5 V Standby Rail Collapses Only Under Load in a 32-inch LED TV
A 32-inch LED TV is brought in dead. With the main board disconnected, the standby 5 V rail measures 5.02 V and the 12 V rail is absent, which is normal for standby. The bulk capacitor reads 315 V DC. When the main board is reconnected and the set is switched on, the 5 V rail falls to 3.1 V and fluctuates by about 0.2 V; the 12 V rail pulses to 4 V and then collapses. The power-supply PWM IC VCC pin moves between 10.8 V and 11.9 V. The customer says the TV sometimes started after several attempts before failing completely. A previous shop replaced all secondary electrolytic capacitors, with no change. The optocoupler, TL431 and a 5 V filter capacitor are accessible. You have an isolation transformer, a true-RMS DMM, an ESR meter and a 100 MHz oscilloscope with a differential probe.
1. Rank the three most probable causes of the 5 V rail collapsing only under load, and explain what measurement would separate them.
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At no load, 5.02 V and 315 V across the bulk capacitor show that the primary switch, transformer and start-up circuit are functioning. The collapse to 3.1 V only when the main board is connected means the supply can source almost no current, or the main board is demanding too much. The previous blanket recap makes simple secondary capacitance less likely but does not clear the 5 V rectifier, the transformer winding resistance, or the feedback loop.
Rank: 1) feedback/regulation fault - TL431 reference divider, optocoupler CTR, or PWM compensation; 2) 5 V secondary rectifier or winding/connection resistance; 3) main-board overcurrent or a short on the 12 V rail loading the supply.
Separation: use an adjustable electronic load on the 5 V rail with the main board disconnected. If 5 V holds at 1 A, the supply is probably good and the main board is suspect. If it collapses, monitor TL431 cathode and optocoupler LED current while raising load. If opto current rises but 5 V still falls, the secondary side is losing regulation. Scope the secondary diode waveform for ringing or a collapsed duty cycle, and confirm VCC stays above UVLO. A current-limited bench supply can also measure main-board current at 5 V.
2. Describe a safe step-by-step procedure to confirm the fault without damaging the main board or yourself.
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Work with the set unplugged, discharge the primary bulk capacitor through a 1 kΩ/5 W resistor, and verify below 10 V before touching anything. Use an isolation transformer for the DUT, and do not ground the scope chassis to the primary side; use a differential probe or an isolation amplifier. Connect a suitable load, preferably an electronic load or power resistor, to the 5 V rail and bring it up slowly.
Measure VCC, TL431 cathode, optocoupler LED current and the 5 V rail simultaneously. Use a series incandescent lamp in the AC input as a current limiter during first power-up. Check solder joints and transformer pins with the power off, and measure the 5 V rectifier and winding resistance. Do not connect the main board until the 5 V rail is proven stable at its rated current. If the main board is suspected, measure its 5 V and 12 V rail resistance and feed it from a current-limited supply. Replace only components shown faulty by measurement; do not power-cycle repeatedly with a failing supply.
3. The owner asks you to 'just replace the whole power board' because it is cheaper than diagnosis. Give a reasoned recommendation, including one check that could change the decision.
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Replacing the whole board can be a valid commercial decision if the board is inexpensive and available, but it is not diagnosis. If the main board has a short or a failed 12 V load, a new power board may fail immediately or run hot. Before agreeing, measure the main-board current on the 5 V rail and the resistance of the 12 V rail to ground with the power off. A 5 V current well above the service value, or a 12 V rail reading near zero, changes the decision: the main board also needs repair.
Also verify that the replacement board is the correct revision and has the same connector pinout and LED backlight drive. If the replacement is a used or counterfeit board, the risk of repeat failure is higher. Offer the customer the measured evidence and a fixed-price diagnosis, and state that a board swap will not recover the existing board's fault history.