I am posting my repair notes in the hope they may save someone else the trouble. Based on searching the web, my issue seems rather uncommon.
A 9-year old Samsung Syncmaster 204T stopped working around 2015-03-26. Increasing audible noise from its internal power supply was noticed in the weeks prior to failure. Then, one day, the bezel buttons would no longer respond, and neither the backlight nor the pixel grid would function. The green LED blinked every 0.54sec with a very brief (barely noticeable) off time. This is quite different from the blink pattern that means "no video signal". An audible chirp from the big transformer accompanied the off phase of the LED. It was immediately noticed that the chirp went away when the scaler card is unplugged from 13V DC supply.
In the course of diagnostic efforts, the display did suddenly turn on normally (2 times out of over 100). In retrospect, I think that was random, and had nothing to do with the changes I made to the circuit. But it did lead my diagnostics on a wild goose chase to repair things that weren't broken.
In the end, the first clue proved right: problem was traced to the scaler card (BN41-00620D, rev. MP1.3). The 13V supply and inverter driver board, after 9 years of almost always being powered on, was still working great. It was the 3.3V rail on the scaler card that was experiencing some sort of glitch, resulting in a periodic restart behaviour. The sudden change in the impedance of the load seen by the 13V supply caused the PWM-induced oscillation in the transformer 's secondary winding to phase-shift abruptly, which explains the chirp. (Although the PWM's oscillator is 66kHz, it does not drive the transformer at these high frequencies. The sawtooth wave on the 13V rail, at that particular load level, was found to be about 300Hz, with 17% duty.)
I have managed to patch over the issue by adding more capacitance to C419, the capacitor that backs the scaler's 3.3V supply. It is one of the 6mm-tall SMD electrolytics, marked 100 16J. I simply soldered a no-name 1000uF 25V cap in parallel (mind the polarity if you do that). The display is working now, though I doubt that the underlying problem is cured. Some component has come out of spec after all these years, and it may not be the C419. If the display should fail once more, I will add a note here.
This hasn't been the story I had hoped for. It would have been neat to spot one big bad capacitor, and conquer the beast in one fell swoop. Instead, after throwing all my brainstorming power at the problem, repeatedly, for 2.5 months, the result feels like a Pyrrhic victory. Life is not neat.
A 9-year old Samsung Syncmaster 204T stopped working around 2015-03-26. Increasing audible noise from its internal power supply was noticed in the weeks prior to failure. Then, one day, the bezel buttons would no longer respond, and neither the backlight nor the pixel grid would function. The green LED blinked every 0.54sec with a very brief (barely noticeable) off time. This is quite different from the blink pattern that means "no video signal". An audible chirp from the big transformer accompanied the off phase of the LED. It was immediately noticed that the chirp went away when the scaler card is unplugged from 13V DC supply.
In the course of diagnostic efforts, the display did suddenly turn on normally (2 times out of over 100). In retrospect, I think that was random, and had nothing to do with the changes I made to the circuit. But it did lead my diagnostics on a wild goose chase to repair things that weren't broken.
In the end, the first clue proved right: problem was traced to the scaler card (BN41-00620D, rev. MP1.3). The 13V supply and inverter driver board, after 9 years of almost always being powered on, was still working great. It was the 3.3V rail on the scaler card that was experiencing some sort of glitch, resulting in a periodic restart behaviour. The sudden change in the impedance of the load seen by the 13V supply caused the PWM-induced oscillation in the transformer 's secondary winding to phase-shift abruptly, which explains the chirp. (Although the PWM's oscillator is 66kHz, it does not drive the transformer at these high frequencies. The sawtooth wave on the 13V rail, at that particular load level, was found to be about 300Hz, with 17% duty.)
I have managed to patch over the issue by adding more capacitance to C419, the capacitor that backs the scaler's 3.3V supply. It is one of the 6mm-tall SMD electrolytics, marked 100 16J. I simply soldered a no-name 1000uF 25V cap in parallel (mind the polarity if you do that). The display is working now, though I doubt that the underlying problem is cured. Some component has come out of spec after all these years, and it may not be the C419. If the display should fail once more, I will add a note here.
This hasn't been the story I had hoped for. It would have been neat to spot one big bad capacitor, and conquer the beast in one fell swoop. Instead, after throwing all my brainstorming power at the problem, repeatedly, for 2.5 months, the result feels like a Pyrrhic victory. Life is not neat.
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