HP-15C speed up


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

180 1.00

100 1.27
82 1.38
47 1.75
39 1.88
33 2.00
27 2.15
22 2.30

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.


have you measured the current drain before and after the changes you have done?


Olá, Artur!

I haven't measured the current drain before the modification. I have just made a couple of measurements on my two units though:

                       ON,  displaying         running  ON/x
0.000000000 self test

2343B (normal speed): 15.5 uA 0.85 mA

2712B (double speed): 27.0 uA 1.50 mA

The calculators have different chip sets but I think this can give you an idea of the overall power consumption increase (about 75%). To be more rigorous I should have used the same battery set but I have assumed they are both good.




Hi, Gerson --

Great post, and thank you for the acknowledgement and link.

After any edits and enhancements, you should place the post in the Articles Forum.

I should get busy with making photos of my work on the HP-11C.


-- KS


Hello Karl,

Thanks for your remarks!

After any edits and enhancements, you should place the post in the Articles Forum.

I would like to invite YOU to do this, since you're a better writer than I am and have a technical expertise on the subject. I don't feel much comfortable writing a technical article as I am not sure of the correct terminology in English. For instance, I don't know which of the following is more widely used: trail, track or trace. You could compile the information in Ken's article, in your earlier posts and other posts about this matter (with the due acknowledgements). You could include both your pictures and mine.

The article should present both speed-up methods - by connecting an inductor in parallel or by replacing the capacitor. The first is easier since you don't have to cut any trail, thus reversible with no change in the cosmetic look of the board. On the other hand, a low-resistance inductor could cause the power consumption to increase. I tried both methods and the latter worked better for me (I think the inductor I could find here was not so suitable, since the flashing low-battery star would show up in two or three days after battery replacement, even for a 2.5x speed-up). The latter method will not present this problem as the overall resistance of the LC circuit will not be changed. Measurements showing the increase in current drain should also be shown.

Before writing the article, a couple of things have to be checked. I forgot to take a picture from the other side of the board, but as I can remember there are no tracks underneath the board connecting the capacitor to anything else. Also, I may have overestimated the capacitance between the tracks: they seem too far apart for 12pF. Perhaps the newer processors have a built-in capacitor, since the earlier Voyagers don't present this additional capacitance.

Best regards,



Hi, Gerson --

You posted,

I would like to invite YOU to do this, since you're a better writer than I am and have a technical expertise on the subject. I don't feel much comfortable writing a technical article as I am not sure of the correct terminology in English.

I appreciate the compliment, but any advantage in writing skill is due primarily to being a native speaker of English. I suspect that your experience and knowledge of practical electronics exceeds mine.

I also believe that what you have posted is already sufficient for a MoHPC article. I still need to make photos of my earlier work for my contributions. Perhaps we can collaborate on a consolidated article at some later time.

Best regards,

-- KS

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