iMac 27" 2015: Display Backlight Failure
Overview
A photographer from Beecroft brought in their iMac 27-inch with a completely black screen. Using a flashlight, a faint image was visibleāconfirming the LCD panel worked but the backlight had failed. Apple no longer services this model, and third-party quotes for display replacement were $600+. We diagnosed and repaired the backlight circuit, restoring full brightness.
Initial Symptoms
- Screen completely black on startup
- Faint image visible with flashlight (LCD working)
- External display worked perfectly via Thunderbolt
- Mac booted normallyāsounds, mouse movement all worked
- Backlight failure was sudden, not gradual dimming
- 9-year-old machine but otherwise working well
Diagnostic Process
iMac 27" Backlight System
The 2015 iMac 27-inch uses LED edge-lit backlighting controlled by:
- LED driver circuit on logic board (LP8550 and supporting components)
- Backlight connector carrying power to display LEDs
- LED strips along display edges
- Feedback circuit for brightness control
Testing
- PPVOUT_S0_LCDBKLT (backlight power): 0Vāshould be ~24V
- Backlight enable signal: Present from logic board
- LP8550 LED driver IC: Not producing output
- Component inspection: One filter capacitor showed signs of failure
Root Cause
A failed capacitor in the backlight driver circuit caused the LP8550 to shut down. This is a protective responseāthe driver detected a fault and disabled output. The LED strips themselves were fine.
Repair Process
Component Replacement
- Identified failed capacitor (10ĀµF ceramic) in backlight circuit
- Removed failed component and tested neighboring parts
- Installed replacement capacitor (same specification)
- Verified backlight voltage now present: 24.2V
Testing
- Display illuminated immediately on power-up
- Brightness adjustment working across full range
- Auto-brightness with ambient light sensor functional
- No flickering or uneven illumination
Outcome
The photographer's iMac was returned with fully functional display. The machine continues in daily use for photo editing 10 months later (February 2025). A $5 capacitor saved a $600+ repair.
Key Takeaways
- Black screen with faint image = backlight failure: Not always a display panel problem.
- Component-level repair often possible: Full display replacement not always necessary.
- Older iMacs still worth repairing: A 9-year-old iMac can still be productive.
- Diagnosis saves money: Understanding the actual fault prevents overspending.
iMac Screen Gone Dark?
A black screen might be a simple backlight repair rather than expensive display replacement.