My built-in flash pops up but fails to flash--have checked all settings so the unit must be broken. What type of flash unit do I need to attach to the hot shoe? I see by the manual that there are SB-910, 900, 800, 700, 600, 400, 300 units that are available. I did try an old flash unit (Sunpack Auto 221) which did work, however, in Auto mode, the flash was too bright so the photo is overexposed. I don't want to spend hundred of dollars for a flash unit.