Amp Problems

Hi All Im new here so i hope im not annoying with my problems. My Fender Ultimate Chorus amp started crackling a few months ago whilst playing at volume, i thought maybe it was the input on my guitar, faulty lead or my effects peddles, but iv tried different guitars, bought new leads and played it without peddles and still have the problem. It seems to be worsening as it was on and off in the past but now its crackling all the time. Can anyone help, or maybe surgest someone who can. Many thanks. Mark