The basic answer is Ohm's Law. This is the red wire from my inspection of your images. That carries the rectifier DC output to the battery and the bike's load. A faulty connector starts to gain resistance: Ohm tells us that P=I²*R - power is current squared times resistance. A connection with zero Ohms makes zero power: this is not possible, of course, as any wire or connection has a resistance.
The spade connection, in perfect condition, has a very low resistance. As it ages that resistance rises, and current through it produces more heat. The heat accelerates corrosion, increasing the resistance, increasing the heat produced. At some temperature the metal of the female connector loses its springiness, contact pressure goes down, resistance goes up, heat increases.
A 1/4" spade connector can safely handle 20 Amps or more when in good condition.
The CB750 alternator, at full output and at riding RPM, makes around 220W or close to 20 Amps.
Apparently your connections are not in good condition.
It is not possible to get more than about 20A out of the alternator. Using a 750A field coil will get a bit more but do you have one?
So the basic fix is to fix the connector. The spade terminals can be removed from the shell and replaced. Careful cleaning of both ends, and adding a bit of tension to the female end helps it make a good connection, but if that has overheated then it has to be replaced. Possibly the wire connection to one of the terminals is the problem.
Or you can solder the wires together with good insulation (this wire is an unfused connection to battery +).
Possibly - unlikely - a rectifier diode has failed short and a very high current is flowing from the battery back to the rectifier. That would cause rapid battery discharging and other serious electrical problems.
