Hi,
I'm sure it is the POT. As it does work, just not all the way through the 'turn'.
Fully anticlockwise, the inverter will show 40Hz, then you turn it and it stays at 40Hz until about 2/3 turning clockwise, then the frequency starts increasing, then when it is fully turned clockwise, the inverter will show 60Hz.
The frequency is correct, but I'd like the frequency to start to increase when you start turning the POT from anticlockwise before you get 2/3 through the turn. It's a 1 turn POT.
Here is the circuit diagram (basic)
**broken link removed**