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?