TI-66 double speed mod

[Nigel (UK)]

For those who are interested, the attached graph shows how the calculator speed varies with resistance.

I'm measuring the speed by counting the number of loops of "+ 1 RST" executed in 20 seconds. I'm doing this by hand, which might explain some of the scatter.

I've kept the 68k resistor that I've already installed in place, and added a 150k trimmer in parallel on a breadboard. The resistance in the graph is the parallel resistance of the trimmer, the 68k resistor, and the built-in 180k resistor. The rightmost graph point is the calculator speed with only the 68k resistor added.

(The number of loops in 20s on the original calculator, with R = 180k, is 94; I've omitted this from the graph to show what happens at smaller resistor values more clearly.)



Rather surprisingly (to me) the graph has a step-like appearance. Decreasing R has only a very slight effect on speed, until a sudden jump upwards takes place. I imagine that if I had removed the 68k resistor and looked at the speed for larger values of resistance I'd find another step.

The three orange crosses near the jump represent erratic functioning. Extra 1's were appended to the number in the display - 1111294 instead of 194, 1300 and 1305 instead of 300 and 305. This didn't always happen, but it does suggest that the calculator is unstable close to the jump.

Below the lowest resistance shown the calculator still turned on, but gave nonsense answers.

It would seem to be sensible to choose a resistor value that lies well away from the jump, or from the final failure point. There's no guarantee that the step up will be at the same point on all calculators, or that it will stay fixed as conditions change. In my initial experimentation I found that a 22k resistor in parallel with the 180k resistor wouldn't work; this time it did, and the failure occurs for resistances less than about 18k. This is quite a drift! (The weather is hot today.)

Nigel (UK)