more info needed.
when you say no uhf, do you mean loopthrough has failed, that you can no longer plug aerial into vcr, then vcr into tv? if that still working, then rf amp ok, so probably modulator is as well.
Sure it's not just your tv lost the channel? try playing tape and scanning all uhf channels on tv.
is ouput channel a screw on back (v old!)? Or set by software, if so has it changed, or software glitched? try unplugging vcr from mains, waiting 1 min, then resetting everything.
if modulator gone then, yes you can probably get spare one, soldering needed, so maybe new vcr better, they're very cheap.
hope helpful.