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Polaroid LCD tv: TLA-04641C(Will not turn on)

This is a discussion on Polaroid LCD tv: TLA-04641C(Will not turn on) within the Electronics forums, part of the Tech Support Forum category. First have no schematic on this set. Have no voltages out of power supply. Fuse good. Cannot hear relay click.

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Old 12-31-2009, 12:20 PM   #1
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First have no schematic on this set.
Have no voltages out of power supply.
Fuse good.
Cannot hear relay click.
No lights on front panel.
Appreciate any help.
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Old 12-31-2009, 01:27 PM   #2
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Hi Rogerb10

Check for continuity on the wire side that is soldered to the power supply pins, if everything checks out follow the trace to where the voltage rectifier is. There are usually 4 pins to the rectifier,you will see on the rectifier + sign a -sign and 2 sine waves. The 2 sign waves are the AC voltage from the outlet. Set your meter on AC and check for outlet voltage, then switch to DC on your meter and connect your leeds to the + and - this will give you DC output. If nothing comes comes out of the rectifier then desolder it and bench test the pin diodes. If just 1 diode junction conducts both ways then replace the rectifier.

Another factor is that you have to consider is that if it's a switching power supply. Switching power supplies behave different because they use an oscillator to trigger the switching action. Depending on the power supply orientation, if there is a small transformer with multiple output pins then it's a clear indication of a switching system. The oscillator can be triggered either from an SCR, or a 4 pin photo-coupler (opto-isolator) which is located near the transformer.

Keep in mind that some power supplies have a 3 pin regulator on a heat sink after the rectifier.

When you press the power on button, the relay is engaged by a transistor driver, it's either an NPN or PNP package type. The transistor is located near the relay, just follow the trace from the coil side of the relay to the transistor, then set your meter in diode mode and test it. If the driver is faulty then replace the transistor.

post back your findings.

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Old 12-31-2009, 03:07 PM   #3
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1. Check for shorts on the low voltage side of the power supply.
Any voltage rail that is shorted will stop the supply from working. To see if the problem is power supply related or main board related you will need to remove the feed from the power supply to any other boards. If the short stays on the power supply board, you should have the experience to figure out where the short is coming from.

2. If you find no shorts, check the Voltage across the main smoothing capacitor. Take extreme care though since the voltages in this area can be lethal!!

3. I have yet to find a modern set that doesn't use a switched mode power supply. Check to see if any of the low voltage power supply rails is showing any signs of voltage. might just be one or two volts and fluctuating, but that will give an idea that the SMPS is trying to do it's job but is being inhibited.

4. Look for any domed capacitors on the secondary side of the power supply that could be causing malfunction. If nothing is obvious it may well be worth trying to replace all electrolytic capacitors on the secondary side of the board, using 105 degree Celsius Capacitors of the same capacitance and Voltage rating. Sometimes they are faulty internally but don't show any signs on the outside.

5. Find the SMPS controller IC. Find the data sheet for it on the web and look to see what electrolytic capacitors have been placed on the SMPS controller for start up. Find these on the Power supply and replace them as stated in 4. above

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