HP-25 resurection project

[wbvreeuwijk]

Hi All,

Just received an HP-25 that is in need of some serious love and care. The HP-25 holds a special place in my heart because it was the first calculator that I remember programming. Entering the lunar lander game and nims.

Here are some pictures and of the internals.
           

Doesn't look too bad, other than some green stuff on the tracks and it appears there are some components missing.

... to be continued.

[Maximilian Hohmann]

Hello!

(12-01-2020 10:44 AM)wbvreeuwijk Wrote:  Doesn't look too bad, other than some green stuff on the tracks and it appears there are some components missing.

It really looks quite nice inside and out (compared to others I have seen) and there are no missing components. I happen to have an opened one right on my desk that I bought as defective from eBay last week (for 10 Euros). It seems to have some contact problems only because I could briefly see "0.00" in the display.

To get rid of the green corrosion you can (safely!) immerse the main PCB in a little bit of vinegar (not the keyboard or display though!). I use it pure because with diluted vinegar it takes much longer. Rinse it very well thereafter and let it dry completely, e.g. with a hairdryer.

I'm sure that you have already come across (many times...) the most important issue regarding these calculators: Never ever power them with anything but batteries or a regulated DC power supply set at 2,5V. Throw away the orinial charger otherwise somebody might plug it in ad fry your calculator... There is no polarity protection either, so mark the "+" and "-" connections clearly.

Good luck with bringing your HP-25 back to life!

Regards
Max

[wbvreeuwijk]

Thanks for the feedback Max.

I do believe there are some components missing from the PSU. Details here:
   

Based on other pictures I've seen of the HP-25 internals there should be some more components here.

Also some examination of the LED display. Not for the faint of heart ;-)

[Maximilian Hohmann]

Hello!

(12-01-2020 04:39 PM)wbvreeuwijk Wrote:  I do believe there are some components missing from the PSU. Details here: ... Based on other pictures I've seen of the HP-25 internals there should be some more components here.

Obviously the "non-C" HP-25 has less components (I guess the extra resistors keep the memory of the 25C powered when it is turned off). Here is a picture that I took just now from a normal HP-25. It looks exactly like yours!

(12-01-2020 04:39 PM)wbvreeuwijk Wrote:  Also some examination of the LED display. Not for the faint of heart ;-) ...

Wow, this looks very green inside. Here is a link to a photo of an HP-25C where the display connections are annotated (it was made by a forum member who unfortunately passed away some years back): http://home.citycable.ch/pierrefleur/Jac...25_WEB.jpg
The "K1" to "K12" connections are the cathodes of the individual digits and "a" to "h" are the anodes of the segments and the decimal points. With that it should be possible to check which segments are still working. Mind you that there are no original replacement parts. You may need to cannibalise another calculator of that series to fix yours...

You have certainly come across the website of "Panamatik" (also a forum member) (http://www.panamatik.de). He designed some repair kits for "Woodstock" calculators like the HP-25 and at one time even made some replacement displays. He might have some left if you are lucky.

Regards
Max

[wbvreeuwijk]

Ah, that is good to know :-)

Glad to see that nothing is missing. I must have looked at images and schematics of an HP-25C.

Regarding the display, I did notice that "Panamatik" has a replacement, but it seems he is out of stock. I designed a replacement PCB and ordered 5 from JLCPCB. I also found the HP5082-7414 bubble displays to populate it with.

The vinegar bath was also on my to-do list :-). Good to hear that that is indeed the recommended approach.

... to be continued

[cjmcc]

(12-01-2020 05:36 PM)Maximilian Hohmann Wrote:  ... Here is a link to a photo of an HP-25C where the display connections are annotated (it was made by a forum member who unfortunately passed away some years back): http://home.citycable.ch/pierrefleur/Jac...25_WEB.jpg
The "K1" to "K12" connections are the cathodes of the individual digits and "a" to "h" are the anodes of the segments and the decimal points. With that it should be possible to check which segments are still working. Mind you that there are no original replacement parts. You may need to cannibalise another calculator of that series to fix yours. ...

Thanks for the link. Fantastic set of images detailing the innards of an HP-25 that should be invaluable when I get around to trying to fix the display of my HP-29C. The top of the website is http://home.citycable.ch/pierrefleur/Jac..._intro.htm .

-Chris-

[AndiGer]

The website on home.citycable.ch to my knowledge has lost information, esp. pictures / drawings. Therefore the guys here on this forum restored more info and published on https://archived.hpcalc.org/laporte/
This is an hommage to Jacques LaPorte who contributed enormous information on these wonderful machines to the public.

[wbvreeuwijk]

So I removed all components from the board and will start populating it. First with the PSU section. I did find out why there was a difference between other HP-25 boards and mine. It seems that the Vdisp is taken from Vss through a resistor. This explains the missing part.
   

Also I had a first go at creating the LED display
   

It's working, but the digits are in the wrong order and the height of the board is a bit to small, so next revision has been ordered.

Panamatik suggested to place the ROM/RAM and ACT in IC sockets, which I hadn't considered yet. Parts should be arriving this week.