The Takács Quartet regretfully must cancel its Ravinia concerts on Aug. 5 and 6 because one of its members recently had minor surgery and the recovery is taking longer than anticipated. Ravinia hopes to reschedule the complete Bartók Quartets by Takács for a future season. Ticket-buyers need take no action, as refunds will be automatically processed via the original purchase method. The Aug. 5 concert by participants in Ravinia’s Steans Music Institute will go on as scheduled. Ravinia will be closed on Aug. 6.