When measuring AC, some meters are distinctly better than others at measuring the AC voltage that is superimposed on a DC voltage, like in this case. Here is a method to check the smoothing without knowing how the meter reacts to DC.ok I did measure the DC voltage from the wart before the incident happened and it reported 18V which is far less than the 50V the blown cap was rated for.
I'm about ready to throw this adapter away and buy a new one, but when I measure ac coming from the adapter, what number am I looking for on the meter to indicate whether the 22uF/50V capacitor I'm about to connect to it is likely to blow up or not?
I suggest you measure the voltage with no capacitor, and then with the 22 μF capacitor only, but with no load. There is little risk of it exploding if the polarity is correct.
If the voltage is larger with the capacitor than without, there is no smoothing inside the wall wart.
Also, if you measure the AC voltage, with the meter on AC, check with the leads both ways round. If the reading is the same or nearly so, and a lot smaller than the DC reading, it's probably correct. If the voltage shown is very different one way round from the other, your meter is no good at measuring mixed AC + DC voltages.