Chirping SMPS in a 21-Inch Colour TV
A 21-inch CRT colour television is dead, but a soft 'chirp-chirp' is heard from the SMPS about twice per second. The mains is 230 V AC. On the secondary side, the main B+ is 63 V instead of the marked 115 V. The horizontal output transistor is not shorted, and the flyback secondary rectifiers show no short. When the horizontal output stage is disconnected and a 60 W incandescent lamp is used as a dummy load, the B+ rises to 78 V and the chirping continues. With the lamp removed, B+ jumps to 132 V for about one second, then the SMPS shuts down and restarts. The secondary filter capacitors have normal capacitance and low ESR. The optocoupler and the TL431 reference are suspected, but the technician also notices a small electrolytic capacitor in the primary startup/feedback network that reads high ESR. The set uses a self-oscillating flyback SMPS. The workshop has an isolation transformer, a variac, an oscilloscope with a differential probe, and a component tester.
1. Is the SMPS failing because of a secondary overload or a regulation/feedback fault? Justify with the lamp-load and no-load observations.
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The evidence favours a primary regulation/feedback fault rather than a simple secondary overload. A 60 W lamp is a much lighter load than a scanning TV, yet B+ only reaches 78 V and the SMPS still chirps. If the secondary were shorted, the lamp test would normally show a low output but not this same hiccup pattern with a light load. With no load, B+ rises to 132 V and then the SMPS shuts down, which suggests the feedback loop cannot keep the output at 115 V and the overvoltage or error amplifier is tripping. A high-ESR capacitor in the primary startup/feedback network can reduce drive, make the oscillation burst, and cause exactly this low-output chirp. The optocoupler and TL431 divider are still possible, so they must be tested, but the primary feedback capacitor is a strong suspect. The horizontal stage and flyback rectifiers should still be checked for leakage, because a partial overload can aggravate a marginal SMPS.
2. Describe a safe test sequence to distinguish primary feedback from secondary load.
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Use an isolation transformer and bring the set up slowly on a variac with a dummy load. Do not rely on the TV’s own horizontal stage for initial tests.
- Check the primary electrolytic capacitor’s capacitance and ESR with a component tester; compare with its marked value. Also inspect the startup resistor and snubber.
- Measure the feedback divider around the TL431 for drift or open resistors. Test the optocoupler by injecting a controlled current into its LED and watching the primary drive, or substitute a known-good part.
- Observe the primary switching waveform with a differential probe. A healthy self-oscillating flyback should show regular, clean bursts; narrow or irregular pulses indicate insufficient drive or a feedback problem.
- Disconnect secondary loads one at a time and use the lamp. If B+ remains low and chirping with only the lamp, the fault is likely primary. If B+ becomes normal when a particular secondary rail is disconnected, trace that load.
- Check the B+ sense divider and the overvoltage protection node; a drifted divider can make the SMPS think the output is high when it is not.
- Do not run a self-oscillating SMPS with no load for long. Use the lamp or a suitable minimum load.
3. What repair would you perform, and what post-repair checks are needed?
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Replace the high-ESR primary startup/feedback capacitor and any drifted feedback divider resistors. If the optocoupler or TL431 is suspect, replace it with a known-good part rather than adjusting the divider blindly. Resolder cracked joints around the transformer, heat sinks, and feedback components. After repair, start on the variac with a lamp load and confirm B+ is 115 V ±2% with acceptable ripple. Then reconnect the horizontal stage through a current-limited supply and check horizontal output current and the flyback waveforms. Run the set for at least an hour and monitor the SMPS switch and transformer temperature. A chirp that disappears only when the TV is fully loaded indicates a remaining loop-stability or minimum-load problem, so do not close the set until the B+ is stable at both light and normal load.