Discounting a short circuit, the electrical brushes need replacing or possibly you need new vacs. Often when the brushes get worn down to the state they blow themselves or they blow the fuses, they have usually damaged the commutator. If this has occurred, simply just replacing the brushes with new ones wont do, as they will spark and disintegrate within minutes. However replacing the brushes is probably the only way to find out if the vac is knacked.
I ran portables for 23 years and found this to be the case every time.
Dave.