Sony KD-49X755F eMMC Failure: Symptoms, Repair, Replacement, and Main Board Swap

Platform and architecture

• SoC: MediaTek Android TV platform of the 2015–2016 generation (Sony BRAVIA ATV2 family). The SoC boots from eMMC boot partitions (boot0/boot1), then mounts Android system/user partitions in the eMMC user area.
• eMMC organization
• Boot partitions: two small hardware partitions (commonly 4 MB each) holding the primary/backup bootloaders.
• RPMB (Replay Protected Memory Block): secure, write‑protected area that stores device-unique cryptographic material (e.g., Widevine/HDCP keys). Not clonable in plain form.
• User area: holds system, vendor, cache, and userdata images.
• Board variants: KD‑55XD7005 mainboards exist in multiple revisions with P/Ns in the 1‑980‑8xx‑xx series; eMMC vendor and capacity can vary (Samsung, Hynix/Skhynix, Kioxia/Toshiba are typical).

Failure mechanisms

• NAND wear-out: Android logging, app updates, and caches produce many small random writes; after years of service, blocks exceed P/E cycle limits.
• FTL (Flash Translation Layer) corruption: unclean power‑downs under write load or degraded blocks corrupt mapping tables and/or boot regions.
• Thermal stress: sustained SoC temperature accelerates aging and increases uncorrectable bit errors.

Field symptoms indicating eMMC trouble

• Boot loop: SONY logo → Android animation → restart.
• Stuck at logo; USB update never starts or stalls.
• Extremely sluggish UI, app crashes, random resets.
• Red‑LED blink codes indicating mainboard/software fault on this platform.
• From service/ADB (when obtainable): read‑only filesystem, repeated I/O errors, failure to complete OTA.

Diagnostic approach (in increasing invasiveness)

• Software level
• Forced factory reset: press/hold TV front‑panel Volume‑ (Minus) + Power while applying AC; release when the status LED turns green/white and a reset starts. If the reset cannot finish, suspect eMMC.
• USB rescue/update: prepare the model‑specific firmware on a FAT32 USB stick; power‑cycle with the USB inserted. Failure to detect or repeatedly fail write stages points to eMMC write/erase problems.
• Electrical checks (board level)
• Verify rails: eMMC VCC ≈ 3.3 V, VCCQ ≈ 1.8 V during boot. Brown‑outs or droop under load can mimic flash faults.
• Bus activity: with a scope or LA, confirm CLK/CMD/DAT toggling within a few hundred ms after power‑on; absent/aborted traffic beyond initial attempts often correlates with bad boot partitions.
• Device health (chip level, if accessible)
• Read EXT_CSD via ISP or socket programmer to inspect:
  • PRE_EOL_INFO (0xB1): 0x01/0x02/0x03 indicates early/warn/critical.
  • DEVICE_LIFE_TIME_EST_TYP_A/B (0xA7/0xA8): stepwise wear indicators.
• Attempt read of boot0/boot1 and small ranges of user area; widespread read failures or very low read speeds corroborate controller/NAND degradation.

• EXT_CSD[179] PARTITION_CONFIG: select the correct boot partition (usually boot0) and enable boot acknowledge as per the original device.
• BOOT_BUS_WIDTH, BUS_WIDTH, HS_TIMING: match the original to allow HS200 after boot.
• It is safe to start in legacy timing and let Android switch to HS200 once the kernel loads.

• 8 GB is original; 16/32 GB eMMC 5.1 devices (BGA‑153) are usually compatible and provide better endurance. The image can be cloned to the start of the device; expand userdata afterward to utilize extra space.

• Adding a thin thermal pad between the SoC shield and the rear cover or a small copper shim on the SoC shield (without shorting) can reduce steady‑state temperature a few degrees, improving lifespan.

• Try forced factory reset and official USB firmware update first.
• If those fail, the most predictable route is mainboard replacement matched by P/N and panel code; this preserves full functionality (including L1/HDCP).

• Tools: BGA rework station with preheater, microscope, quality flux, eMMC programmer (UFI, Easy‑JTAG, Medusa, etc.), BGA‑153 socket.
• Workflow
  1. Back up what you can in‑circuit (ISP) or after removal (socket): attempt boot0/boot1/RPMB auth data export if the tool supports it.
  2. Program the new eMMC in a socket with a clean model‑correct image (boot0+boot1+user). Configure EXT_CSD (partition config, boot bus width, HS timing).
  3. Reball/solder with a controlled thermal profile; avoid over‑temp to protect nearby SoC and PMICs.
  4. First boot on bench with minimal loads; verify update/rescue accepts the official firmware, then factory‑reset from service menu.
• Validation
• Cold/warm boot cycles without loop.
• Storage self‑tests via ADB (fsck clean, no I/O errors in dmesg/logcat).
• App store operation and streaming playback (resolution/DRM status).

• If original RPMB is unreadable, explain to the owner that premium streaming may be limited post‑repair.
• Keep ESD protection and shield masks; PCB warpage is a common cause of lifted pads on these boards.

• Modern TV/mainboard boot chains (very common on MediaTek- and Amlogic-based boards, including many Sony/Android TV designs) protect their UART/loader shell behind a challenge–response gate to prevent unauthorized firmware access and key extraction.

• Your challenge is Base64; 24 characters with “==” padding indicates 16 raw bytes. On receipt, an authorized tool decodes those bytes and computes a response with a device/model secret and a fixed algorithm (commonly HMAC-SHA256, AES-CMAC, or an AES encrypt-then-encode variant). The response is typically returned as Base64 or hex.

1. Boot ROM/first-stage loader prints challenge=’…’.
2. Service software decodes the challenge, derives or fetches the device key (locally stored/obfuscated, in OTP/efuse, or via service portal), computes MAC/signature.
3. Tool sends response=. If it matches, privileged UART commands are enabled.

• The secret is not on the UART; it is in secure storage (OTP/efuse, RPMB, trusted firmware) or in the service ecosystem. The space of possible keys is cryptographically large; brute force is infeasible, and many loaders impose lockouts after a few failures.

• You can log pre-auth output (boot banners, early diagnostics). Once the challenge appears, further privileged interaction is blocked until authenticated.

• You can confirm its size locally:
• Linux/macOS: echo 'rTnzTFzPmQF5UvgPiECykQ==' | base64 -d | xxd
• Expect 16 bytes of pseudo-random data.

• Python (HMAC-SHA256 + Base64):
• import base64, hmac, hashlib ch = base64.b64decode('rTnzTFzPmQF5UvgPiECykQ==') key = b'\x00…' # device/model-specific secret mac = hmac.new(key, ch, hashlib.sha256).digest() resp = base64.b64encode(mac).decode() print(resp)
• Many real implementations also concatenate device IDs (eMMC CID, serial, board ID) before MACing, or use AES-CMAC rather than HMAC.

• Use ISP (In-System Programming) to the eMMC:
• Locate test pads: CLK, CMD, DAT0 (minimum), GND, and verify I/O voltage (often 1.8 V for VCCQ, 3.3 V for VCC).
• Use a capable programmer (Easy-JTAG, UFI, RT809H, Medusa Pro).
• Always make full backups first: boot0, boot1, user area; dump RPMB if your tool supports authenticated reads.
• Program a known-good image for the exact board P/N and panel pairing; keep original key partitions intact to preserve DRM and panel calibration.
• Chip-off (socket) is a fallback when ISP is unstable.

• Capture the entire UART log from power-on to see pre-challenge messages (baud is often 115200 8N1; level is 3.3 V or 1.8 V—confirm before connecting).
• Avoid rapid repeated wrong responses; some loaders enforce delays/lockouts.

• Engage an authorized service center; provide the board P/N, SoC family, and serial. They can authenticate and perform secure procedures without exposing keys.

• Don’t attempt random guesses or publish captured challenge/response pairs; they are per-session nonces and won’t help, and failed attempts can slow or lock the interface.
• Don’t short eMMC lines indiscriminately to force alternate boot modes; this risks SoC or eMMC damage.