This plug had a cable that had been pluged out by force many times, so eventually it had all seven inner cables making a shortcut with each other and causing the handpiece not to work at all. So with some drops of super glue and a piece of sugru I managed to isolate the inner cables and stabilize the plug. This should probably have saved the dentist more than 123 euros with VAT.
Take a look at how simple it is.
Step 1: Check If All Pins Are Working Properly
To do this just use a multimeter in buzzer mode and check the conductivity for each pin.
In my case all worked just fine...lucky me!