I have acquired a Sharp sm-25sb60 TV (circa 1990) without remote or
paperwork. I do have a universal remote that will operate basic
functions but a few of the channels have been locked out of the channel
up/down and I can't find a way to add them back in. I can access those
channels by number but not with channel up or down. Controls on the set
are minimal, vol up/down, channel up/down, antenna/cable switch and a
power button. I tried leaving the set unplugged all night to see if it
would reset but didn't so the settings must be in non-volatile memory.

Without trying to find a original remote how might I add channels back
in?