Check to see if its charging? 12 volts plus, upto 14.8 is the max you would want.
Or if its a 24v upto 28 is OK.
Is the drive belt tight and in good condition.
you say you've checked the connections, have you removed and cleaned them.
There is loads you can check before buying a new altenator, try taking the motor to a lucas or vehicle electrician on the premise that you will be buying the replacement part from him, most will check your sysyem for free and then you go and order and fit the item yourself.
Lee