The HP Series 10 calculators use two versions of the CPU. The first one was 1LF5, and the second one was 1LM2. The 1LM2 version can run faster than the 1LF5 version. Below are two tables of speed up values, one for the 1LM2, and one for the 1LF5. The 1LM2 table is more complete, because that is the one I own, and its the first one I tried.
To find out which version of the CPU you have, open up the calculator (there are 4 screws under the rubber feet) and look at the chip in the upper right corner as you look at the back of the calculator. If your calculator has black plastic covering the circuit board, and the LCD display driver, you probably have the 1LF5 CPU. Otherwise, the CPU chip will either have 1LF5 or 1LM2 printed somewhere on the chip.
When you open up the calculator, be careful not to lose the two springs that connect the back cover plate to ground.
In its original configuration, the oscillator is a parallel LC circuit, with a capacitor value of 180pF, and an inductor value of 180µH. According to the laws of LC circuits, the resonant frequency is:
f = 1 / (2*π*sqrt(L*C))
In the normal case, this means that:
f = 1 / (2*π*sqrt(180pF*180µH))
f = 1 / (6.2832*sqrt(32.4E15))
f = 1 / (6.2832*180E9)
f = 1 / 1.131E6
f = 884.19 KHz
There are 3 downsides to the speedup:
The speedup involves attaching an inductor in parallel with the LC circuit located just below the batteries. Here is a table of inductances I used and the speedup I achieved with it on the 1LM2 CPU:
inductor value  180 µH  100 µH  82 µH  47 µH  33 µH  28 µH  22 µH  15 µH  10 µH  8.2 µH 

speedup factor  1.45x  1.75x  2.0x  2.43x  2.75x  3.05x  3.3x  4.0x  4.8x  5.3x 
I tried using 6.8µH, but the calculator didn't respond to key presses correctly. However, it did turn on, with some big number in the display.
I would recommend using a 28µH inductor on the 1LM2 version of the CPU. This gives adequate performance, and seems to be well within the limits of the device. I have run the repeating self test for 1 hour, and the calculator reported no errors.
Here is a table of inductances I used and the speedup I achieved with it on the 1LF5 CPU:
inductor value  82 µH  47 µH  33 µH  12 µH 

speedup factor  1.8x  2.2x  2.5x  4.0x 
I tried a 8.2µH inductor, but the calculator wouldn't turn on. Also, after I removed the 8.2µH inductor, I still had trouble turning the calculator on. Shorting the battery terminals momentarily, (as suggested in the 15C owners manual) fixed the problem, and the calculator turned on (but continuous memory was cleared).
For the 1LF5 CPU, I would use a 47µH inductor, for the same reasons that I picked the 28µH inductor for the 1LM2 CPU.
I seem to remember I determined the speedup by using a stopwatch to time how long it took to run the selftest sequence. I think I ran it three times and averaged the result. Hence, the accuracy of the speedup factors is not absolute. I did not own a frequency counter at the time, and I was working in a software lab at HP at the time, so didn't know where to borrow one.
In later HP12C prodution, the CPU and RAM/ROM/Display chip were integrated into a single chip, the 1LQ9. I do not know whether any other Voyager models were ever made using the 1LQ9. It is probably possible to speed up a Voyager using the 1LQ9 by the same technique, but neither Ken nor I have tried it.
Even more recently, the design of the HP12C changed to use a single coin cell rather than the traditional three button cells. Since the operating voltage is reduced, they may have had to redesign the electronics yet again. I have not opened one of these models.
The newer HP 12C Platinum uses a Sunplus SPLB20D microcontroller containing a 65C02 equivalent CPU core. Some information was posted to a thread on the Museum of HP Calculators forum.
Last updated January 27, 2004 Copyright 1990, 2004 Ken Sumrall 
