if you have a local Maplins and your lucky to find a nice shop assistant who has some idea of electronics he'll tell you which diod to replace, or maybe a old type tv repair man would piont you in the right direction..
ofcourse it's not always the pektron units fault sometimes it's the blasted master switch
[8 whole cylinders worth of punch to ram the world through the windshield and out the rear view mirror. Car & Driver]
1981 Grinnall TR7 v8
1981 Black FHC
1979 & 1980 Black Premium FHC(laid to rest) Sorry
John