7" LCD backlight problem - LMS700KF07-004

[Onur Karagül]

Hi guys,

Here i have one issue with my 7" Lcd backlight is not working. I was using it in the car but after few days stop to work. I've checked with a flashlight and i can see picture. Nothing is burned on the board, a coil and microcontroller is hotter then before. I am noob for electronic things but learning. The monitor is LMS700KF07-004, and i bought it from ebay. Its hard to contact to seller for circuit of the board. Could you please tell me where can be the problem ?



**broken link removed**

Thanks in advance,

 

[udayan92]

a coil and microcontroller is hotter then before

This means that somewhere,on the PCB,shorting is taking place.The coil and micro-controller are getting hot due to excess current being driven through them,and the cause for excess current being pulled is a short circuit,somewhere.I suggest you to check for short circuit between connections,using the circuit diagram of the system for reference.
Also in the picture you uploaded,two black capacitors have been highlighted,with a red square around them.Is it that those two capacitors are the ones which are getting hot,or is it something else,associated with them?

 

[burt007]

Check the voltage going to the back light if voltage is present than the tube may be gone. If no voltage is there then power supply.

Hi guys,

Here i have one issue with my 7" Lcd backlight is not working. I was using it in the car but after few days stop to work. I've checked with a flashlight and i can see picture. Nothing is burned on the board, a coil and microcontroller is hotter then before. I am noob for electronic things but learning. The monitor is LMS700KF07-004, and i bought it from ebay. Its hard to contact to seller for circuit of the board. Could you please tell me where can be the problem ?

View attachment 104123

**broken link removed**

Thanks in advance,

 

[Onur Karagül]

Re: 7" LCD backlight problem - LMS700KF07-004

This means that somewhere,on the PCB,shorting is taking place.The coil and micro-controller are getting hot due to excess current being driven through them,and the cause for excess current being pulled is a short circuit,somewhere.I suggest you to check for short circuit between connections,using the circuit diagram of the system for reference.
Also in the picture you uploaded,two black capacitors have been highlighted,with a red square around them.Is it that those two capacitors are the ones which are getting hot,or is it something else,associated with them?

Hi, highlighted capacitor and coil is getting hot. And i changed highlighted capacitors. What i realized now, if you take a look under highlighted capacitor, there is small micro-controller named "S142749" which i can not find any details on internet. This micro-controller is incredible hot. Coil "220" as well. The main question is, there is 6 pin cable goes to the LCD. and i am getting around 5volt for LCD backlight. Isn't it too low ?

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Check the voltage going to the back light if voltage is present than the tube may be gone. If no voltage is there then power supply.

Yes i checked, around 5 volt for each pin goes to the backlight. there is 25 Leds as backlight. I am sure 5v is not enough.

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By the way, seller told me that this monitor works within cigarette port on any car (12v) and warned me that use 2A fuse for this. I checked car's voltage is 12.2v during standby, but after start-up it gives 14.6v. Maybe thats the way monitor get over voltage. And i am gonna make voltage regulator for this.

 

[burt007]

Re: 7" LCD backlight problem - LMS700KF07-004

No need to make voltage regulator for car, they know this, it is normal and it is not the problem. Also disconnect the backlight and than measure the voltage if it goes higher and how high? Also it is possible that back light itself may be the problem, so if you can supply external voltage to the backlight, do it and see if it lights up. Make sure you observe the polarity of the voltage and have a variable powersupply so start with 5 volts and gradualy increase to see if it turns on.

Hi, highlighted capacitor and coil is getting hot. And i changed highlighted capacitors. What i realized now, if you take a look under highlighted capacitor, there is small micro-controller named "S142749" which i can not find any details on internet. This micro-controller is incredible hot. Coil "220" as well. The main question is, there is 6 pin cable goes to the LCD. and i am getting around 5volt for LCD backlight. Isn't it too low ?

- - - Updated - - -



Yes i checked, around 5 volt for each pin goes to the backlight. there is 25 Leds as backlight. I am sure 5v is not enough.

- - - Updated - - -

By the way, seller told me that this monitor works within cigarette port on any car (12v) and warned me that use 2A fuse for this. I checked car's voltage is 12.2v during standby, but after start-up it gives 14.6v. Maybe thats the way monitor get over voltage. And i am gonna make voltage regulator for this.

 

[Onur Karagül]

Re: 7" LCD backlight problem - LMS700KF07-004

No need to make voltage regulator for car, they know this, it is normal and it is not the problem. Also disconnect the backlight and than measure the voltage if it goes higher and how high? Also it is possible that back light itself may be the problem, so if you can supply external voltage to the backlight, do it and see if it lights up. Make sure you observe the polarity of the voltage and have a variable powersupply so start with 5 volts and gradualy increase to see if it turns on.

Now i did whatever you told, disconnected backlight and measure the voltage, still 5v. Gave external voltage to the backlight started as 5v and increase, no light. So that means led lights gonna be changed.

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I take out the led strip, and tested each led gives response. But when i give power to parallel leds, wont work (more than 1 led). Led strip has no resistors. I am little bit confused now. The problem is led strip or resistors on the board ?

 

[Onur Karagül]

 

[burt007]

Re: 7" LCD backlight problem - LMS700KF07-004

To me it seems the led strip is the problem but not all may be one of them is bad but when you apply the power to them they must light up. You will have to do a little bit of work as to find out which one is bad and replace either that one or the whole strip. But if I were you I would concentrate on led lights because that is where is the problem. I mean voltage is there and they don't turn on. Find out if they are in series or parallel I guess they must be in series so if one is open none of them work so you can find out which one is gone and then just short that one just for testing purpose to see if they all light up and than get either one or the whole strip but don't use it with shorted led because the rest will have a slightly higher voltage and may destroy the one more and so on. So test the rest of them and see which one is bad.

Now i did whatever you told, disconnected backlight and measure the voltage, still 5v. Gave external voltage to the backlight started as 5v and increase, no light. So that means led lights gonna be changed.

- - - Updated - - -

I take out the led strip, and tested each led gives response. But when i give power to parallel leds, wont work (more than 1 led). Led strip has no resistors. I am little bit confused now. The problem is led strip or resistors on the board ?

 

[Onur Karagül]

Re: 7" LCD backlight problem - LMS700KF07-004

Hi guys, sorry for late response due to illness and moving home staff. We found the problem comes from one component, and this is MIC2297-42YML 40V Led driver. Which was incredible hot and gives 5v to led strips. And this part is not easy to find so I ordered for 5$ from Hong Kong again. Once it came, i will share the resolution.

Thank you all for your responses and being interest.
Onur Karagul