John Cavanaugh of 412 Wood St.;Belle Vernon;died Tuesday;Sept.28;1982;at 6 a.m. in Monongahela Valley Hospital;Carroll Twp.  Mr.Cavanaugh was 75.  He was born Aug.3;1907;in Fayette City;a son of the late Thomas and Margaret Long Cavanaugh.  A resident of Belle Vernon for the past 73 years;he was a former member of Belle Vernon Borough Council.  Mr.Cavanaugh was a member of St.Sebastian Roman Catholic Church in Belle Vernon.  He retired as a chemist in  1970 from the Page Plant of the American Chain and Cable Company in Monessen.  Mr.Cavanaugh was also a member of the Belle Vernon Legion Club and the Belle Vernon Musical Society.  He is survived by his widow;Thelma Riskey Cavanaugh;one daughter;Mrs.Ronald (Yvonne) Koeper of Moon Twp.;one son;William Hixenbaugh of Grindstone;one sister;Mrs.Margaret Martin of Houston;Texas;and eight grandchildren.   Friends will be received in the Shelby D.Ferguson Funeral Home;700 Broad Ave.;Belle Vernon;Wednesday from 2 to 4 and 7 to 9 p.m.  Service will be held in the funeral home Thursday;Sept.30;at 10 a.m.  Burial will be in Belle Vernon Cemetery.  Shelby D.Ferguson;funeral director;is in charge of arrangements.