks for that matter.

The full monte involves:
1) Cutting loose the keyboard to free the keys.
2) Remove the LED lens for spare parts
3) Cover over the keyboard holes with a piece of plastic
4) Add stick on rubber feet
5) Remove all the parts from the PCB except the charge resistor and diode. Save for spare parts.
6) Add an LED/current limit resistor across the charge resistor for charge indication. Mount it in a hole drilled in the bottom of the case next to the battery box.
7) Reassemble and voila...

You should only do this for well and truly dead/ugly/engraved/, etc machines. Nicer machines can be used pretty much as is.


Actaally, battery chargers exist for (almost?) all the the calculator batteries. HP called them 'Reserve Power Packs'. They consist of a plastic box to hold the battery with some very simple circuitry inside (often close to a copy of the calculator charger circuitry). You connect them to the same AC adapter as you'd use with the calculator (well, with one exception -- the classic-series reserve power pack uses the 3 pin adapter as you'd expect, but of course it can also charge the battery in the HP75 which itself uses the 8V 3VA adapter).
These Reserve Power Packs certainly exist for the Classic, Woodstock, Topcat and Spice models. The Classic one seems to be the easiest to find, the Spice is by far the rarest (in fact I'd never seen it mentioned until I found one).


Speaking of rare (probably non-existant) reserve power packs. Has anyone ever seen or heard of one for the Stings (10C and 19C)? I'd sure love to have one of those!


Unfortunately the Woodstock reserve power packs (and all the others for that matter) are rather hard to come by. Ugly and dead Woodstocks are all too common. My favorite charger for classic battery packs is an HP55 without a battery door.

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