 The Mk . II was used for only a short time when the team replaced its transmitter section with one from the ASV Mk . I , which used the new Micropup valves . The new AI Mk . III sets were experimentally fitted to about twenty Blenheim IFs in April 1940 , where they demonstrated an improved maximum range of 3 to 4 miles ( 4 @.@ 8 – 6 @.@ 4 km ) . However , they still suffered from a long minimum range , from 800 to 1 @,@ 500 ft depending on how the receiver was adjusted . 
