I have a similar problem with an old Mitsubishi CS-210--my housemate accidentally set a lock code without knowing how and locked out channel 7. The "Mute+8" definitely did NOT work for me (not sure why it would have, and sounded wrong, but I was willing to try anything and thought it couldn't hurt).
I gave up on the TV and did an end-run--I access the locked-out channels through my VCR. (I put the cable in my VCR's input, my VCR's output in my TV's input) and do all the channel changing through the VCR remote. I'm not sure if your tv can even read the line in (since I don't know if you mean "unlock [channels]" or something more serious) but you might try something similar.
FMR, January 2005
