ACT available

[PANAMATIK]

A very special repair in the new year!

The story:

In my collection of woodstock calculators, there was residing one item, an HP-25, that I bought last year as defective, an attribute, which I like for nearly all of my aquisitions. But this one refused to be repaired by the new ACT and all other means. Its display driver chips, both the cathode driver and anode driver, were damaged. Also the internal power supply didn't work. It was a hopeless case and was laying around for nearly one year, waiting for what to come.

But meanwhile, thanks to another HP-25 repair, which I had promised to a member here, I had to learn the hard way how to repair the woodstock power supply, by replacing active components like transistors, diodes and condensators by their modern equivalents. And I got its power supply repaired. Accordingly I could repair the power supply of my calculator as well.

But how to repair the display driver chips? I needed a replacement. I unsoldered both chips from another calculator of mine, a defective HP-19C, which was beyond repair for other reasons, but its display drivers seemed to be still working. I gave it a try. Yes, I got a running display, but not completely running, unfortunately some LED segments always were lit dimly, only the cathode driver was ok, and this was not a successful repair. The precious anode driver spare part was not completely healthy. I realized, that I was short in original display drivers.

Left the display with dot and segment e always on, right the display driver chips on sockets.

With the idea in mind, that I never should give up and finally have to repair all of my calculators, I was thinking about a display driver replacement, based on the knowledge about the timing betweeen the chips I had. I modified an newACT circuit and reversed its input and outputs, replaced many components and wrote a completely new program, which was able to read the display digits serially from the bus and to set the LED 7-segment patterns with exact timing. I found, that the 16MHz PIC processor was way too slow for the task listening to all of the 200kHz signals. I couldn't use interrupts because they were not fast enough, only polling of the clock signal was a chance and I had to count each machine instruction to get a result. Every embedded programmer knows, what it means to relinquish on any interrupts.

After several days working with the oscilloscope, I finally succeeded. The prototype was working. And it was no problem to integrate the small circuit into the calculator. I repaired my poor calculator, which was waiting so long for getting alive again.

Here is the source code for this approach:

https://drive.google.com/open?id=0Bwx8KU...0prbHFXTlU

The following images show the prototype board attached to the pins of the anode driver chip and the running display. The brightness can be adjusted by the resistor values for each segment, I choose 470 Ohm to get a bright display for my calculator. This my HP-25 is fully functioning again.

On the right the final placement

Don't be disappointed, but this anode driver replacement concept will not become available for public, because it replaces only the anode driver and therefore is not good enough. But I'm planning a design, now I have more knowledge, which will replace both display driver chips and has even more advantages. It will be available in late spring, as I can hope, and you will be really surprised about its features.

Happy new year!
Bernhard