Lenovo li2341twf Monitor - No Power

[mikey32230]

TLDR: Brand new monitor turned on for a few seconds, wont power on anymore. Motherboard has open unused spots for capacitors... (I don't know what that means). Where should I start / Advice on what to do?

Longer verison:
Hello, so I bought a Lenovo li2341twf touch screen monitor about a year ago. Upon opening and first installation, it powered on for only a few seconds and then turned back off. We ordered a replacement, but lenovo never wanted the original monitor back.. so now I have this broken one laying around and I would love to be able to repair it.

I am a bit of a noob when it comes to this, but I love to learn.
I have torn the monitor apart, and observed what i could. All the capacitors look physically okay, no bulging, but I did notice that there are a few places on the board marked for capacitors that do not have any capacitors in place. I do not know what voltage or uF capacity should go there because... nothing was there before.

Should I even try to install capacitors there? Where should I start / what should I do?

[Porto]

Re: Lenovo li2341twf Monitor - No Power

If there aren't any caps present at that spot, it don't uses those caps.
Probably that board is used in multiple screens in their assortment; probably those caps are used for circuitry your specific monitor don't use / have.
You can open your working monitor and use that as a reference, not only to find out those caps are also not present in your working monitor but also for present voltages on certain points.
Maybe you can lay the monitors side by side, both monitors powered up, measuring voltages at certain points and verify that voltage on the broken board.
Maybe only some mosfet or voltage regulator has blown.


Is the fuse still good?

That monitor uses an external power supply, did you test it with your working monitor?

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

If there aren't any caps present at that spot, it don't uses those caps.
Probably that board is used in multiple screens in their assortment; probably those caps are used for circuitry your specific monitor don't use / have.
You can open your working monitor and use that as a reference, not only to find out those caps are also not present in your working monitor but also for present voltages on certain points.
Maybe you can lay the monitors side by side, both monitors powered up, measuring voltages at certain points and verify that voltage on the broken board.
Maybe only some mosfet or voltage regulator has blown.


Is the fuse still good?

That monitor uses an external power supply, did you test it with your working monitor?

Yes, I tested the external power supply, that is not the problem.

Unfortunately, I have no idea what the fuse is on this board. It does not have any fuses that I am familiar with.

I dont know what a mosfet is, or where it is on this board.. from what the pictures look like on google. It looks like there are a few mosfets on this board.

Can you help me find the fuse? and if we find it how can it test it? do i need desolder it first?

[budm]

Re: Lenovo li2341twf Monitor - No Power

Is that the only circuit board in the unit? How about the picture that show the whole back side so we can see how all the boards are connected together?
I can see the main SDM fuse F901 next to C919 which is not installed.
There may be more than one fuse on that board, so look for Fxx. F = Fuse

[Porto]

Re: Lenovo li2341twf Monitor - No Power

Fuse don't have to be desoldered to test.
If you have a digital multimeter, set it on the Ohm (resistance) range and measure the fuse with black probe on one end and the red probe on the other; it must give some 0.2-0.3 Ohm for a good fuse.

Testing mosfets are a bit more tricky but I can't identify the mosfets on your board because of the resolution. But since this is not the power supply itself, maybe the culprit is rather more a blown diode or a voltage regulator (LMxxxx) for one of the power rails.

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

Is that the only circuit board in the unit? How about the picture that show the whole back side so we can see how all the boards are connected together?
I can see the main SDM fuse F901 next to C919 which is not installed.
There may be more than one fuse on that board, so look for Fxx. F = Fuse

Fuse don't have to be desoldered to test.
If you have a digital multimeter, set it on the Ohm (resistance) range and measure the fuse with black probe on one end and the red probe on the other; it must give some 0.2-0.3 Ohm for a good fuse.

Testing mosfets are a bit more tricky but I can't identify the mosfets on your board because of the resolution. But since this is not the power supply itself, maybe the culprit is rather more a blown diode or a voltage regulator (LMxxxx) for one of the power rails.

Here are some better pictures possibly? I will try to test any fuses i find.

How would I find a blown diode / voltage regulator?

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

Fuse don't have to be desoldered to test.
If you have a digital multimeter, set it on the Ohm (resistance) range and measure the fuse with black probe on one end and the red probe on the other; it must give some 0.2-0.3 Ohm for a good fuse.

Testing mosfets are a bit more tricky but I can't identify the mosfets on your board because of the resolution. But since this is not the power supply itself, maybe the culprit is rather more a blown diode or a voltage regulator (LMxxxx) for one of the power rails.

The fuse seems to be good. There only appears to be one fuse. Which is marked K on the lower right hand corner of the board

[Porto]

Re: Lenovo li2341twf Monitor - No Power

I believe I see two voltage regulators, the ones with '1117' on it, the parts with the center tab attached to those pink/orange colored spots.

pin 1 is ground, pin 2 is output and pin 3 is input.

Can you measure the voltages on pin 2 and 3 of those regulators?

Without the schematics it's a wild guess what voltage it has to output but my guess is either 3.3v or 5v to power some IC.

The diodes are the ones beginning with a 'D' on the board. You can measure them with the multimeter on Diode mode, the black probe on the side with the mark (most of the time a white line) and the red probe on the other; a good diode must show its forward voltage, 0.3 to 0.7v mostly. The probes reversed must show nothing or a few volts (when measuring in-circuit). Diode testing better be done out of circuit but that can be tricky when dealing with SMD diodes and you don't have the proper soldering tools to (de)solder SMD components (hot air station).

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

I believe I see two voltage regulators, the ones with '1117' on it, the parts with the center tab attached to those pink/orange colored spots.

pin 1 is ground, pin 2 is output and pin 3 is input.

Can you measure the voltages on pin 2 and 3 of those regulators?

Without the schematics it's a wild guess what voltage it has to output but my guess is either 3.3v or 5v to power some IC.

The diodes are the ones beginning with a 'D' on the board. You can measure them with the multimeter on Diode mode, the black probe on the side with the mark (most of the time a white line) and the red probe on the other; a good diode must show its forward voltage, 0.3 to 0.7v mostly. The probes reversed must show nothing or a few volts (when measuring in-circuit). Diode testing better be done out of circuit but that can be tricky when dealing with SMD diodes and you don't have the proper soldering tools to (de)solder SMD components (hot air station).

Im sorry, can you circle the 1117 parts you see. I'm not seeing those any where.

[Porto]

Re: Lenovo li2341twf Monitor - No Power

I've circled the two (sot-223 package) voltage regulators which I believe are LD1117AC types.

[budm]

Re: Lenovo li2341twf Monitor - No Power

"The fuse seems to be good. There only appears to be one fuse. Which is marked K on the lower right hand corner of the board" Are you taking about the fues I ask you to check? You get DCV at both ends against GND? You should verify that the center pin of the DC Input jack is connected to F901.
The 'K' fuse F502 looks to be the fuse for the USP ports.

Is that the only circuit board in the unit? How about the picture that show the whole back side so we can see how all the boards are connected together?
I can see the main SDM fuse F901 next to C919 which is not installed.
There may be more than one fuse on that board, so look for Fxx. F = Fuse

BTW, there is also a green fuse on the LED driver board.

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

"The fuse seems to be good. There only appears to be one fuse. Which is marked K on the lower right hand corner of the board" Are you taking about the fues I ask you to check? You get DCV at both ends against GND? You should verify that the center pin of the DC Input jack is connected to F901.
The 'K' fuse F502 looks to be the fuse for the USP ports.

I missed fuse F901, but just checked it and it also seems okay. I'm not terribly sure I'm testing them correctly. I'm setting my multimeter to Alarm, and I putting both ends against each solder point. I hear the alarm so that mean current is getting through. I then move the multimeter to OHM (200) and I'm getting readings of about .2

I believe the center pin (power) is connected to fuse F901, when I do the alarm test with the mutimeter one end to center pin, the other end to each side of the fuse I get the alarm sound. I also get about 5.0-8.0 with OHM 200

I'm not sure how to test when you say DCV at both ends against ground.

I just tested the green fuse the same way I said I was testing them above. And I got the same results. It appears okay?

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

"The fuse seems to be good. There only appears to be one fuse. Which is marked K on the lower right hand corner of the board" Are you taking about the fues I ask you to check? You get DCV at both ends against GND? You should verify that the center pin of the DC Input jack is connected to F901.
The 'K' fuse F502 looks to be the fuse for the USP ports.



BTW, there is also a green fuse on the LED driver board.

Okay so I tried to Study up basic electronics so I have a slightly better understanding of things. And I also made a few more discoveries that might be more helpful.
All the fuses appear to be good, they all have continuity.

The power supply is 19V, when applied to the board approximately 19V run through both ends of fuse F901 when shorted to ground.

When I power the board and put my probes to both leads of The two capacitors on the bottom left of the board C908 and C907, they both show 19V On the multimeter.

Something new I noticed is that when I power on the board and connect the board with the power button/menu/etc all the LEDs now light up and, but the chip on that board gets extremely hot. This Happens specifically when the brown and yellow wires are connected on that board.

What does this mean? Is the full 19V getting all the way to that board because of a faulty resistor or capacitor?

[budm]

Re: Lenovo li2341twf Monitor - No Power

'Something new I noticed is that when I power on the board and connect the board with the power button/menu/etc all the LEDs now light up and, but the chip on that board gets extremely hot. This Happens specifically when the brown and yellow wires are connected on that board. '
So the Brown and the Yellow wires were not connected before? Were they like that from the beginning or someone disconnected the wires before you start working on this monitor?
The IC should never get too hot to the touch.

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

'Something new I noticed is that when I power on the board and connect the board with the power button/menu/etc all the LEDs now light up and, but the chip on that board gets extremely hot. This Happens specifically when the brown and yellow wires are connected on that board. '
So the Brown and the Yellow wires were not connected before? Were they like that from the beginning or someone disconnected the wires before you start working on this monitor?
The IC should never get too hot to the touch.

Well it's been like that from the beginning, but I must have forgotten that occurred. I had not fully connected the board together For a long time. I narrowed it down to the yellow and brown wires by taking each wire out of the plastic connector on the power button board and plugging them in. In different combinations.

I know it shouldn't get hot, something is shorted or some resistive component is failing and allowing to much current or voltage to that board

[budm]

Re: Lenovo li2341twf Monitor - No Power

The IC gets hot is due to internal shorts, the IC draws current from the power supply, so if it has internal shorts inside the IC then it will try to draw lots of current from the power supply so it gets hot.
Those wires going to that switch board one of them should be for the power supply VCC fro running the IC, and then it should have a GROUND wire for the current return back to the power supply.
Can you read the P/N on top of the IC? You cn then get the spec sheet of the IC so you can locate the VCC pin and the Ground pin of the IC, at which time you can then check the resistance between the VCC and the GND pin to see if it shows LOW resistance.
You can also disconnect the connector from the board and check the DCV of each pin, I would expect the VCC pin feeding that board to be either 3.3VDC or 5VDC.
That C702 looks to be the filter cap for the VCC feeding that switch board (one end of the cap should be connected to the circuit ground (chassis) and another end should be connected directly to one of the wires. Check the resistance between the two legs of the CAP C702, then check the DCV between the two legs of C702, that C702, no power applied to the board when do the resistance testing.

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

The IC gets hot is due to internal shorts, the IC draws current from the power supply, so if it has internal shorts inside the IC then it will try to draw lots of current from the power supply so it gets hot.
Those wires going to that switch board one of them should be for the power supply VCC fro running the IC, and then it should have a GROUND wire for the current return back to the power supply.
Can you read the P/N on top of the IC? You cn then get the spec sheet of the IC so you can locate the VCC pin and the Ground pin of the IC, at which time you can then check the resistance between the VCC and the GND pin to see if it shows LOW resistance.
You can also disconnect the connector from the board and check the DCV of each pin, I would expect the VCC pin feeding that board to be either 3.3VDC or 5VDC.
That C702 looks to be the filter cap for the VCC feeding that switch board (one end of the cap should be connected to the circuit ground (chassis) and another end should be connected directly to one of the wires. Check the resistance between the two legs of the CAP C702, then check the DCV between the two legs of C702, that C702, no power applied to the board when do the resistance testing.

1) Okay, so for starters the P/N on the IC is:
3584A2-7B
H-N7RP912
ACA130328

I have no idea where to find schematics for any parts in this monitor. I have tried to google various part numbers and nothing comes up. Is there a schematics database you know of that may have any of these schematics?

2) I took off the plastic connector piece (switchboard side) that houses the 6 colored wires. I measured DCV for each pin (red probe on pin, black probe to GND (metal chassis)). This is what the voltages were:

Yellow 2.8-2.9V
Green 1.9-2.0V
Black 1.9-2.0V
Brown 0V
Grey 0V
Red 2.3V

*Brown and Grey seem to a return for power on its way to the mainboard / power supply. A circuit seems to complete itself when both yellow and brown are connected, thats when all the LEDs turn on. I also took these voltages:

Between Yellow & Gray 0.7V
Between Yellow & Brown 3.0V

3) Resistance between 2 legs of C702 = 2.6 Ohms
Voltage between 2 legs of C702 = 2.94Volts

[budm]

Re: Lenovo li2341twf Monitor - No Power

'3) Resistance between 2 legs of C702 = 2.6 Ohms
Voltage between 2 legs of C702 = 2.94Volts'

2.6 Ohms reading, that is bad. Did you measure this without the connector to the switch board in place?
Are you sure now that the IC gets really hot?
So far I have no luck finding the spec sheet for this IC 3584A2-7B.
http://nl.aliexpress.com/store/produ...toreId=1771244

[mikey32230]

Re: Lenovo li2341twf Monitor - No Power

'3) Resistance between 2 legs of C702 = 2.6 Ohms
Voltage between 2 legs of C702 = 2.94Volts'

2.6 Ohms reading, that is bad. Did you measure this without the connector to the switch board in place?
Are you sure now that the IC gets really hot?
So far I have no luck finding the spec sheet for this IC 3584A2-7B.
http://nl.aliexpress.com/store/produ...toreId=1771244

Correct, I measured it without power to the switchboard.

I'm sure that that area of the board gets extremely hot. I'm was not 100% positive the IC is the component that is causing the heat, i thought it could be any of the smaller capacitors, or resistors there that are heating up the entire area including the IC.

So, I just applied power again and i tried to measure heat with a IR thermometer. I'm not sure how precise it is i could get with which component I was measuring, but based on where the laser dot was on the thermometer it appears that it is not the IC that gets the hottest, I think it might be that C702 capacitor or something else closer to the top right of that corner. I was getting temperatures of between 150-200 degrees fahrenheit.

I also double checked the IC P/N and that is the correct number.

[budm]

Re: Lenovo li2341twf Monitor - No Power

The failure mode of the MLCC (Multi Layer ceramic Capacitor) type capacitor is shorted circuit, so try removing that cap and see if the low resistance will go away