Template:Repair Guide/Example

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.