Dead 19" LG Flatron

[Solder Boy]

Hi guys been awhile since I've been here. I have a dead LCD monitor here and just looking to see if my gut instinct is right in suspecting a bad bridge rectifier.

Model L1953TQ.

Original Symptom- No power no blinking light. Cracked it open and found a bulged cap in the output area. Tested the other caps all were fine except for one small one so I replaced that too. Applied power to unit.. still nothing.

AC voltage at BR is 120-123. Checked DC voltage at the big capacitor.. reading was 0V. Checked the voltages to the video board.. on the 12V all I got was 3V something and the 5V rail read 0.3V

Turning the power on and off made no difference on the AC side of the BR voltage remained the same at 120V.

Checked the main fuse.. it was fine.

This is for a client of mine.

[Rtech]

Re: Dead 19" LG Flatron

Chances are that should there have been a short cct anywhere, then the fuse would probably have blown,so the Bridge certainly looks favourite,HOWEVER don'tunderstand how you can measure 3volts after the big cap when there is 0 volts on the cap ?????
if no DC on the Cap then there is none anywhere else ???

[Solder Boy]

Re: Dead 19" LG Flatron

I don't understand it either which is why I came here seeking input.

I forgot to mention that for the hell of it I turned the selector on the DC section of my DVM to way below 20V and I did get a reading but it was very low. Go back to 20V and I get zero.

[jetadm123]

Re: Dead 19" LG Flatron

Are measuring directly across the + and - leads of the large cap? If yes, is your meter working correctly? Try measuring a 9 volt battery to verify it's working ok.

[Th3_uN1Qu3]

Re: Dead 19" LG Flatron

Are you sure you don't get ANY voltage to the primary cap? If so, you have either a blown trace between the bridge rectifier and the cap, bad soldering, or an OPEN bridge rectifier (use diode test on your multimeter to confirm this). They rarely fail open but it happens sometimes.

[Solder Boy]

Re: Dead 19" LG Flatron

Hi.. yes my DV meter is working. Tested the 9V battery from my capacitance meter and it read 9V.

Edit: Ok I took another look at this thing... Gawd I feel like a dum dum.... I get about 165VDC on the primary cap.. I can't tell what the capacitance is supposed to be but the primary cap says 450V.. I'm guessing something is obviously out of whack but not quite sure what? Transformer maybe?

[PlainBill]

Re: Dead 19" LG Flatron

Hi.. yes my DV meter is working. Tested the 9V battery from my capacitance meter and it read 9V.

Edit: Ok I took another look at this thing... Gawd I feel like a dum dum.... I get about 165VDC on the primary cap.. I can't tell what the capacitance is supposed to be but the primary cap says 450V.. I'm guessing something is obviously out of whack but not quite sure what? Transformer maybe?

Problem is likely on the 5V supply rail. With power off, measure the resistance from the 5V rail to ground. If it reads less than 10 ohms, remove D202 and check it.

PlainBill

[Th3_uN1Qu3]

Re: Dead 19" LG Flatron

Green caps... aren't those Samxon GF? If so, those may be your issue. If the diode PlainBill asked you to check is okay, then change out those caps on the 5v rail. Even if they look fine.

[Solder Boy]

Re: Dead 19" LG Flatron

Ok I changed out all those caps.. did not change a thing.. rechecked the VDC on the primary cap.. still 167VDC not 450.

I pulled out D202 to check it out of circuit.. Part# MBRF10100CT

>. reading is 210 with Diode selection on DVM (+ -)
>.reading is open (-+)

.< reading is 210 (. +)
.< reading is open (+ .)

Not sure if I did this right or if I am missing a step

[Th3_uN1Qu3]

Re: Dead 19" LG Flatron

The diode checks good.

As for the primary cap - that power supply does not have Active PFC. You live in a 120v area, right? The primary cap is 450v for 230v countries - and even there it only gets to see 320v or so, but it seems the demand for Active PFC in power supplies have driven down the prices of 400 and 450v caps hence why they are used even in supplies that would have did fine with a lower rating.

[PlainBill]

Re: Dead 19" LG Flatron

Ok I changed out all those caps.. did not change a thing.. rechecked the VDC on the primary cap.. still 167VDC not 450.

I pulled out D202 to check it out of circuit.. Part# MBRF10100CT

>. reading is 210 with Diode selection on DVM (+ -)
>.reading is open (-+)

.< reading is 210 (. +)
.< reading is open (+ .)

Not sure if I did this right or if I am missing a step

Something just percolated through the gray matter. In an earlier problem, one of the inverter drivers was defective, but not shorted. It overloaded the 12V supply.

The picture of the top is too small to really help, so I'm forced to work from the picture of the back side.

Referring to the 11 pin connector, the two bottom pins are 12V, the next two are Gnd, then next two are 5V. With power off, measure the resistance from Gnd to 5V and from Gnd to 12V. In each case, I would expect a resistance of over 20 ohms.

If you haven't already done so, reinstall D202, since that tested good.

I'm having a problem tracing the circuitry around U201. I can't figure out if the 12V or the 5V rail is regulated.

PlainBill

[Th3_uN1Qu3]

Re: Dead 19" LG Flatron

I'm having a problem tracing the circuitry around U201. I can't figure out if the 12V or the 5V rail is regulated.

C'mon now, i wasn't expecting to hear something like that from you. The 5v always is, the 12v usually isn't.

[Solder Boy]

Re: Dead 19" LG Flatron

Thanks for the info Bill I will do a follow up tomorrow..

[PlainBill]

Re: Dead 19" LG Flatron

C'mon now, i wasn't expecting to hear something like that from you. The 5v always is, the 12v usually isn't.

Not always. Samsung has regulated the 12V (15V?) supply in some of their monitors.

PlainBill

[Solder Boy]

Re: Dead 19" LG Flatron

Just a follow up from yesterday.. haven't done that other test yet that you mentioned Bill but this morning I found something that looks like a Picofuse.. it's labeled FB101 on the board. I don't recognize the symbol.

I also took a better shot of that U101 IC you mentioned as well.

Please see attachments.

[jetadm123]

Re: Dead 19" LG Flatron

FB101 is not a fuse, it's a Ferrite Bead (what the "FB" stands for). Can't read the part number on U101. Can you please tell us what the part number is?

[Solder Boy]

Re: Dead 19" LG Flatron

Part # for U101 is: FAN7601 VG51

Part # PC201: CC642 EL8

Thanks for the info about FB101

Update.. checked the otherl diodes and Schottkey Diodes they are fine.. just have a question about this other part. It was labeled Q101 on the board.. pulled it out tried googling the numbers on it.. no luck. FM G46 FQPF 8N60C.

[PlainBill]

Re: Dead 19" LG Flatron

Part # for U101 is: FAN7601 VG51

Part # PC201: CC642 EL8

Thanks for the info about FB101

Update.. checked the otherl diodes and Schottkey Diodes they are fine.. just have a question about this other part. It was labeled Q101 on the board.. pulled it out tried googling the numbers on it.. no luck. FM G46 FQPF 8N60C.

FQPF8N60C is the relevant number. Hint: Google each block of numbers and the word datasheet. The first hit when Googling 8N60C datasheet brings you to alldatasheet.com, which reports that FQPF8N60CF is a 600 Volt mosfet.

PlainBill

[Solder Boy]

Re: Dead 19" LG Flatron

Thanks Bill I spoke to the owner of this thing yesterday and told him I'm determined to find the culrpit which is preventing this thing from firing up lol.

At this point I'm suspecting this MOSFET or the two IC chips or all 3.

[PlainBill]

Re: Dead 19" LG Flatron

Thanks Bill I spoke to the owner of this thing yesterday and told him I'm determined to find the culrpit which is preventing this thing from firing up lol.

At this point I'm suspecting this MOSFET or the two IC chips or all 3.

Probably none of them. The fact that you do get some output voltage indicates the problem is on the secondary side, or the loads.

I see two approaches. The preferred approach may take a little while. I have marked off an area of the bottom of the power supply in the attached picture. I would like a good quality picture of the area within the yellow lines. I need it sharp and clear because I am going to have to follow some circuit paths.

The riskier approach is to unsolder one end of J1 (circled in black). This will disable the inverter. If the power supply now works, the problem is in the inverter.

PlainBill