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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.
⚙️ Technical Conclusion
The U4801/U5701 keyboard SPI controller contains firmware/configuration specific to each board model. These chips are not freely interchangeable between different MacBook series (A1278, A1502, A1398, etc.).
⚠️ Using an incompatible IC can cause:
Reversed Caps Lock LED logic.
Noticeable input delay on keys input.
Malfunctions with the trackpad or power button.
✅ Key Takeaway
“The keyboard IC (U4801/U5701) must come from a firmware-compatible logic board. Although they are electrically identical, their internal configuration depends on the board model and its interaction with the SMC.” — Diógenes el Perro
🛠️ Note: Neither the BIOS nor the SMC were reprogrammed during this repair.