yes.maibe something else is wrong.i will change the optos..check the resistences and change the igbt driver..maibe check after that...4 boards like this with the same problem.I have found all the variations of the MC2100, they are basically identical in general design, just some component and circuits are different, but they all obtain the same result.
Are you sure the 20Hz is set, you can even vary this to see if there are any changes, It would seem obvious the boards you show seem to use exactly the same method of control, i.e. 20Hz PWM.