Right, many thanks to both Pete and John for the insights. The good news is that the fan motor and the MDK-039 PCB it's directly attached to are both functional. The fault, whatever the cause, is that there's no power from the connection at the white board near the motherboard. I tried Pete's test powering the lead unit PCB from the transformer and that runs the fans.
Power output at both the lead and trailing units' PCBs measures slightly more than 4 VDC and the fans don't spin at breakneck speed. Toggling the smoke unit on/off keys (and a soft reset) has no effect on either unit although it makes sense to me as John suggested that the smoke outputs would have been repurposed for the fan motors. I have not tracked down K-Line diesel reset codes so I have not tried doing that. P.S. Even with a Legacy command base connected, the fan speed varies with transformer lever movement so I guess that it might not be command controlled at all.
So, as the PCBs appear to be OK, and unless this might cause another issue, I'll wire the lead unit PCB to track power. I really don't know how the PCB is feeding DC to the fan motor as there's no rectifier on that board (but there are several diodes on it, which I'm guessing have the same purpose).
Just as a matter of interest, opening up the trailing unit shows a set of electronics that I don't think I seen before (but I am no expert), especially the big blue caps. There is a tether between the units so I guess that this is a slave board arrangement of some kind:
Many thanks again.
