TI-66 double speed mod

[Nigel (UK)]

(07-14-2022 07:08 AM)EdS2 Wrote:  That's a very interesting and quite unexpected result.

Any electronics circuit people here who could hazard a guess as to what's going on? How could this have a step function like that?

It surprised me too - as I reduced the parallel resistance and saw no change I was certain that there must be a loose connection! The three observed speeds (slowest with no added resistor, then the two levels shown in the graph) are roughly in the ratio 1:2:3, which suggests harmonics, although there doesn't appear to be a crystal or a ceramic resonator on the circuit board. In any case, I wouldn't expect changing a resistor to excite different harmonics. Ideas?

Nigel (UK)