Hi All,
The tempo input is a switched jack, so inserting a cable disconnects the internal clock from the divider.
The input directly feeds the divider, so it must be a fast clock around +10V, so most eurorack 5V clock signals will not work.
The LEDs are connected to the 2 slowest divisions (/32 and /64), so if the clock connected is not very fast, it will take quite a few pulses before the LEDs light.