SDG #017 Benq LCD Monitor Teardown and Repair

A dead BenQ TFT LCD monitor with an LG panel makes a good case study in mains side switch mode power supply faults, because the reported symptom, repeatedly blowing 5A mains fuses before finally refusing to power up at all, points directly at a hard short rather than a subtle fault. That distinction matters: a monitor that simply fails to light usually has degraded electrolytics or a failed backlight inverter, while one that destroys fuses on plug-in almost always has a shorted switching device, bridge rectifier or input side component drawing excessive current the moment the primary reservoir tries to charge.

The repair follows a sensible order of operations. The monitor is stripped down to the bare power supply and controller boards, the primary side is inspected for the visible signatures of a failure event such as scorching, cracked packages and lifted tracks, then components are checked out of circuit rather than trusted on a visual pass. An ESR meter is valuable here, since primary and secondary electrolytics can measure correctly for capacitance while having lost the low impedance the converter needs, and an isolation transformer is used so that live measurements can be taken safely on a supply whose primary side sits at mains potential.

For anyone weighing repair against replacement, the episode shows how much of the diagnosis is achievable with a decent multimeter, a component tester and an oscilloscope for verifying switching waveforms once the short is cleared. Mains side faults are also where corner cutting is genuinely dangerous, so the emphasis on isolation, discharging bulk capacitors and refitting a correctly rated fuse rather than a heavier one is the part worth copying.