The ACT repair Kit for your HP-25

After many years without possibility of repair, there is finally a replacement available for the famous PMOS ACT (Arithmetic and Control Timing) chip for all HP woodstock calculators.

Many ACT chips of the “Woodstock” series HP calculators were damaged while using its power supply without batteries, due to a design flaw of the original power switching unit.

In most cases of a non functioning woodstock calculator only the ACT Chip is defective, but power supply, ROM, RAM and display drivers are still  working. The new ‘chip’ takes over all signals on the board, communicates with the components and establishes a running display and keyboard matrix while emulating the arithmetic logic unit.

The new ACT supports HP-21 HP-22 HP-25 HP-25C HP-27 HP29C and HP-67* calculators!

It executes either the original ROM code from the on board ROM chips or internal ROM code. The RAM chips are no longer needed after the replacement, because the new ACT uses internal RAM

There will be no visible difference in the behaviour to the original, except for additional features.

*HP-67E ACT doesn’t support card reader, instead it has 22*224 program steps and 186 registers in Continuous Memory

HP-25E Extended ACT Version

1.) Automatic Power Save

There will be an automatic Power Save Mode when there is no keyboard activity for a while. The display will show only a single moving decimal point, even while a program is running. The next key will show the normal display again. This Power Save Option saves about 50 mA, a third of the total power consumption, and makes your battery less often empty.

2.) Show Logo and your serial number

Show the calculator Logo and the serial number of your calculator at power up.

3.) Continuous Memory

If you run your HP-25 calculator with the “new ACT”, you will automatically  own an HP-25C with “Continuous Memory”! Your program steps and registers are saved in Flash Memory. Even replacing the batteries will keep your data in “Continuous Memory” with modern technology.

4.) Internal and External ROM

You can decide to run the firmware from the original external ROM chips or from internal ROM which is faster and can be used when your external ROM is defective.

All new features can be individually switched on and off by keys at power up and will be saved in “Continuous Memory”.

5.) Stopwatch

There will be an 8-digit Stopwatch including a Chess Clock and two independent timers. It displays up to 99 hours with 1/100 of seconds resolution. Lap times can be stored and recalled and there is even a calibration procedure.

6.) Program collection

There will be more than 5000 program steps available in “Continuous Memory”, containing the complete HP-25 application collection, predefined constants and many free space for your own program library.

7.) Undo

You can “smart insert” and “smart delete” program steps and you have an Undo button.

8.) Auto Repeat

Some buttons have Autorepeat function and you will see many more improvements, which make your vintage HP-25 calculator a very special one.

9.) Multitasking and programs of up to 5000 steps size

There is no  49 step barrier any more. Write programs as big as you want.  Since Version 1.12 you can run your programs in the background and run other functions simultaneously.

Download the Quick Guide for having an idea of the new features.

Download the detailed English User Manual to have a complete reference.

Price 80,- Euro + shipping (order by mailing to


HP-25A Advanced ACT Version  --- out of stock

This new low cost ACT repair kit is available since march 2019. It restores your HP-21, HP-22, HP-25, HP-25C, HP-27 calculator like the HP-25E to its original functionality, but has no additional key sequences and only a few functions, which can be enabled/disabled. If all functions are disabled you can hardly distinguish the calculator from the original.

Especially for the non programmable models HP-21,HP-22 and HP-27, which just need a repair but no extras, this low cost alternative could be a good choice.

This ACT board version has assembled a smaller PIC16F1516 processor which has only half the memory compared to the PIC16F1518 of the HP-25E Extended version. These boards were assembled unintentionally with the wrong processor and will be offered as long as stock is available.

The ACT HP-25A repair kit will be shipped without Quick Guide.

Download the User Manual to have a complete reference.

Price 50,- Euro + shipping (order by mailing to out of stock


HP-25E Ir, HP-29E Ir, HP-34E Ir, HP-67E Ir ACT version

This Infrared printing version of the ACT contains all “Extended ACT” features and allows you to print to the HP82240 Infrared printer.

1.) Printing your program collection

 You can print your programs with their mnemonics and program code.

2.) Tracing manual calculations

You can activate tracing your manual keyboard entries and calculations, which shows you the operators and their results as you are typing.

The Ir ACT contains an already mounted amplifier transistor to control an IR diode and the manual includes instructions how to fix the IR diode inside your HP-calculator.

The trace calculation print functionality is available for all models from HP-21 to HP-29C, but especially the programmable models HP-25 HP-29C can benefit from printing also its programs.




Price 119,- Euro + shipping

or the same as above and including Update Kit and three overlays

Price 139,- Euro + shipping (order by mailing to

Update Kit and Overlays

The update Kit allows you to change the firmware of the ACT. Not only programming a new version but also the emulation of another calculator is possible. Imagine, you can convert your HP-21/HP-25 to one of the high end models HP-29E, HP-34E or even HP-67E.

It consists of an USB/TTL RS232 Converter and programming cable, which fits directly into the ACT circuit.

You will get also a set of three vinyl overlays, which are available for HP-25C, HP-29E, HP-34E, HP-67E and HP-67. The labels will fit perfectly to any woodstock calculator. the self adhesive vinyl is easy to apply and the fresh colors make your calculator look like new. The HP-67 overlay has a different size and will  refurbish your real HP-67 calculator.


Price 30,- Euro + shipping (order by mailing to

The overlays can also be ordered seperately. Each vinyl overlay is 8,- Euro, the set of three is the same price 24,- Euro. 2,- € shipping


Front Labels for Classic calculators 35 45 55 65 as shown above are out of stock!

LED Module repair kit

This is a new replacement for the 7-segment LED display modules of any of the woodstock calculators from HP-21 to HP-29C. You can replace it without soldering, just by sticking it to the gold plated prongs of the display connector.

If a single or two LED segments of your original LED module are missing, this is normally beyond repair, because the very tiny bond wires have lost contact or the LED segments itself are corroded.

Now you can use this new LED module. Its digits are slightly smaller than the original and the impression of the digits gives it a different look, which is a beautiful new experience and you will have a complete new display again.

        Original HP-25 Display                                   new HP-25 Display

The display module uses three 4-digit bubble display components. These are not manufactured any more today. But we found a last quantity of them to supply you with this new repair possibility for woodstock calculators.

All LED modules are fully assembled, tested and the bubble displays are adjusted and preselected for having the same brightness.

Unfortunately the bubble display modules has become very expensive, since they are not longer sold regularly, this explains the new higher price.

Price 60,- Euro + shipping (order by mailing to

out of stock!