Well, the replacement stepper controller chips arrived early this morning. Didn’t take too long to replace them and connect everything up for a test.
The good news is that after a slight adjustment of VR2 the stepper appeared to be controlled by the rotary encoder. It made the right noises.
Try as I may, I wasn’t able to carry out the initial setup procedure as outlined by Loftur. I couldn’t find a peak in the noise.
After applying a portable SWR analyser directly to the loop when I thought I was at the lowest frequency point the loop would tune, I discovered there was a dip around 23 MHz! I was nearer the other extreme of what I think this loop’s range should be. So I’m thinking I may have the stepper motors wired wrongly with the result that the stepper goes in the opposite direction to what the controller thinks it should be.
After switching one pair of wires at the controller end, the stepper made a terrible noise as it hit the end of the capacitor’s travel. Checking it tentatively it didn’t seem to distinguish between clockwise and counterclockwise movement of the encoder. Whatever you did, the stepper turned in the same direction.
Time for further closer inspection of the wiring around the newly re-oriented common mode chokes and their associated bypass capacitors. This kind of behaviour may have something to do with a missing or failed connection in this part of the circuit.
Stepper only appears to turn CCW which turns the VVC CW. I’m now hoping the glitch is pretty obvious in this part of the board or associated wiring.