What's involved in the rebuild of a manual tuner? A spray of tuner cleaner
or something like that was all I ever did the few times I came across one.

Many decades ago (in my high-school days) I did a bit of TV-tuner
servicing. The VHF-channel tuners used a turret design - rotating the
knob into position rotated a channel-specific resonator on the turret
into the contact position. Each resonator had a fine-tuning reactance
(either a movable-slug inductor or a variable capacitor, I don't
recall which), whose adjusting shaft ended in a plastic gear that
meshed with the "fine tune" control (typically a ring running around
the outside of the channel knob).

Over time, the plastic gears on the turret, and the plastic gear or
planetary ring on the fine-tune control would wear, and they wouldn't
mesh properly. As I recall, the failure would usually occur when the
fine-tuning adjustment was rotated to one end of its range - the
control teeth would no longer mesh with the worn ends of the teeth on
the turret gear, and the fine-tune control would be "stuck" for that
channel (typically at a position where the channel wasn't watchable).

I could clean the turret contacts well enough, and manually return the
worn-gear "stuck" channel adjustments back to a point where they'd
mesh with the control gearing and work again, but the problem would
recur if somebody rotated the fine-tuning knob too far once again.

A real rebuild would have required replacing all of the worn gears. I
didn't have the ability to do that, at the time.