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HP-25 resurrection adventure – Part 1: The Keyboard
09-20-2021, 03:32 PM
Post: #1
HP-25 resurrection adventure – Part 1: The Keyboard
Well hello folks! I've been absent from the community quite a while now but (of course) never lost my passion for all things HP calcs. ;-)

I just stumbled upon the fantastic ACT replacement hardware by Bernhard Emese (PANAMATIK) for the HP-25 (and others). This made me take my good old HP-25 out of the display cabinet and check it out.

Turns out: the obvious. Corrosion, leakage, the usual. After cleaning with IPA everything, including the keyboard, it seemed to work fine again. When turning it on it shows the well-known "0.00" at the left. Entering numbers works flawlessly now.

However, a strange thing is happening that renders the unit unusable: upon pressing almost any key (besides the numbers 0-9 but including the decimal point) or invoking any math function the display goes dark for about 0.5s and then shows "0.000000000".

There's one exception: setting the FIX, SCI or ENG modes works. So the f key works, too, obviously. I bet all the keys work fine but they're not doing what they should.

Is this a typical sign of a fried ACT?

I did all the measurements Bernhard suggests in his ACT manual – everything seems to look fine, even the frequencies measured at the pins 16, 17 and 20 of the ACT.

So what might be a next step here for me? Anything I can investigate further? Or am I already in for a new ACT from Bernhard? Any advice very welcome! :-)
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09-20-2021, 03:41 PM
Post: #2
RE: HP-25 resurrection adventure – Part 1: The Keyboard
Sounds like the keys rows are not getting scanned properly. Open the unit and clean the connection pins and corresponding holes carefully with 400 grit sandpaper. Did corrosion get into the keyboard? If so, you can try spraying a bit of contact cleaner into the holes beneath each key, and pressing the keys repeatedly. If necessary, you 'can' disassemble and clean the keyboard. You must drill out the heat stakes, clean the contacts, then carefully reassemble and reglue. If the calc still fails, I'd try Bernhard's ACT-E. It worked great for me.

Regards,

Dave
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09-20-2021, 10:37 PM
Post: #3
RE: HP-25 resurrection adventure – Part 1: The Keyboard
If the numbers work, and you say the [f] key seems to work, do the shifted functions work on the number keys?

If they do, then perhaps the other keys and the contacts between the circuit boards (including the holes in the board) need a good clean.

If not, then it could be a hardware issue. ie ROM, ACT etc.

cheers

Tony
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Yesterday, 09:07 PM (This post was last modified: Yesterday 09:08 PM by metawops.)
Post: #4
RE: HP-25 resurrection adventure – Part 1: The Keyboard
(09-20-2021 10:37 PM)teenix Wrote:  If the numbers work, and you say the [f] key seems to work, do the shifted functions work on the number keys?

If they do, then perhaps the other keys and the contacts between the circuit boards (including the holes in the board) need a good clean.

If not, then it could be a hardware issue. ie ROM, ACT etc.

cheers

Tony

Hi Tony & thanks for caring. ❤️
Nope, the f-shifted functions of the number keys don't work. They produce that "0.000000000", too.

I'm pretty sure now that this is either a bad ACT, ROM or RAM or a combination of these. Waiting for Bernhard to get through his emails after being away. Eager to hear what he thinks.
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Yesterday, 11:19 PM
Post: #5
RE: HP-25 resurrection adventure – Part 1: The Keyboard
Maybe have a look at the pin connections from the IC's to the circuit board.

If you have a soldering iron, you could try re-soldering them. Try not to touch any circuity, static from your body may damage things.

cheers

Tony
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Today, 03:44 PM
Post: #6
RE: HP-25 resurrection adventure – Part 1: The Keyboard
"display goes dark for about 0.5s and then shows "0.000000000"."

I had this identical behavior on one of my 25s after the Panamatik ACT-E conversion. The 'external' 25 ROM gives the behavior you described. The 'internal' ROM (on the Panamatik chip) works perfectly. It's the ACT chip. Put the new ACT-E in. If you get this behavior, hold down EEX while switching on. This toggles between internal and external ROM.

Regards,

Dave
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