TI-66 double speed mod

[Nigel (UK)]

(07-07-2022 08:30 AM)John Garza (3665) Wrote:  Thanks Nigel,

I took your advice and made the upgrade.
Only issue for me was I had a physically larger (1/2W) 68K resistor, so I wired it in and nestled it in some heat-shrink tubing next to the batteries. Works great!

The machine is now pleasant to use. The delay used to be annoying even in manual calculations.

It makes a nice handy desk calculator for me. I grew up (high-school) with a TI-59, so the programming paradigm is familiar and comfortable. And I don't have to take out my 59 and bang it around in daily use.

-J

I'm pleased to hear that the modification works on another machine without problems!

I've been trying smaller resistors this morning to see how much further the speed can increase. Changing the 68 k resistor to 47k produced only a small further increase in speed. However, a parallel combination with a value of 31.4 k doubled the speed again; a further decrease to 30.3 k and the machine would no longer turn on.

Based on this, you can safely try resistors smaller than the 68k I initially used. However, unless you take the time to tune things carefully you risk either not going far enough to make much of a difference, or going too far and stopping the machine from working altogether. It's probable that the exact threshold varies from calculator to calculator; it might also vary with battery voltage. Personally I'm happy with the factor of two speed increase that the 68k resistor gives. It makes a real difference to the feel of the calculator while leaving an adequate safety margin.

One last comment - no matter what value you try for the parallel resistor, it doesn't seem to damage the calculator in any way. If it doesn't work with one value of resistance, remove the resistor and everything is all right once more!

Nigel (UK)