HP-25 not working

[Peval]

Hi all,
I'm new in this forum so it could be I'm submitting something already discussed that I didn't find.
in any case, I own a very old HP-25 (1974 or 1975 I'm not sure).
I would like to resume it working.
it is not working by 40 years probably.
I do not remember what was the cause that force it to stop working.
Maybe a try to use it connected to the power supply but without batteries or maybe something else.
What I did, following some suggestion found on the net, was to remove the plastic "bridge" between batteries, remove the old rechargeable batteries and place two Alkaline AA instead.
Still not turns on.
I have read that in this case one of the possible problems could be the ACT chip.
I'm ready to buy from panamatik (http://www.panamatik.de/html/hp_calculat...r_kit.html) the repair kit.
But if there are some other possible reason why it does not turns on, please give some advice what to check and how.
Many thanks in advance for your help.
Best regards to all.
Peval

[Steve Simpkin]

A couple of quick notes.

1. Never plug the AC Charger power supply into the HP-25 without a know good battery installed! This will likely damage the ACT chip instantly. These models use the batteries to regulate the charger voltage. It is best not to use the charger at all. For long time use, charge the batteries outside of the calculator.

2. When using Alkaline batteries, be aware that the positive terminal of the battery may not make good contact with the PCB contact point (rivit) in the battery compartment. You may need to try it a couple of times before it works. It is best to use flat-top rechargeable batteries.

3. Observe proper polarity of the batteries as shown in the following picture.
https://www.hpmuseum.org/batts/batt20.jpg

[Peval]

Thanks Steve for the notes.

But the mistake and the consequent damage was done 40 years ago (more or less).
So, now I just want to get it back working.

If the only cause is the damage of the ACT chip, I think that the solution from panamatik (http://www.panamatik.de/html/hp_calculat...r_kit.html) repair kit should be nice.

I need to know if there is something else to check.

I'm just afraid to buy the ACT repair kit, replace the damaged one with the new and still not seeing HP-25 beck to life.

Many thanks.
Best regards
Peval

[teenix]

Apart from checking for visual problems like corrosion or cracks on the circuit board, I'm not sure there is much more you can do. Unplug the keyboard and display from the CPU board and then reconnect them. There may be a connection problem. Clean the battery terminals.

If the calculator has been sitting unused for 40 years, then maybe some capacitors are on their way out.

The best option would be if you have a multimeter and/or oscilloscope, or know someone who has, then you can check the power supply voltages and signals on the data buses.

cheers

Tony

[Peval]

Thanks Tony,
I have a friend of mine that has an high level electronic company so he has all the needed equipments to check what you have suggested.

The PCB schematic should be useful in this case.
Do someone has it?

Many thanks,
Best regards.
Peval

[AndiGer]

Link to Tony Duell's schematics is linked here:
https://hpmuseum.org/forum/thread-13740-...#pid121606
Andi

[Hiwi]

Check the current, it should be 160-180 mA when turned on, ok only with display, but if it is zero, there can be a contact problem. Then check voltages VGG VDD and VSS. You can find schematics in the net. I recently fixed a 25C which has no continuous memory. There was the typical problem with corrosion. The Pwon line was interupted under the memory chip. So if turned off, the memory had no battery on Pin 1.

In most cases, when users plug in the charger without battery the ACT Chip could be damaged, so there is often a 0.00 or flashing numbers on the display. (No keyboard action) If there is nothing on the display this could be a faulty Cathode /Anode driver.

Ralf

[BobVA]

You might also consider the Woodstock Low Power repair kit. This replaces many of the internal components (including the power supply), so it can fix more drastic problems. You can download the manual from Panamatik for the details.

[kohli_vie]

i had the same problem.
i buy (for my memory corner from the 70ths) a HP-25 which doesnt work.

as some user wrote previous, corrosion (maybe from accumulator acid) was the problem, blue tracks)
after some reengineering (to easier find the points to measure) i remove the wrong repair try from the previos owner, and find 2 of the internal power tracks are damaged

you find a lot of information in the WWW
in my opinion, the best is (because you have all pin numbers from the ICs):
https://archived.hpcalc.org/laporte/Wood..._intro.htm

good luck