If the internal fuse has blown then a fault - broken component - will have been the cause. A transistor perhaps - maybe a diode or capacitor - unlikely to be a resistor but you never know. Maybe even a transformer.
With the meter handy you will have to take pot luck testing components one at a time as you go.
You may find videos on youTube that show you the basics - components soldered in-circuit will nearly always give different test meter-results than out of circuit - you may have to unsolder any suspect parts to be sure.
A new charger will cost about £45 - was your friend being generous to you when he gave you the charger or was it a disposal exercise for him?
Good luck..., June 2015
