lg 2234s no power at all

[vagos_89]

Hello all and sorry for my english.

I found an lg w2234s monitor without power board and i bought one from ebay. He said tha was 100% tested working but the monitor doesn't work. I dont have the 12V and 5V to the output connector. I tested alla diodes on board and are good as the electrolytic capacitors are good. Can anyone help me? What could i look?
Also on the big electrolytic capacitor(150μF/450V) i have only 20V. I think that is no good..
I could take and clear pictures if necessary..

Thans in advance!

[vagos_89]

Re: lg 2234s no power at all

I forgot to say that i tested the ccfl lamps with an inverter and are all good.

[Porto]

Re: lg 2234s no power at all

Well, if the big filter cap on the primary side is not reaching 160v or 320v, then I bet that powerboard is no good at all. Try measuring the input voltage from the terminals itself to the rectifier and look what that pin 1 on the rectifier is giving you as output voltage to the filter cap, if it is also 20v then I guess the rectifier itself or something before that is at fault. Also measure between pin 2 and 3 on AC voltage setting, it has to be your inlet voltage. The board can't start with 20v mains so solve that low input voltage first.

[budm]

Re: lg 2234s no power at all

"Also on the big electrolytic capacitor(150μF/450V) i have only 20V. I think that is no good.." What do you use for GND ref point for the meter? You are working in the primary side of the circuit so you cannot use chassis as the GND ref. for the meter, the negative leg of the main filter cap is the GDN ref for the primary circuit.

[vagos_89]

Re: lg 2234s no power at all

Well, if the big filter cap on the primary side is not reaching 160v or 320v, then I bet that powerboard is no good at all. Try measuring the input voltage from the terminals itself to the rectifier and look what that pin 1 on the rectifier is giving you as output voltage to the filter cap, if it is also 20v then I guess the rectifier itself or something before that is at fault.


Thanks for your response!

The fuse F801 in the primary is good. I tested the bridge rectifier and is good too. In the pin1 of the rectifier i have 12V, pin2 120V, pin3 115V and pin4 11V.


Also measure between pin 2 and 3 on AC voltage setting, it has to be your inlet voltage. The board can't start with 20v mains so solve that low input voltage first.

If i understand you mean the AC voltage which enters into the board from the main socket. I have 120V too.

Also i found this mosfet on the board

but the service manual says that Q804 mosfet is this

I'll post and two pics from board and service manual link

[vagos_89]

Re: lg 2234s no power at all

"Also on the big electrolytic capacitor(150μF/450V) i have only 20V. I think that is no good.." What do you use for GND ref point for the meter? You are working in the primary side of the circuit so you cannot use chassis as the GND ref. for the meter, the negative leg of the main filter cap is the GDN ref for the primary circuit.

I dont use chassis for my measurements. I use the negative leg of the cap as you said.

[budm]

Re: lg 2234s no power at all

"pin2 120V, pin3 115V" those are two AC input pins for the rectifier which are connected to the incoming AC Line and Neutral so you are not using the correct GDN ref point, if you measure between pin 2 and 3 you will see 120VAC.

[vagos_89]

Re: lg 2234s no power at all

two pics of the board

[budm]

Re: lg 2234s no power at all

I dont use chassis for my measurements. I use the negative leg of the cap as you said.

"In the pin1 of the rectifier i have 12V, pin2 120V, pin3 115V and pin4 11V."
That cannot be since the negative leg of the cap is tied directly to the (-) out of the rectifier pin 4.
You have measurement error.

[vagos_89]

Re: lg 2234s no power at all

"pin2 120V, pin3 115V" those are two AC input pins for the rectifier which are connected to the incoming AC Line and Neutral so you are not using the correct GDN ref point, if you measure between pin 2 and 3 you will see 120VAC.

pin2 and pin 3 i mean the pins in rectifier as it seems in second pic

[vagos_89]

Re: lg 2234s no power at all

"In the pin1 of the rectifier i have 12V, pin2 120V, pin3 115V and pin4 11V."
That cannot be since the negative leg of the cap is tied directly to the (-) out of the rectifier pin 4.
You have measurement error.

You have right. i have pin1 0V pin2,3 120V and pin4 i cant take measurement the voltage alterate continuously.

[budm]

Re: lg 2234s no power at all

"You have right. i have pin1 0V pin2,3 120V and pin4 i cant take measurement the voltage alterate continuously."
So what DCV do you get between the two outer pins (pin 1 (+) and pin 4 (-)) of the bridge rectifier? are you sure your meter is in DCV mode? if it is in DCV then you may have really bad filter cap or bad rectifier.

[Porto]

Re: lg 2234s no power at all

If you have 0v on pin 1 of the rectifier, I would change that first and see if you get the needed 160v (DC) output between pin 1 (+) and 4 (-). My guess is the rectifier is open and thus not passing the voltage internally from pins 2/3 to pin 1.

Those black rectifiers are very common, if you have a broken power supply from a PC or other mains input apparatus, desolder it from there. I just don't hope the problem is in the transformer right next to it?!

[budm]

Re: lg 2234s no power at all

I wonder if he count the pin correctly since pin 1 is the (+) output of the bridge and pin 4 is (-) output which is connected to the (-) pin of the main filter so if he is saying he is using the (-) pin of the main filter cap as the GND ref for the meter, then there is no way he will have Voltage reading at pin 4 since pin 4 is tied to the Negative leg of the main filter cap. It is hard to help when the measurement are done wrong.

[Porto]

Re: lg 2234s no power at all

I always use the ground, pin 4, of the rectifier als ref ground on the primary.
On the secondary I use the ground of the output, but it's also important to use the right measuring method, DC or AC, on the DMM because the values will be quite different and can lead someone on the wrong track. Took myself an whole hour before I discovered this when I started with electronics.

[vagos_89]

Re: lg 2234s no power at all

So i think now i take correct measurements. Between pin2 and 3 of recrtifier i have 226V AC and between pin1 and 4 i have 320V DC. I remove dthe rectifier i tested and is good. between negative leg of the capacitor and pins of the rectifier i have: pin4 0V pin3 105V, pin2 105V and pin1 320V.

I see that the capacitor(150μF/450V) doesnt discharge completely when i stop the power, holds about 20-30V even an hour later.

Thanks for your advices and your patience!

[Porto]

Re: lg 2234s no power at all

ah oke, now we're getting somewhere!

Your countries mains voltage is 230v so 320v on the filter cap is correct.
This is also saying that the primary is working.

Maybe next step is checking that N-chan mosfet (Q804, AP2761i)
Desolder it and check for shorts / continuity between all the (three) leads.
(Recently, I had an Acer monitor with exactly this mosfet which was overheating so maybe trying another just to be sure is a good idea).

A mosfet can be checked this way:

- put DMM in diode mode
- fix the black probe on pin 3 (source)
- put the red probe on pin 1 for a few seconds (gate)
- put the red probe on pin 2 (drain) and see it's been charged.
- keep the red probe on pin 2 and briefly touch pin 1 with the black probe and put it back on pin 3 again... if you get zero (0) charge on your DMM the mosfet is working OK.

I once built this very simple mosfet tester to test them quickly... no hassle anymore with those DMM probes. Can be build perfectly on a breadboard.

Also the mosfet driver IC (IC802, LD7522) can be broken but I don't know how to test those, I just replace them when I suspect a broken one.

I just don't hope for you that the main transformer is shorted!
Do you have a voltage on the transformer pins on the secundary side?

[budm]

Re: lg 2234s no power at all

The picture of the bottom side of the board is cut off and cannot see the pins of the connector that goes to the main board.
It may have bad startup-running cap c807, you need to measure the DCV between the TWO LEGS of that cap, be real careful not to touch anything since you are working in the primary side.
Also check the DCV on the two red wires of the connector against the secondary side GND without having the cable connected to the main board.

[vagos_89]

Re: lg 2234s no power at all

As i said to the post #5 i found FQPF8N60C mosfet instead AP2761I-A as service manual.(i bought the biard from ebay) i'm in work now when i'll go home i'll test the mosfet, the transformer on the secondary and i'll measure the DC of c807 as budm says..

also i'll take a better picture of bottom side.

To the connector with two red wires i dont have any voltage. I tested with main board connected and not already. Should i have ~12V on power pins(red wires) and 5V(pin on/off), right??

[Porto]

Re: lg 2234s no power at all

Ah, sorry, forgot about that mosfet being another one.

Maybe the board was already repaired once, hence the other mosfet type.
I would now surely test it because maybe it failed again.