Arduino Pro Mini PWM issue
Thanks to comments I received from several people here I quadruple checked the wiring and eventualy found the problem. My recent Arduino Uno tests were done on a breadboard, but I had the Pro Mini prototype soldered on the prototyping board which has the holes connected in columns on the copper side. I made the cuts in all the necessary places ... but one! That one omission caused a short between pin 6 and the drain from the other channel's mosfet. The problem with these boards is that the parts are on one side and while flipping the board to the copper side, it is too easy to get lost in all the columns and miss something.
The morales of this story are: &quot;Never be too certain about your wiring/soldering&quot; and &quot;Arduino is pretty resistent to my attempts to fry it :)&quot;.