Greetings Hampshire,
Unfortunately, I don't have manual or schematic for your particular tractor. Shooting from the hip, I'm *guessing* it could be several things. Since we know the bulb is good, I'd recommend checking in the following order:
1. - bad fuse either under the dash or down by the starter
2. - broken or pinched wire or bad system, dash, or light socket ground
3. - bad or corroded key switch
4. - bad ground on the voltage regulator (are the mounting bolts loose?)
5. - bad voltage regulator
You might consider checking with a test light or voltmeter just to be 100% sure that you don't have a bad socket or bad ground at the socket. Testing this way, at least you'll know for sure you don't have voltage to the socket. Also, it would be advisable to get a wiring diagram for the tractor to help trace the circuit in question. Would be easiest to locate the problem that way and you'd know which wires to check based on color.
Is the tractor charging the battery correctly? Have you ever put a voltmeter on the battery both with it stopped, as well as running, to verify they charging circuit is working? Finally, have you ever checked the output of the dynamo to verify that it's working correctly? (test the dynamo using the AC setting on your voltmeter) I'm asking these questions because if the tractor isn't charging the battery properly it would most likely point to either a blown fuse, bad key switch or bad voltage regulator (assuming the dynamo, wiring and grounds are good).
Finally, I notice you also own a B7100. You might consider comparing the charging circuits between the two as I believe they're pretty close. Since both use a dynamo, it should be fairly straight forward to trace which circuits have power when the key is on and, if you get lucky, you might even be able to swap parts for testing to figure out exactly what you need to replace. (I'm thinking voltage regulator)
Hope this helps.
Good luck!
Steve
