http://www.jblproservice.com/pdf/Vin...76LNmanual.pdf
Look at D schematic page (bottom right corner)
Remove strapping jumpers on the primary for 120V service which is 1-2 and 3-4.
To set to 240V, remove strapping as above and attach one on 2-3. This should be the only jumper strap you have.
Worst case you can replace the transformer with a 35VA torodial transformer from here
Hairball Audio