had had its boiler removed and was designated 252 FX. The non-boilered vacuum braked locos were 252 BV and when dual braked became 252 EX. Class 25/3 Number sequence (original) D7598–D7677 (TOPS) 25 248–25 327 The final batch of locomotives were designated Class 25/3 and was to be built by BR's Derby Works and Beyer, Peacock and Company of Manchester. However, because of financial problems Beyer, Peacock was unable to complete the final 18 locomotives and these were transferred to BR Derby for construction. Though these locomotives still carried a RTB 15656 generator, this variant was a ten pole machine with a modified assembly incompatible with earlier equipment. The regulated (full hp) part of its characteristic was substantially the same as before but the unloading point, that is the point at which full power could no longer be utilised, was altered to 900 A, 910 V (819 kW) from 1,050 A, 780 V (819 kW). Only two stages of field weakening were employed, previous machines had six, and this provided ‘full power' at speeds between 7 and , and maximum tractive effort was reduced to . The latter half of the 1960s had seen the widespread introduction of solid state electronics and these locomotives incorporated a control system where speed was detected electronically rather than mechanically. A signal from a tachogenerator was used to close contactors in sequence at given speeds to activate the motor's field weakening process, rather than through contacts and relays as in earlier types. The control system ensured the traction motors and main generator were all operated within the continuous rating of the machines except in full field conditions when the driver was able to judge how long to remain in the short-term rating condition. There were two variants of the Class 25/3 sub-class. Early 25/3 AV locomotives were fitted with vacuum brakes and in due course many of these were dual braked and redesignated 25/3 BX. By the time the last few locomotives were under construction dual braking had become the norm and ten of the last batch from Derby were built new as 25/3 BX locomotives for work out of Willesden on the recently upgraded West Coast Main Line. Class 25/9 At the end of 1985 twelve of the remaining Class 25/3 locomotives were designated as 25/9, the intention being that they would operate on traffic won for the Industrial Minerals Division of Railfreight