Hitachi multisystem TV

[darkdog]

Hi all,
I went through 360 post about Hitachi TV but I couldn’t find the solution so I decided to write my TV problem.
I have a CRT TV Hitachi model#C21-RM37S a full multisystem (that what it written on TV) but when I switch to NTSC (American) signal I can have a color but no sound so I went to check the sound (5.5, 6.0, 6.5) but there is a missing code for NTSC sound code (M)
Everything else is working through the RF input except the sound.
If I switch to AV input there is a color and sound too. I was wonder do I have to replace the RF tuner of TV or there is a solution to add the code (M) through the service meny.

Appreciate for help.