To deal with increasing reliability problems, the Class 50 fleet was refurbished at Doncaster Works between 1979 and 1984. Doncaster had taken responsibility for the fleet after BR completed the purchase of the locomotives from English Electric. The work involved simplifying the complex electronics and removing redundant features such as slow speed control and rheostatic braking. In addition, the air intake fan arrangement was modified, because the original setup often prevented fresh air from entering the engine room and stale, oil mist-filled air from escaping, leading to many main generator failures. This was in part due to the moisture in the air in the UK: dust and other particles would lodge in the filter system and become 'gummed up' with moisture, preventing circulation which in turn also hampered the intended engine compartment pressure levels which then meant 'filtered' air could not be evacuated by the intended means. The filtration system was fundamentally sound and widely used in other countries; the problems arose because relative humidity had not been taken into account at the design stage. This modification eliminated the characteristic droning "sucking" noise which had earned the "Hoover" nickname.