At more than 15 percent of his fund’s assets, John Burnham, manager of the $136 million Burnham Fund, has a larger stake in Apple Inc than any other diversified fund. Yet that devotion for Apple is a problem for Burnham and some other managers of so-called diversified funds like his – they want more Apple than they can buy under self-imposed risk-reducing guidelines that typically have them holding no more than 5 percent of their assets in any one company.