Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

[Quantum IC]

Hello every one,
This TV came to me with the problem that it was totally dead.
The problem was the two MOSFETs and a fuse resistor in the power supply.
The original MOSFETs were KF7N50F and I repalced them with SLF65R700S2.
The original fuse resistor was 0.1 Ohm 1/2W. I didn't find a similar fuse resistor and I replaced it with 0.1 Ohm 5W ceramic resistor.
When I plug the TV the ST-BY LED turns on, when I press the power button on the TV, the ST-BY LED turns off and the TV doesn't turn on until the second time I press the power button is when the TV turns on, but for a span of about 10 seconds the thermistor gets too hot. It is unbearable to the touch. Burn the finger.
I turn the TV off before somthing bad happens, but the ST-BY LED doesn't light, when I press the power button again, the TV turns on but the thermistor reheats again.
If I unplug the TV and plug it again the ST-BY LED lights up.
So I ask you guys
What causes the thermistor to get too hot?
Why doesn't the TV turn on the first time I press the power button?

I have 389V on the main filter.
I have the 12.3V and 25V.

When I bridge between ST-BY and POWERON the SMPS works fine and the thermistor doesn't get hot. I assume it's because the SMPS has no load.

I attached a similar diagram because I couldn't find the diagram for this SMPS.
Similar schematic diagram is on page 28.
I you need more info just let me know.

I hope you understandme. I'm still improving my English. Sorry

Best regards.

[dskall]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

A good picture of your board would help. Your thermistor is probably bad.

[R_J]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

installing a 5 watt resistor for a .5 watt is like replacing a 1 amp fuse with a 30 amp fuse. If there is a fault the fuse resistor will not open like it is designed to do.
The thermistor will get hot, as its temperature goes up its resistance will go down. It could be there is still a fault causing the circuit to draw too much current and over heating the thermistor, normally if there was a fault the fuse resistor would open but not when it is a 5watt ceramic resistor.

If the mosfets shorted, they may have damaged the gate resistors or diodes, this would cause the circuit to not operate properly.

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

A good picture of your board would help. Your thermistor is probably bad.

I couldn't find any similar thermistor to prove whit a new one. I removed the thermistor from the power supply, heated it with my hot air gun and its resistance goes down to 0 Ohm and as it cools its resistance goes up. I did this three times and I hade the same results.
I turned on the tv and measured the current with my multimeter in series with the thermistor and got 600, 680, 700mA.
I expected to get a higher consumption.
I hope the pic are goods and you can continue helping me.
If you need better pics let me know.

Regards.

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

installing a 5 watt resistor for a .5 watt is like replacing a 1 amp fuse with a 30 amp fuse. If there is a fault the fuse resistor will not open like it is designed to do.
The thermistor will get hot, as its temperature goes up its resistance will go down. It could be there is still a fault causing the circuit to draw too much current and over heating the thermistor, normally if there was a fault the fuse resistor would open but not when it is a 5watt ceramic resistor.

If the mosfets shorted, they may have damaged the gate resistors or diodes, this would cause the circuit to not operate properly.

I checked the gate resistors and diodes again and the diodes are OK, the resistors are within their value with their respective tolerance.

I couldn't find any similar thermistor to prove whit a new one. I removed the thermistor from the power supply, heated it with my hot air gun and its resistance goes down to 0 Ohm and as it cools its resistance goes up. I did this three times and I hade the same results.

I turned on the tv and measured the current with my multimeter in series with the thermistor and got 600, 680, 700mA.
I expected to get a higher consumption.

Regards.

[dskall]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

With mainboard disconnected your power board doesn't overheat? Have you checked your main board for possible short?

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

With mainboard disconnected your power board doesn't overheat? Have you checked your main board for possible short?

I disconnected the inverter board and only leave connected the main board to power supply and doesn't overheat. Of course, I have 6 blinks meaning backlight because the inverter board is disconnected.
This TV is drivin' me crazy

[dkneyle]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Yeah, I was just working through in my head and got to the same point: LED inverter issue. The SMPS is only drawing too much current when the LED inverter is connected. Of course, this is the main power draw in the TV as well.
I haven't looked close enough at the schematics, but have a close look at what is going on there. Failed FET/s in the inverter would be my first port of call? They do the hard switching work so are most likely to fail.

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Yeah, I was just working through in my head and got to the same point: LED inverter issue. The SMPS is only drawing too much current when the LED inverter is connected. Of course, this is the main power draw in the TV as well.
I haven't looked close enough at the schematics, but have a close look at what is going on there. Failed FET/s in the inverter would be my first port of call? They do the hard switching work so are most likely to fail.

I thought the same but
If the SMPS draws too much current, why when I measure the DC current in series with the thermistor I get 600~700mA?

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

When I activate the SMPS manually by making a jumper between the STBY and POWER ON pins, I get 389V in the main filter, but when the SMPS is mounted on the TV and with everything connected, I get 375V in the main filter.

[dskall]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

What are the voltages when tv is on?

[dkneyle]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

I thought the same but
If the SMPS draws too much current, why when I measure the DC current in series with the thermistor I get 600~700mA?

That's about 250 watts drain, which seems much too much for a 40 inch LED TV.

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

What are the voltages when tv is on?

I made a mistake measuring the PFC voltage, sorry. I was taking measurements of the heat sink which is not a good ground.

I have 388.5V on the main filter when the TV is on and 158 when the TV is off.

Attached images of the voltages obtained in the connectors.

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

That's about 250 watts drain, which seems much too much for a 40 inch LED TV.

I think you're wrong. Of course, 250W is too much, it's even above the consumption marked on the label of this television.
But I got 600~700mA (0.6~0.7A) and if I'm not mistaken that means around 66~77W.
Mi line voltage is 110V.
The label on this TV says a power consumption of 158 W.
It's an LCD TV. Its backlight is by CCFL lamps.

[dskall]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Are voltages the same when the tv doesn't come on?

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Are voltages the same when the tv doesn't come on?

Do you mean the first time I press the power button?

[Quantum IC]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

When I press the power button for the first time after plugging in the TV, the voltages are present for about 1-2 seconds and then drop to 0V, except for the STBY 3.3V and AC OFF DET 3V voltages, which are always present.
Then when I press the power button again the TV turns on and all the voltages are present.

[dskall]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

I wouldn't think all voltages are the same if tv shuts back down. Tv sensing some problem the first time.

[budwich]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Question about your "thermistor". Normally, it is used to sense temperature of the surrounding area not how much current is going thru it. Having said that, it appears that yours is an "NTC" type going to 0 when heated. I have seen a motor thermistor go bad whereby the "junction" would heat up causing the device to "trigger" even though the surroundings were basically cold. A thermal laser unit could detect a temperature "spike" at the lead connection going into the device while the device itself was basically room temperature. Replacing the device solved the issue... that might be your best route.

[nomoresonys]

Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Maybe ohm out the, what is it 3 transformers on the inverter board, readings are taken with tv unplugged from power. Thinking they should be close in readings to each other, careful very high voltage can be present even when tv is unplugged if recently powered, not same tv but video example there: https://www.youtube.com/watch?v=lvBo3vetuGs