LG Flatron L1942T-PF

[mikey5791]

Got this LG Flatron L1942T-PF monitor free from a friend but the problem is it does not have display power (power board model LGP-002L) and lacks a main cpu at Q101. After searching thru service manual,I installed a mosfet SSS7N60B at Q101, replaced all the secondary filter caps. Then I connect a 60W light bulb in series at F101,switched on mains power,the light bulb flash a second,then lights off. This indicating there is no shorted component.

FYI, I have a digitalmultimeter(DMM), capacitor meter and a 40W soldering
iron with basic skills in replacing bulging capacitors and revive dead LCD monitors.

First,on the powerboard, I need to be sure there is poweroutput from P201 white connector where I perform DC voltage test by touching red probe to label 5V and black pin to cold ground (screw pad) but see no volt reading(zero volt) on DMM,the 5V standby is not present.
Later on I perform the same DCvoltage test(red probe to cathode) on the following with the corresponding reading:

C101 main cap=286vDC
Q101(middle pin)=108VDC
D102 diode= -109vDC( negative reading?)
D107 diode=109vDC
D106=109vDC
D101=109vDC
D105 & D104= no DC at all

On power IC at U101(img1817)it is label LAF0001,pin 1=Vstr,pin 2=CS/FB,pin 5=Ground,pin 7=Vcc
Putting my red probe to pin 7,black pin to cold ground yield a DC reading -109vDC(negative?)

The opto at PC201(img1817) is label 817C,on diode test,one end on left facing D102 reads "1"= infinite while the other end reads 1335. Is the opto good?

Refer img 1818 On inverter transformer at T301, I set my DMM to resistance test on 2k ohms scale. Reading on one end (1&2) is 1028 while pin 3&4 reads 1024. Guess the trans is good.
BTW, how do I test if the yellow transformer beside maincap is good with my limited DMM. I also attached some photos and service manual.
Hope someone can help me in solving the no power issue. Many thanks in advance.

[mikey5791]

Re: LG Flatron L1942T-PF

Hi,

Done a DC voltage test by red probe to each of the pins on U101 (PWM controller LAF0001) while black probe to ground screw pad on right side of the board. On pin 1 (with a circle denote pin1),which is Vstr the DMM reads 74vDC then slowly drops to zero. On pin 5 Grd,the DMM reads -112vDC, i got same reading -112vDC on pin 6,7,8 as well. Refer img 1769.

Does the above result conclude the U101 is bad since according to Fairchild pinout diagram (as attached), Vcc(pin 7) absolute maximum ratings should be at 20V while pin8(Vref) should be 5V. Also I did DC voltage test on diode D102, putting the DMM red probe to diode cathode(diode end with black strip) while the black probe to negative leg of main cap C101 or to cold ground(screw pad). The DMM reads zero. Is the diode D102 bad? Please advise. Thanks and I am still trying to get power from this power board.

[budm]

Re: LG Flatron L1942T-PF

When you make the DCV readings in the DC primary side of the circuit, you have to use the negative leg of the main filter cap in the primary side as the GND ref. for your meter.
The problem can be as simple as the small startup/running electrolytics caps C103, 105 in the primary side.
Was the original power MOSFET Q101 bad? The solder works need to be clean up, it does not look good at all.
We will need the DCV readings again using the correct GND ref point in the primary side.
You can use chassis and the GND ref. point when you make DCV measurement in the cold side of the circuit.

[mikey5791]

Re: LG Flatron L1942T-PF

Hi budm,

On the primary side of circuit and by using black DMM probe to negative leg of the main filter cap at C101,positive red DMM probe to positive leg of main cap while 60 watt series light bulb in place of fuse, DCV reading is 308vDC.

Later on I proceed using red probe to cathode(black strip)of diode as follows with DCV reading (black probe still on negative leg of main cap) :

D107=308vDC
D106=308vDC
D105=124vDC
D104=136vDC
D101=308vDC
Q101(mosfet SSS7N60B middle pin)=308vDC
D102= zero reading
IC U101 (pin 1)=252vDC
All the other 7 pins=zero reading

On the secondary side of circuit and by using black DMM probe to chassis screw pad as ground , positive red DMM probe to cathode side of diode D102, the reading is (negative)-121vDC. However, if the same red probe touched U101 pin 1, the DMM reading is from 21vDC then drops to zero.Then the red probe touched U101 pin 2 to pin 8, each of the pin reads at negative -119vDC. I attached img1799 where I marked HOT ground and COLD ground as reference point. Am I probing correctly? Hope if I am wrong here, please direct me. Thank you for helping to point out in earlier post.

[budm]

Re: LG Flatron L1942T-PF

Hi budm,

On the primary side of circuit and by using black DMM probe to negative leg of the main filter cap at C101,positive red DMM probe to positive leg of main cap while 60 watt series light bulb in place of fuse, DCV reading is 308vDC.

Later on I proceed using red probe to cathode(black strip)of diode as follows with DCV reading (black probe still on negative leg of main cap) :

D107=308vDC
D106=308vDC
D105=124vDC
D104=136vDC
D101=308vDC
Q101(mosfet SSS7N60B middle pin)=308vDC
D102= zero reading
IC U101 (pin 1)=252vDC
All the other 7 pins=zero reading

On the secondary side of circuit and by using black DMM probe to chassis screw pad as ground , positive red DMM probe to cathode side of diode D102, the reading is (negative)-121vDC. However, if the same red probe touched U101 pin 1, the DMM reading is from 21vDC then drops to zero.Then the red probe touched U101 pin 2 to pin 8, each of the pin reads at negative -119vDC. I attached img1799 where I marked HOT ground and COLD ground as reference point. Am I probing correctly? Hope if I am wrong here, please direct me. Thank you for helping to point out in earlier post.

OK based on your DCV readings at the SMPS IC, you do have Voltage feeding the Vstart pin 1 to feed the charge pump circuit to charge the startup cap connected to the VCC pin 7, that cap should charge up till it reaches 12V startup Voltage for the IC to start running, but you are getting 0 VDC on the VCC pin, so it is either bad IC, or there is something loading down that VCC pin. You need to check the resistance between VCC pin and GND pin to see what you have. More likely the IC is bad.

[mikey5791]

Re: LG Flatron L1942T-PF

Hi budm,

Checked resistance by setting DMM to ohm scale at 200, then probe to VCC pin 7 and other probe to GND pin 5 and DMM show 64.8 ohm

Curiously, I also checked resistor R107(Grey,Red,Yellow,Gold) and DMM show 840K on 2000K ohm scale. The reading of 840K keeps going up and increasing while I probed on resistor still on circuit. Is it because the resistor both legs are affected by other components on board?

Checked smd resistor R117 &R118(both had tiny mark 244),DMM reads 220K on 2000k ohm scale. Both resistors are at the back part of power board. Kindly advise & thanks.

[budm]

Re: LG Flatron L1942T-PF

Hi budm,

Checked resistance by setting DMM to ohm scale at 200, then probe to VCC pin 7 and other probe to GND pin 5 and DMM show 64.8 ohm

Curiously, I also checked resistor R107(Grey,Red,Yellow,Gold) and DMM show 840K on 2000K ohm scale. The reading of 840K keeps going up and increasing while I probed on resistor still on circuit. Is it because the resistor both legs are affected by other components on board?

Checked smd resistor R117 &R118(both had tiny mark 244),DMM reads 220K on 2000k ohm scale. Both resistors are at the back part of power board. Kindly advise & thanks.

64.8 Ohms between VCC and GND is definitely bad, you need to lift the VCC pin off the board or remove the IC then check the empty VCC pad and GND to see if it still show around 60 Ohms or not, if not then the IC is bad.

[mikey5791]

Re: LG Flatron L1942T-PF

Hi budm,

I have not yet lift the VCC pin off the board and measure empty VCC pad and GND but surely will do the IC unsolder soon.

Out of suspicion, i measured resistance of those tiny smd resistor at the back side of power board and noticed three resistors worth further checking.
Setting my DMM to 200 ohm scale, I got the two following readings (the resistor marking are in bracket for your reference):

R122(4R7)= DMM reads 80.8 ohm- is too high ? (but 4R7 actual value is 4.7 ohm)
R106(472)= DMM reads 100.1 ohm- is too low ? (but 472 actual value is 4.7k ohm)

Both the above two resistors are slightly below Mosfet Q101 and the resistance value is way off. Does this affect any output power or obstruct the output?
Another resistor is at R102 which i can't see the marking but the schematic list it as 5.8k ohm, my DMM reads 6k ohm. Guess this is still ok.

Kindly advise if I need to change any of the above resistor. I hope this power board will be an easy fix and save the monitor from landfill. Thank you again for assisting the repair.

[mikey5791]

Re: LG Flatron L1942T-PF

Hi,

It seems like the IC at U101 being LAF0001 needs a replacement; which is not easy to get at our part of the country. I have checked the local store but they do not have it.
So i checked online at utsource.net, the cost for 1 piece is USD 1.00 but shipping is USD 20.00; this is way too much a bomb and over my expected budget !

I am thinking "ghetto way" of connecting an external power supply and re using the power board (let it "as is" condition without fuse) as a connecting board to the vga display board. Has anyone done this before?
My choice of external power supply is either a 12 V(6000mA) power adapter or Liteon 19V (3.42A) laptop power adapter or a switching ATX 250W power supply supplying at DC 12V (9A). Pictures of the power board and external power as attached. Which one is the best option as external power supply and which part or position I need to connect to provide the required 22V and 5V.
Will connect the external power to say C207 (desolder the cap at C207,then connect external power positive to cap positive position, negative power to cap negative position) be suffice ?

This is my last desperate option to supply power to the board. If this way is not workable, the power board will go to the bin.
Hoping to hear any suggestion. Thank you.