Any ideas on the tuner dial for the 780? How do I unstick it?
1. Remove the cover and have a look inside.
Does anything look wrong or out of place?
Is the string intact and in place? Compare it to the the drawing in the service manual.
Do you see dirt or corrosion in or around the dial knob shaft, the pullies over which the string passes, or the tuner capacitor?
A careful spritz of non-residue, plastic-compatible contact cleaner may help to remove any dirt, etc. from between parts.
Remove the tuning knob (loosen metric set screw holding it to shaft) to inspect behind it.
If visual inspection reveals no obvious problems, then judicious application of a small amount of the appropriate lubricant to points of rotating contact may help.
The pullies are likely plastic.
You will want to use a lubricant that will not hurt them.
Wiser souls than I will chime in with recommendations of the proper oil/lube.
I would go slow, be careful, and use a paper towel, etc. to avoid getting lube where it does not belong.
Search for the thread that documents how to clean and lube the variable capacitor if you suspect corrosion there.