"Diff'rent Strokes" star Todd Bridges is inconsolable in the wake of Conrad Bain's death -- telling TMZ, he just spoke to Bain last week and everything seemed fine.
Bridges tells us, "I am deeply saddened by the sudden passing of Conrad as we were looking forward to celebrating his 90th birthday next month."
He added, "In addition to being a positive and supportive father figure both on and off-screen, Conrad was well-loved and made going to work each day enjoyable for all of us. He will be missed, but never forgotten."
TMZ broke the story ... Bain -- who played Phillip Drummond on the iconic show -- passed away Monday at a retirement home in Livermore, CA. He was 89.