As an owner of a 1977 cb750a, I would suggest you pop the left side cover off and have a look at the fuse box and fuses. The old glass type fuse clips can corrode and cause resistance and get the fusebox hot enough to melt around the clips and melt the solder out of the ends of the fuse(s). If the clips are dull in color, clean them up with some scotch-brite or emery-cloth. You can do a search for alot of good info just use the word "FUSE" etc....