Isn't that a 200 NOT a 200A with the speakers on the chassis not in the lid?
If it's a ground issue any green wire bolted to the points on the chassis should be taken apart and some cleaner and light sand paper to burnish the connection areas, (washer, nut, chassis) to get a better ground. There are also special connectors going to the amp and preamp that might need cleaning and tightening. If you don't trust yourself to work on things you might want to get someone who is rather than risk making more issues that will be a problem to get fixed.
How to remove hum from you Wurlizter guide.
If it's a ground issue any green wire bolted to the points on the chassis should be taken apart and some cleaner and light sand paper to burnish the connection areas, (washer, nut, chassis) to get a better ground. There are also special connectors going to the amp and preamp that might need cleaning and tightening. If you don't trust yourself to work on things you might want to get someone who is rather than risk making more issues that will be a problem to get fixed.
How to remove hum from you Wurlizter guide.