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820-3662 NO NUMERIC KEYS / CAPS LOCK INVERTED / INPUT DELAY - TOUCHPAD ISSUES
The device arrived with the numeric keys on the internal keyboard non‐functional, despite a complete keyboard replacement. Visible moisture and sulfation were detected near the keyboard’s SPI controller (U4801).
🔍 Step-by-Step Diagnosis
Visual inspection: Corrosion was found near the keyboard/trackpad controller area (U4801).
Voltage drop across R4814 (relevant resistor in the SPI circuit) was checked and confirmed expected values.
Chip replacement attempts:
First attempt:
Replaced U4801 with a donor chip from board 820-3476 (MacBook A1502).
🟡 Result: Keyboard worked, but Caps Lock LED was reversed (lit when lowercase, off when uppercase).
Second attempt:
Used a chip from MacBook A1278 (Unibody).
🔴 Result: Severe input delay, no trackpad and overall poor keyboard behavior.
Final solution:
Replaced with U5701 from donor board 820-3332.
✅ Result: Keyboard fully functional, Caps Lock LED working properly, no delay, trackpad also fully operational.