Getting back to this one (after spending time on other projects, radio and real-world alike):
If I let the rig in question thoroughly warm up, it won't tune at all in either direction with the VFO - though other tuning methods are fine. Commonality in theory looks to be the voltage fed to the optocouplers or a questionable solder joint on that board itself. Time to dig into the Technical Supplement and see what's what.
73 - Fred, N8YX
