Be sure to open up the top cover and then the electrical box and check the condition of the capacitors. I also have the original AC on my 78 however the old timey capacitors were finally giving up the ghost. Each capacitor body should be crisp and well formed like it was just made, not bulged and/or possibly leaking. You should have 3 capacitors in each unit:
Compressor Run Capacitor
Compressor Start Capacitor
While a new capacitor may be the same electrical value, they are physically much smaller than they were in the 70's due to improved manufacturing processes. Values depend on the Mfg/model of the AC but are written on the side of each capacitor.
Make sure the evaporator and condensor coils are clean of dirt, bugs, bug nests and the fins are in good shape (not bent reducing air flow through the transfer coils).
Take some time and clean all electrical connections topside under the AC cover. Time and humidity take their toll on the connections reducing voltage/current to the fan & compressor resulting in premature failure because they have to work harder.