Well, at the moment, that pot is doing precisely nothing and there's no trace visible as per the photo I posted. There *is* a 100Mhz signal going into RF input, though.
Fortunately I have a spare HP SA (can't recall the model number but it goes 9kHz to 12Ghz.) that did come in handy for the YTO issue even if its specs aren't close to those of the 8566B. Looks like it might come in handy again - at some future point when I have the time to deal in detail with this unforeseen hitch!
Jinxie