HP-15C Speed-up

[matthiaspaul]

(01-20-2016 09:26 PM)matthiaspaul Wrote:  By looking at the resonator's equivalent circuit diagram I figured it should be possible to make it resonate at a much higher speed by adding an inductor in parallel (and in series with the miniature switch - something that wouldn't work for much higher speeds, but still worked fine at those low frequencies). After experimenting with a few inductors from my spare parts collection, even the baud rates matched nicely at a factor of 2, but I found it more reliable to switch back to the default speed when doing longer serial communications.
(I don't think I had to add a capacitor as well, but I'm not completely sure about that any more after all these decades - unfortunately, I can't find my old notes right now.)

I meanwhile found several ring binders with many of my original notes on the Sharp PC1403 internals, including listings and circuit diagrams (and a local newspaper cut out of a long forgotten article reporting about that wind energy data logging project ;-). Unfortunately, the c't article drafts are not in there, but they might be stored elsewhere. Anyway, it's a lot of stuff and it would need way more time to prepare this for upload than I have right now, so this will have to wait for somewhen in the future - sorry.

According to my old notes, the inductor for the "turbo mode" would ideally have had a value of 60..65uH, a not very common value. Instead, I used a 56uH inductor and added a small ceramic trimmer cap 5..30pF in parallel to tune the frequency. While the calculator worked reliable at 1.536 MHz, it started to behave erratic when further increasing the frequency above ca. 1.6 MHz. The power consumption was about 2.5 to 3 times higher in turbo mode, while in theory it should not have been much higher than factor 2. Most probably this is at least in part down to the unconventional resonator tweaking method...

Hope it helps,

Matthias