HP-15C Speed-up

[Alex Liggett]

There are a number of threads from the old forum regarding modifications to speed up old Voyagersaurus calculators, such as Gerson's here, Karl Schneider's here and Lyuka's here (with a link to his site here). Unfortunately however most of these had no pictures or the links to them were dead (although Gerson was kind enough to email me his pictures, which were very helpful). In case it is still of interest in the age of the C-LE and the DM-15L I thought I'd document my recent 15C fiddling.

For the uninitiated, the idea is to modify the LC circuit to increase its resonant frequency and hence increase the clock speed. This comes at the cost of reduced battery life and operating temperature range, although this is all relative given the very generous parameters of the original.

The subject calculator is SN 2423A46491, which I picked up while on holiday in the US last year. This unit has the single PCB, but with all of the components of the LC circuit mounted on the top side of the board:



As an aside this contrasts with most of the pictures I have seen of other (later) units which have them on the bottom or back of the board, like the ones shown in Lyuka's article:



A further variation appears to turn up still later in which the components disappear to the other side of the board again, but with a very different track layout, as in this image of Geoff Quickfall's from elsewhere on the forum showing a 1989 unit:



I wonder what the rationale behind such substantial hardware changes was - obviously one coincided with the elimination of the second chip, but the other..?

One method of altering the LC circuit is to replace the capacitor as per Gerson's project. For still greater rewards, one could undertake the more drastic modifications shown in Lyuka's thread. Because of the layout, any component changes would have involved undoing the heat stakes to get to the front of the PCB. Therefore I decided to go with the simplest option, which is to add an inductor in parallel with the circuit, as per Karl's thread.
I added a 33μH inductor on to the reverse of the existing inductor like so:



My first attempt at soldering onto a PCB (after a few practice runs on a less precious subject), so pardon the slightly untidy work:



The subject unit also had a dodgy 2 key, which was sorted using the method discussed in this thread (IPA down the hole in the PCB, then clear out using a piece of soft wire.

The modified calculator yields a Savage Benchmark result of approximately 2233 seconds, compared to the ~5840 seconds of the unmodified machine - 2.6 times faster. I am very pleased with this machine, which will now be my main office number cruncher.

Thanks to all of the contributors whose threads are linked above, especially Gerson for taking the trouble to email through his photos.