TI-66 double speed mod

[Nigel (UK)]

The TI-66 calculator is a nice machine to look at (see this thread for pictures and a discussion) but it is just so slow! After entering each program instruction there's a big pause while the calculator sorts itself out; even the simple counting program below only counts up to 46 in 10 seconds.

I couldn't find anything online about how to make it go faster so I decided to try myself. There is only one plausible component to play with - a 180k resistor near one corner that connects two processor pins.

• Shorting this resistor leads to a non-functioning calculator.
• Adding a 22k resistor in parallel with it gives a non-blank display, but nothing works.
• Adding a 68k resistor in parallel (see picture) leads to a working calculator with a speed roughly two times faster than without the resistor. Yessss!
  

I decided to quit while I was ahead. Maybe 56k or even 47k might work, but it seems likely that any further speed-up would be small.



Running the program "+ 1 RST" for ten seconds gives 46 on the unmodified calculator compared with 89 on the speedy version.

The pauses when entering program code are noticeably less on the faster calculator, making it more pleasant to use.

Current consumption for both calculators is zero when off and \(25\,\mu\rm A\) when waiting for key input. When a program is running, current consumption rises to \(75\,\mu\rm A\) on the unmodified calculator and \(111\,\mu\rm A\) on the faster machine. This is less of a rise than I expected and the current draw remains tiny. I don't expect any noticeable effect on battery life.

Of course, if you have the printer that goes with this calculator I assume that the interface will no longer work. For most TI-66 owners this won't be a problem!

Enjoy.

Nigel (UK)