Just got this 22" TV in today, the blue power light sometime it will be on but the touch control to turn on the TV does not work, sometime the Blue power light does not come on at all.
Took it apart to inspect for bad caps, burnt parts, loose connections, they all look OK.
I then check the DC output, the 5.1V was only 4.30V, the 15V was only about 13.25V. Load or no load made no difference, I load it down with light and heavy loads and it maintains the voltages real well, so it is regulating but at the wrong voltage, since both 5v and the 15v are off by the same percentage, then it must be something common to both supply. I down load the spec of the SMPS IC, 3BR1065. At first I thought it may be the SMPS IC is bad but it is able to maintain the regulation real well, so I start looking at the feedback signal from the secondary side which drive the Opto, the feedback circuits is typical TL431 precision adjustable reference diode with opto for isolation between primary and secondary side, the regulation is done by how hard the LED inside is turn on (if the output voltage tries to go up, it will turn on the LED more, so more current transfer on the Opto transistor receiving side), the less the primary will drive the transformer.
So it can either be the Opto or the TL431, Opto does not fail very often since they are just LED and photo transistor inside. I test my theory by solder in a 10k pot in parallel with the LED of the Opto so I can divert the drive current away from the LED, the more I lower the resistance (less current go to the LED) the higher the output voltage goes up, I set the pot so I can get 5v and 15v back, then the TV starts working again. so I decide to change the TL431 instead, and that was it, bad precision reference Diode.
It is sad that TV broke down 2 months after warranty expired due to 50 cents part.
Took it apart to inspect for bad caps, burnt parts, loose connections, they all look OK.
I then check the DC output, the 5.1V was only 4.30V, the 15V was only about 13.25V. Load or no load made no difference, I load it down with light and heavy loads and it maintains the voltages real well, so it is regulating but at the wrong voltage, since both 5v and the 15v are off by the same percentage, then it must be something common to both supply. I down load the spec of the SMPS IC, 3BR1065. At first I thought it may be the SMPS IC is bad but it is able to maintain the regulation real well, so I start looking at the feedback signal from the secondary side which drive the Opto, the feedback circuits is typical TL431 precision adjustable reference diode with opto for isolation between primary and secondary side, the regulation is done by how hard the LED inside is turn on (if the output voltage tries to go up, it will turn on the LED more, so more current transfer on the Opto transistor receiving side), the less the primary will drive the transformer.
So it can either be the Opto or the TL431, Opto does not fail very often since they are just LED and photo transistor inside. I test my theory by solder in a 10k pot in parallel with the LED of the Opto so I can divert the drive current away from the LED, the more I lower the resistance (less current go to the LED) the higher the output voltage goes up, I set the pot so I can get 5v and 15v back, then the TV starts working again. so I decide to change the TL431 instead, and that was it, bad precision reference Diode.
It is sad that TV broke down 2 months after warranty expired due to 50 cents part.
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