You can test the AC/DC converter circuit using a multimeter set on DC Voltage to test the output from the converter.
Yes, that is correct. If you are not getting the correct voltage then the converter is faulty and will need to be replaced.
Finding the faulty component will be difficult, without using schematics and spending a lot of time trying to diagnose which part has failed.
Usually the entire converter is just replaced for a faster, cheaper and more reliable fix.
If you have schematics and some test tools, then you can debug the circuit and find that faulty part.Or a local electronics repair tech could do it for you.However the cost for labor to do that repair often exceeds the cost of a new converter, so most people will simply replace it.Pete.
Without schematics the only other possibility would be to use a similar flash unit to take some voltage readings from and compare.