 New Blenheims eventually arrived at Martlesham , these having been experimentally converted to heavy fighters with the addition of four .303 British Browning machine guns and four 20mm Hispano autocannon , while removing the mid @-@ upper turret to reduce weight by 800 lb ( 360 kg ) and drag by a small amount . These arrived without any of the racking or other fittings required to mount the radar , which had to be constructed by local fitters . Further deliveries were not the Blenheim Mk . IF and IIF models originally provided , but the new Mk . IVF versions with a longer and redesigned nose . The gear had to be re @-@ fitted for the new aircraft , and the receivers and CRTs were mounted in the enlarged nose , allowing the operator to indicate corrections to the pilot through hand signals as a backup if the intercom failed . 
