It says "broken Datassette". It doesn't say "broken C64" so I think the answer with schematics for the C64 was wrong. Are you sure that the Datassette is broken? If you write a simple program like:
10 PRINT "HI!"
and save it with:
and then load it with:
If the above works ok but you can't load programs that were saved earlier and which used to load without problems, then the Datassette is probably misaligned, which is pretty easy to fix if you have a very small screwdriver and an alignment program running on the C64. The tricky part is to load an alignment program to the C64 when the Datassette is mis-aligned. For that, you probably need a diskdrive, a correctly aligned Datassette or 64HDD.