Badcaps.net Forum
Go Back   Badcaps Forums > Troubleshooting Hardware & Devices and Electronics Theory > Troubleshooting TVs and Video Sources
Register FAQ Calendar Search Today's Posts Mark Forums Read

 
Thread Tools Display Modes
Old 01-14-2022, 12:06 AM   #1
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

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.
Attached Images
File Type: jpg fuse_resitor.jpg (806.3 KB, 42 views)
Attached Files
File Type: pdf kdl32bx320.pdf (2.42 MB, 35 views)

Last edited by Quantum IC; 01-14-2022 at 12:27 AM..
Quantum IC is offline   Reply With Quote
Old 01-14-2022, 11:00 AM   #2
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,642
Default 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 assume no responsibility for any stupid suggestions I might post.

Last edited by dskall; 01-14-2022 at 11:11 AM..
dskall is offline   Reply With Quote
Old 01-14-2022, 11:10 AM   #3
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 7,962
Default 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.

Last edited by R_J; 01-14-2022 at 11:26 AM..
R_J is offline   Reply With Quote
Old 01-16-2022, 08:32 PM   #4
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by dskall View Post
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.
Attached Images
File Type: jpg SMPSup.jpg (445.2 KB, 16 views)
File Type: jpg smpspry_1.jpg (573.5 KB, 14 views)
File Type: jpg smpssec.jpg (581.1 KB, 12 views)
File Type: jpg smpsup_pry.jpg (559.6 KB, 13 views)
File Type: jpg 5.jpg (746.3 KB, 16 views)
File Type: jpg 9.jpg (604.6 KB, 10 views)
File Type: jpg 11.jpg (767.9 KB, 8 views)
File Type: jpg 12.jpg (962.4 KB, 8 views)

Last edited by Quantum IC; 01-16-2022 at 08:47 PM..
Quantum IC is offline   Reply With Quote
Old 01-16-2022, 08:44 PM   #5
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by R_J View Post
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.

Last edited by Quantum IC; 01-16-2022 at 08:48 PM..
Quantum IC is offline   Reply With Quote
Old 01-16-2022, 11:10 PM   #6
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,642
Default 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?
dskall is offline   Reply With Quote
Old 01-16-2022, 11:37 PM   #7
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by dskall View Post
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

Last edited by Quantum IC; 01-16-2022 at 11:41 PM..
Quantum IC is offline   Reply With Quote
Old 01-17-2022, 04:47 AM   #8
dkneyle
Badcaps Veteran
 
Join Date: Dec 2013
City & State: Tathra, New South Wales
My Country: Australia
Line Voltage: 230VAC 50Hz
I'm a: Knowledge Seeker
Posts: 398
Default 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.
dkneyle is offline   Reply With Quote
Old 01-23-2022, 08:09 PM   #9
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by dkneyle View Post
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 is offline   Reply With Quote
Old 01-23-2022, 08:28 PM   #10
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default 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.
Quantum IC is offline   Reply With Quote
Old 01-23-2022, 09:56 PM   #11
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,642
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

What are the voltages when tv is on?
dskall is offline   Reply With Quote
Old 01-23-2022, 11:22 PM   #12
dkneyle
Badcaps Veteran
 
Join Date: Dec 2013
City & State: Tathra, New South Wales
My Country: Australia
Line Voltage: 230VAC 50Hz
I'm a: Knowledge Seeker
Posts: 398
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by Quantum IC View Post
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.
dkneyle is offline   Reply With Quote
Old 01-24-2022, 07:07 PM   #13
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by dskall View Post
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.
Attached Images
File Type: jpg Voltages CN6401.jpg (49.4 KB, 7 views)
File Type: jpg Voltages CN6402.jpg (24.8 KB, 7 views)

Last edited by Quantum IC; 01-24-2022 at 07:59 PM..
Quantum IC is offline   Reply With Quote
Old 01-24-2022, 07:58 PM   #14
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by dkneyle View Post
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.
Quantum IC is offline   Reply With Quote
Old 01-24-2022, 08:30 PM   #15
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,642
Default 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?
dskall is offline   Reply With Quote
Old 01-24-2022, 09:01 PM   #16
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default Re: Sony KDL-40BX450 LCD. Thermistor gets too hot 10 seconds after turning on the TV

Quote:
Originally Posted by dskall View Post
Are voltages the same when the tv doesn't come on?
Do you mean the first time I press the power button?
Quantum IC is offline   Reply With Quote
Old 01-26-2022, 04:05 PM   #17
Quantum IC
New Member
 
Join Date: Sep 2021
City & State: New York
My Country: United States
Line Voltage: 110VAC 60Hz
I'm a: Knowledge Seeker
Posts: 16
Default 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.

Last edited by Quantum IC; 01-26-2022 at 04:07 PM..
Quantum IC is offline   Reply With Quote
Old 01-26-2022, 04:30 PM   #18
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,642
Default 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.
dskall is offline   Reply With Quote
Old 01-26-2022, 07:18 PM   #19
budwich
Badcaps Veteran
 
Join Date: Jul 2015
City & State: Kanata Ontario
My Country: Canada
I'm a: Knowledge Seeker
Posts: 2,901
Default 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.
budwich is offline   Reply With Quote
Old 01-27-2022, 03:59 PM   #20
nomoresonys
Badcaps Veteran
 
Join Date: Jan 2013
City & State: midwest
My Country: U.S.
I'm a: Knowledge Seeker
Posts: 7,628
Default 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

Last edited by nomoresonys; 01-27-2022 at 04:26 PM..
nomoresonys is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump



Badcaps.net Technical Forums 2003 - 2022
Powered by vBulletin ®
Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
All times are GMT -6. The time now is 12:16 AM.
Did you find this forum helpful?