Total Repair

[PANAMATIK]

No, this is not the story of a secret agent on mars, saving the world.

But it is the Total Repair story of my last non functioning HP-21 in my Woodstock collection.

This HP-21 calculator had a non functional switching power supply unit, measuring only 2 Volt instead of the usual 6,4 Volt. My repair skills for power supplies were not good enough at the time when I bought this calculator one year ago, and I decided to use it as a display driver and ACT spare part unit for three other Woodstock calculators, which could be repaired. At last all chips were removed, leaving a bare board as result, waiting for what to come.



HP-21 bare board. right: the display drivers occupying the space below the LED display, left: the ACT also removed.

Now after having made a replacement not only for the ACT but also for both display drivers, I thought I could replace all three chips and bring this calculator back to life again. The idea was to replace the ACT by the new ACT and anode driver and cathode driver by a modified ACT hardware, and let communicate them like the original parts.

Before I could be successful the power supply unit had to be repaired. Normally the two transistors had to be replaced by new ones. When I started to replace the transistors I recognized, that they were not original! Not the usual 4-071 and 4-055, but a normal todays BC337 NPN type. Then I recognized, that they were put in the wrong way, emitter and collector were exchanged. Somebody has tried to repair this unit and did it not well. Not knowing the condition of these transistors, I replaced them by BC338, which I had at hand. But still it didn't work. Then I removed the Zener diode by a new Zener 7,2 Volt. This made the power supply work again. And finally after having connected many wires to connect the three replacement boards I could repair this calculator.



Top and bottom wired with three replacement boards. Most of the wires are the 12 digits and 8 segments of the display, then the communication lines ISA, SYNC, RCD STR and PHI1 clock.

I used an HP-25 keyboard and programmed the HP-25E firmware, which upgraded the HP-21 to an HP-25, but I can go back to HP-21 or forward to an HP-29C as well.

Then I realized, that there were no more PMOS chips inside the calculator - all were replaced by todays semiconductors. And I realized, that the -12V and 6.4Volt were not longer necessary at all. The circuits could run with the single battery power. And further more, I could replace the replacements by my HP-01 board, which was already designed to drive a 9-digit display, and had three spare I/O pins for driving a 12-digit display. After some moments I started to completely remove all wires and solder new wires from a HP-01 board to all segments and digits. And after another half day of soldering I got this result.



Now this is the "Total Repair" of an HP-21, inside an HP-01 board, running the HP-25 firmware. And a surprising new feature!! As I could disconnect the internal switching power supply completely, the calculator drives only 20 mA, when display is on, compared to 180 mA original, and only Microamps when display is off.

Now I have to finish the software for my first Low Power HP-25E. Equipped with two AA batteries it will last for 100 hours and for years when using sleep mode and the processor register state is always saved. Still the software is not finished, but there is no doubt, it is the first Low Power LED calculator of the Woodstock series. Perhaps not the last.

Bernhard