How can I fix my Mitsubishi Black Diamond VCR model HS761V?

I have a six head Mitsubishi Black Diamond VCR model HS761V with NICAM stereo.

On inserting a video cassette, the cassette goes in half way and the system then powers off automatically. Occassionally the tape goes all the way in and then powers off. I can see no foreign objects jamming the works (such as kids toys).

Manually putting slight pressure on the drive band that draws in the tape input mechanism, causes the power down, thus I am guessing it is a safety cut-off and something is jamming the input mechanism.

This problem has occurred twice in the past and been fixed by an expert, but it costs too much each time so I want to fix it myself.

Any advise on what to look for to fix this would be appreciated.

Thanks