|HP-15C speed up|
Message #1 Posted by Gerson W. Barbosa on 7 Sept 2006, 6:20 p.m.
As I opened up a 15C to fix a bad key I took the time to speed it up. Instead of connecting an inductor in parallel to the LC circuit I replaced the small 180 pF capacitor with a 27 pF one:
This is the original capacitor:
It is normally soldered underneath the board. As I had detached the board I decided to solder the capacitor on the other side for a later replacement. Of course it is not necessary to sever the board just to replace the capacitor: just cut out the traces in the indicated yellow lines to isolate the original capacitor and solder the new capacitor on the lower or upper pads:
The speedup factor obtained with a 27 pF capacitor is about 2.15x. Previously I had obtained 1.7x with 50 pF (two SMD 100 pF capacitors connected in series):
There seems to be a 12 pF capacitance between the traces that should be taken into account when calculating the capacitors for our desired speedup factors:
C (pF) speedup
While writing this (table calculated with help of a small program on this very modified 15C), I realize 33 pF gives the exact 2x factor I was looking for. Back to the soldering iron, after replacing the 27 pF capacitor with one of ten 33 pF capacitor in the package I bought yesterday (the only one which read 34 pF on the meter - all others 37 and 38), I have a perfect double speed 15C: the selt test on this unit (1212B) runs in 13.02 seconds (mean of 13.06 and 12.97 s) while on my first 15C (2343B) it takes 26 seconds to run (mean of 26.06 and 25.94 s).
By the way, this is the same 15C that was on sale at eBay some days ago. I shipped the buyer a mint one after his agreement because the '+' key became sticky and would not get back to its normal position. Looks like its ok now. I am still looking for a pristine 15C. I will keep this one though.
- I am not responsible for anything that happens to your vintage Voyager if you decide to follow these instructions. Do this only if you're used to this;
- Take care not to lose the two springs when opening up the calculator: they ground the bezel, the faceplace and the back plate together.
In case the pictures don't show up, try later.
For more information take a look at these links:
Earlier thread about an 11C speedup (started by Karl Schneider)
http://www.brouhaha.com/~eric/hpcalc/voyager/speedup.html (by Ken Sumrall, and Eric Smith's notes)
Edited: 7 Sept 2006, 6:34 p.m.