LCD tv power/inverter board

[Franc]

Totally new at pcb repair but think there's a possibility in doing the repair myself, just not sure what to do next. Any help or suggestions would be appreciated.

Problem, 19 inch haier tv powers up, red green flashing light, then constant green but no image or sound.
I've located a burn chip (me4542) on the power/inverter board. To make a long story short, I have acquired another identical board but thins one does not power up at all. I am hoping that the inverter part of the second pcb is functional and that I can repair the power supply with parts from the first board. The only thing is I do not know how to test the components on the board.
Any tutorial geared toward what I need to do? Any suggestions on how to start?

Board is a jsi-150211-050. The chip that is burned on the original board is part of the inverter and is surface mount type.
I figure repairing the power supply on the second board would be easier than trying to solder a new chip 8sop chip.

 

[Franc]

Follow up question

To test the different component on the ps side of the board is it possible to do this while still on board or do I have to remove each one and test them individually.

So far I get 120 ac coming into from mains then I get a 60 vac after the transformer(step down I believe). After there I am a little lost.
Any help, suggestion or reading material(links) would really help out a newbie.

 

[jackorocko]

Check the bridge rectifier. We are still talking about the second one that doesn't work at all right?

 

[davenn]

hi Franc,
welcome to the forums

show us sharp and easy to view pics of the PSU boards... the original and this second one you have

one of us may see some obvious faults

Dave

 

[Franc]

Yes we are still talking about the second, the one that is not functioning. The bridge rectifier test ok. Just to be sure i tested both boards with the BR onboard.(hope this is ok)

Here are a few pics.
the board on the right is the original and the other is the replacement which i got for free and doesnt work.( but it was supposed to)
As you can see the original inverter had a burned chip and i damaged the board when i was trying to remove it.
This board still puts out the required 12v dc and powers on the lcd but makes a ticking sound after with no picture and no sound.
The second one does nothing at all.
Hope this helps in an effort to help me.
Thx for the replies.

 

[jackorocko]

What is that black stuff on the bottom of the second board with the missing chip? Is this the second board you where talking about?

edit: guess i should read before posting, you already explained which board was what.

 

[Franc]

The black stuff near the bar code is the remnants of the me4542 chip. It exploded from it corner.
Also if you look at the top right side of the same board you'll notice a burn mark from the transformer arcing. Not sure if this is serious, definitely not good but the ps still works.

Pictures aren't super clear but considering the damage(I've done) should I try to repair and replace the burned chip or continue trying to get the one on the left working even though I am not sure the inverter works?

 

[davenn]

.........
So far I get 120 ac coming into from mains then I get a 60 vac after the transformer(step down I believe). After there I am a little lost.
Any help, suggestion or reading material(links) would really help out a newbie.

its a little more complex than that, this is called a switch mode power supply, SMPS for short.
The 120V AC coming in gets rectified and you have ~ 220VDC that is switched very fast (40kHz or so) by that transistor on the heatsink to the right of the yellow taped transformer.
it is then fed to the transformer then stepped down and rectified to as mant regulated voltages rails as designed for that PSU.

there is also feedback circuitry from the secondary to the switching side that is also used to control voltage regulation.

Dave

 

[davenn]

you could carefully measure the voltage across that cap and see what it is ... I would expect ~ 200 - 250VDC


now that other section on the top side of the board that is labelled COLD, in fact is an area that can bite! that is the 1 to 3kV generator for the fluro tubes for the LCD screen

when you look on the bottom side atthe board that had cooked that IC just to the left of the vertical white stripe, those 2 IC's are part of the oscillator cct that drives that small flat transformer on the left edge of the PCB.


Without some years of experience, you are really biting off a bit more than you can chew trying to fix these boards. I have done a lot of LCD monitor type repairs and these days, if I cant see an obvious fix within 30 mins to an hr, I will toss the board and get a replacement (assuming a replacement doesnt cost more than 1/2 the cost of the monitor.
its really just not worth the screwing around with them, they will drive you crazy!!!

Dave

 

[Franc]

Thanks for all the useful info, it is greatly appreciated. It does seem like I am biting off to much to chew but think I will keep at it for sometime.
I did test the big cap on both boards and got 168vdc.
I did some other poking around with my multimeter and found that I got 10vdc coming from the chip pc815(left of the yellow transformer) on the original board but not the second one. This chip straddles the hot/cold line. I am guessing the problem is either this chip or something before it.

 

[davenn]

...I did some other poking around with my multimeter and found that I got 10vdc coming from the chip pc815(left of the yellow transformer) on the original board but not the second one. This chip straddles the hot/cold line. I am guessing the problem is either this chip or something before it.

Thats an opto-coupler its providing an isolated feedback signal from the LVDC side of the supply to the HVDC side. It in itself doesnt generate a voltage You are measuring that 10V across a collector-emitter junction of a transistor ( in this opto ... its a darlington transistor configuration. The LED inside the opto is in the LVDC side of the circuit its brightness will vary in accordance to its monitoring of one of the DC output rails. That varying LED brightness will cause the C/E junction to conduct more or less. This will control the PWM oscillator on the HVDC primary side and will adjust the duty cycle of the main switching transistor.

cheers
Dave

 

[Franc]

Here's an updated.
Finally decided to bring the 2 boards to some one that knows what they are doing. Turns out there was an ic on the hot side that was burnt and also a dead diode near the opto-coupler. So we made one board from two.*
Backlight now functions, as in I see it light up but unfortunately I still do not get any picture. When I power it up I get a green light, see the backlight flicker and hear the speakers crackle on. After that I get nothing.*
From reading numerous forums and how-to(s) I have come the the assumption that it must be the t-con board. I have checked the fuses(2) on this board (they are good). So basically I am I am stuck again.
Any quick tips for checking a t-con that I might not of read? Should I just cut my loses at this point and junk it. A screen is too expensive to justify buying one. *
Either way it has been a learning experience.*

 

[(*steve*)]

What is the t-con board?

Do you have any idea of what the failure mode may have been (e.g. just loss of power, c.f. unregulated power to the boards)?

Are the power rails now correct?

 

[Franc]

If by the rails you mean the 12vdc to the main board then yes, everything tests fine from the power supply/inverter board.
The tcon board is the timer control board of the actual LCD screen.
I figure if I see the backlight power up, hear the speakers, it mustn't be the inverter or main board.
Am I right in thinking this.

 

[(*steve*)]

Not sure. If it was just something to do with the display then you should be able to tune in a channel and hear the sound.