Thomson T55FSL5140 TV No Power, No LED – PSU Board 40-L141H4-PWC1CG Faults

User question

Thomson T55FSL5140 не включается телевизор и не горит индикатор, что может быть

Artificial Intelligence Response

Direct answer to the question

• The absence of both picture/sound and the standby LED almost always means that the TV does not receive the 5 V (or 3.3 V) “stand-by” supply coming from the internal Power Supply Unit (PSU).
• Most frequent causes: blown AC fuse, open-circuit power cord, shorted primary-side components (bridge rectifier, MOSFET, PFC circuitry), dried-out electrolytic capacitors in the PSU, or—far more rarely—an open standby voltage regulator on the mainboard.

Detailed problem analysis

1. External power path
   • Wall outlet → Power cord → AC input filter → Line fuse → Primary rectifier and PFC → Stand-by SMPS → 5 V_SB rail → Main-board MCU & LED.
   If any device in this chain is open or in protection, the LED stays dark.

2. Model-specific data (Thomson T55FSL5140)
   • Typical PSU board P/N: 40-L141H4-PWC1CG (may vary by production batch).
   • Stand-by converter built around PWM IC OB2269 or FA6A01; large 400 V capacitor 100–150 µF tends to lose capacity; secondary-side 5 V rectifier usually an SS14/MBR1100 Schottky.
   • The mainboard will not even boot without a solid 5 V_SB; thus black screen + no LED in 90 % of cases = PSU failure, 8 % = power cord / socket, <2 % = mainboard short.

3. Typical failure modes
   a. Blown T 3.15 A/250 V line fuse after surge → indicates shorted MOSFET or bridge.
   b. Bulged electrolytics (especially 10 µF/50 V on VCC pin of PWM) → converter never starts.
   c. High ESR of the 400 V bulk capacitor → standby attempts but clicks off (still no LED).
   d. Cracked solder joints around transformer or heatsinks → intermittent loss of 5 V_SB.

4. Diagnostic flow (safe sequence)
   1) Verify wall outlet with another load; bypass any power strip.
   2) Inspect power cord for kinks/cuts; ohm-check from plug to IEC pins (<0.5 Ω).
   3) Unplug TV, wait 20 min, remove back cover, locate PSU.
   4) With set unplugged check fuse for continuity. If open, measure bridge/MOSFET for short.
   5) If fuse good, power up through an isolation transformer or bulb limiter, measure 5 V_SB pin on PSU–MB connector. Expected: 4.8-5.2 V DC.
   – 0 V → PSU primary not starting → focus on start-up cap, PWM IC, open startup resistor.
   – <1 V pulsing → over-current or short on 5 V line; disconnect mainboard ribbon and retest to isolate.
   6) If stable 5 V appears with mainboard disconnected, mainboard is shorted (often 1.1 V buck regulator IC).

Current information and trends

• Board-level replacement is now common; complete PSU for this chassis sells for ≈ 25-45 € on EU market (Q2-2024).
• Component-level repair kits (electrolytic set + PWM IC) cost <10 €.
• Some workshops retrofit universal “12 V notebook brick + inverter board”, but picture quality may suffer.
• Manufacturers progressively integrate PSU and mainboard; future sets will make discrete PSU repair impossible, so board swap skills remain relevant in 2024.

Supporting explanations and details

• Why firmware is unlikely: firmware lives in eMMC/Flash powered by 3.3 V derived from the 5 V_SB; without the latter, the MCU never executes code, therefore LED cannot light.
• Analogy: Think of 5 V_SB as the “ignition key”; if the key is missing, the engine (mainboard) never cranks.

Ethical and legal aspects

• High-voltage safety: primary side carries up to 325 V DC. Lethal if mishandled.
• Warranty: opening the back cover voids any remaining guarantee.
• Environmental: prefer component-level repair or board recycling instead of discarding a 55″ panel.

Practical guidelines

• Tools: insulated screwdrivers, digital multimeter (CAT III 600 V), ESR meter for capacitor health, optionally isolation transformer + 100 W bulb limiter.
• Best practice: always discharge primary cap via 100 kΩ resistor before soldering.
• Common pitfalls: replacing only the fuse, or using low-temperature-rated capacitors (<105 °C).
• Challenge: some PSUs use proprietary PWM ICs; if unavailable, board swap is faster.

Possible disclaimers or additional notes

• Statistics above are empirical; an individual unit may deviate.
• If you lack HV experience, outsource the repair—cost of professional PSU swap is usually <1/3 price of new TV.

Suggestions for further research

• Download PSU schematic for 40-L141H4-PWC1CG (available on elektrotanya.com).
• Study application notes on quasi-resonant flyback (ON-Semi NCP-series) to understand standby supply design.
• Explore passive PFC failure analysis; good resource: IEEE paper “Failure Mechanisms in LCD TV PSUs” (2022).

Brief summary

No standby LED on the Thomson T55FSL5140 almost certainly points to loss of the 5 V stand-by rail produced by the PSU. Begin with external power checks, then test the line fuse and measure 5 V_SB. In most cases the cure is either (a) replace bulged electrolytics / failed PWM IC, or (b) swap the entire PSU board (P/N 40-L141H4-PWC1CG). Observe high-voltage safety; if unsure, entrust the job to a qualified technician.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.