TI-66 double speed mod

[Gerson W. Barbosa]

It seems the clock speed depends on the voltage level of the batteries as well.
I tried 524287 on the primality test program here. The author says it takes less than eight minutes on his TI-66, so I was expecting less than four minutes on mine (2.09x). I was disappointed it took 4m9.5s. However, after installing fresh batteries it dropped down to 3m51.6s. I haven’t checked the voltage level of the new Maxell LR44 batteries, but the old ones read 2.88V (1.42V + 1.46V).