HP-29C repaired

[PANAMATIK]

One, Two, Three

The HP-29C is the high end scientific calculator of the "Woodstock" series. Its value as a collectible can be significant and sometimes is more than 200$. For my collection of LED calculators I bought three HP-29C over the last two years, all of them sold "as is" or as defective. The positive aspect was the price, which was one for 31,- $ , and two for 75,- $ each.

As it turned out, the first HP-29C was working when I inserted new batteries. This was very lucky indeed and I was the proud owner of a nicely working HP-29C calculator.

But the remaining two HP-29Cs were not working. And their repair waited for some reason until today.

The second HP-29C did power up and showed 0.00 when switched on, but the left keyboard row did not respond at all. You just might think of a contact problem. But as it turned out, it was not a contact problem of the keyboard row, because I tested another keyboard and finally I measured the key signal at the corresponding ACT (the Woodstock arithmetik processor) pin. It was there. The only conclusion could be, that this signal was not handled properly inside the original ACT chip, despite it seemed to work flawlessly and could do calculations. But clearly a calculator needs all of its buttons, otherwise it would be worthless. It was an easy decision to replace the ACT by a new HP-29E ACT. And indeed the keyboard problem was solved and this HP-29C was completely repaired.

The third HP-29C has a different story. Its ACT was obviously defective and I replaced it soon after purchase. But still the display wasn't working properly. When switched on it showed a curious pattern of segments and didn't react to key entries. When I measured some signals, I found that the anode display driver, which normally generates a Strobe (STR) signal for the cathode driver, didn't generate a good STR signal. Obviously this chip was partially defective. Because I didn't have a replacement I had the idea to reprogram the new ACT and let it generate the STR signal, which normally was the task of the anode driver. It was easy to add the STR signal to the ACT firmware and connect a free pin to the cathode driver. And miraculously the HP-29C was working and I was very happy about this special repair. The calculator keyboard and everything else was in immaculate condition.

But my disappointment was even larger, when some days later I discovered, that the calculator didn't work any more. The only reason could be that the partially working anode driver now completely had ceased to work for some reason. I put it away and thought "easy come easy go". This happened one year ago.

Now I remembered this HP-29C calculator again, because recently I developed a replacement for the HP-25 anode display driver as described in another thread here http://www.hpmuseum.org/forum/thread-244...l#pid66339. And this could be working in an HP-29C as well. Today I unsoldered the driver chip and put the replacement inside. And success! I am really happy again.

Now all my HP-29Cs are repaired and they are lucky to be good friends together. From left to right:
The today repaired HP-29C.
The second one with replaced ACT has still the original display driver, therefore it is not capable to show 'HP-29C', because the letter 'P' cannot be displayed.
The original HP-29C.
The fourth HP-29C calculator at the right actually is an HP-21 hardware, which is converted to a "HP-29E GPS" by a keyboard vinyl overlay.



Three of the above calculators have 30*98 program steps, which can be combined together in a single contiguous program by using the unique dynamic program loading feature of the new ACT rev. 1.10.

Bernhard