How I Repaired SONY TV Power Supply Model: T370XW02 Made In Taiwan
This TV was made in Taiwan and I received it in my lab with panel problem but after one or two time on/off, the TV can’t be turned On again.
Shorted wires of lvds cable made this problem for this panel
Burned pixels on screen due to bad lvds cable
The TV already 15 years old and eligible to have many problems like bad caps and cold joints and so forth. Since the TV can’t be turned On so the main suspect was power supply.
The power supply was made by SUNKEN co. in Indonesia and assembled in Sony co. in Taiwan.
The first component after AC terminal input was VDR ( volt dependent resistor ). If the unwanted voltage at the entrance of power supply are going up more than limitation of VDR, the resistance of VDR will gets higher to protect the entire circuit against higher voltage. Usually VDRs are getting short most of the time but in this case, VDR was not shorted. But what if this VDR was open? Actually there was no difference between an open VDR and a good one because our normal multimeter can’t test it. If you want to test for short circuit, you can use analogue mutlimeter set to 10 kOhm range and it should not have any reading. I decided to replace VDR with a new one, but after replacing it there was no progress at all.
On the way down to the bridge rectifier there was two toroid coils that were intact, these toroid coils protect the power supply against unwanted frequencies of AC voltages. The next components were two MKT CAPs that one of them showed very high ESR about 65 ohm but comparing to same kind cap and with same value I convinced that the MKT CAP was also good and the ESR is in the tolerance range. The next component in this direction was a black NTC and this is another protective component in AC entrance line to the TV.
The NTC is based on Negative Temperature Coefficient and this means if the temperature of component goes up the resistance value comes down. This is how I perform the test on the NTC.
I hooked the pins of NTC to the probe of analog multimeter and selector must be on 1 ohm and the multimeter showed 5 ohms. With the help of a hot soldering iron and placing it close to the NTC, a good NTC will react to the heat and the resistance started to decrease. But in my case the NTC did not react to the heat. This means the NTC is open circuited. By replacing the NTC with the same rating, 220 volts are reaching to AC pins of Bridge Rectifier (BR) but very low DC volts on output of BR (about 6 volts) so I can assumed that BR was not good.
So I replaced the BR with a new Bridge rectifier same kind but with different part number. Now I have proper voltage DC out of BR.
By measuring the voltages on the pins of the big filter capacitor, I got about 200 volt DC which was low. It supposed to have about 300VDC. The value of cap was 270 microfarad but ESR meter read this cap just 240 microfarad and ESR of 2 ohm. 2 Ohms ESR means the capacitor is bad. I replaced this capacitor and now have about 300 volt DC across the two pins of the big filter capacitor.
I forgot to explain also that one relay before NTC was tested good. I replaced a resistor with printed value 4.7 ohm ceramic ½ watt that was open and it acted as a fuse in the board .
Now I have 3.3 volts in output terminal of power supply (psu) to feed CPU and start the TV.
Bad TV parts
And yes I have to find a way or make a way to solve the panel problem of this TV as well from now.
This article was prepared for you by Mr Beh from Iran
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Note: You can check his previous article on How 10“ Wired Video Intercom System Doorbell With Camera And Monitor Plus Memory Door Security System TABA Is Repaired model: TVD-1035