Many people all around the world love the old HP LED calculators. There’s helpful discussion forums at the HP Museum. The folks on the forums are knowledgeable and friendly. It was on these forums that I learned of Panamatik in Germany. This company has produced 2 amazing products:
The first is an ACT (arithmetic, control, timing) chip for use in the HP Woodstock series (HP 21, 22, 25, 25C, 27, 29C) calculators. ACT chip failure is a common problem with the Woodstock series, and the Panamatik ACT chip has brought HP25s owned by me and a few of my friends back to life. This chip can also be used to upgrade Woodstock calculators, and provide many new functions including printing to a compatible HP printer.
The second product from Panamatik is even more amazing. The Woodstock LP board replaces all the original internal parts in the Woodstock series, and replaces them with a modern processor, and memory. A whole new world is contained in the LP circuit. BTW, LP means ‘low power’ It draws very low power when operating. The circuit never turns off, and draws microamps in the sleep mode. Battery life is outstanding! You can communicate with the LP circuit through an external pin jack, and save and load programs.
There’s also an LP circuit for the Spice series (HP 31, 32, 34C, 37, 38). I have modified both an HP25 and an HP34 to use the LP circuit. These calculators are even more amazing that the ones with the modified ACT chip. I encourage a visit to the Panamatik site.