I write the following hoping to help other new users with electronic DC motor speed control. When I bought my lathe and mill which are similar to the CX machines discussed here, the supplier was most insistent that when starting the machine the speed potentiometer (knob) should be turn all the way to the minimum setting. Then the machine is started and the speed increased to the setting desired. Likewise when stopping the machine, turn the speed to minimum and switch off. This was explained to avoid damage to the control board. I wonder if this might be an inrush current problem that could overload the Triacs and power diodes, thus releasing magic smoke.
Apart from the board failure that resulted in me changing to a genuine KB Electronics board, which I suspect was caused by water ingress from a leak in the roof from a storm, I have followed that advice and have trouble free operation.
I should mention that though I ordered the machines from the UK yet live in France, I did not really expect much support and after I found the Chinese control board faulty I preferred to find my own solution rather than have another of the same boards.
Paul