You disconnect the power. Look carfefully for the screws that hold the box together (removing the wrong ones can be a problem). They are usually around the edge of the box. Once you have the lid off you will see the tape deck. It will have a great big cicular thing on it. That is the picture head. Leave it alone unless you really know what you are doing. You will see how the tape moves through a serious of post and rollers. The thick vertical thinhg that doesn't look like a post or roller ids the sounfd head. Use a cotton bud with preferably de-ionised water to clean the brown crud off it. They dry thoroughly with a clean bud.
Having said all that. If you have some tapes that are Hi-Fi the sound head for them is on that big circular thing. You could try outputting your sound for them through the two Phono (RCA) jacks.
Terry
June 2006