LG Flatron L17NT-A no power

[Elmojo]

Hi all,
I just stumbled upon this site and am impressed by the knowledge and helpfulness of the members here.
I'm hoping you LCD gurus can help me as well.
I was given a nonfunctional LG Flatron L17NT-A monitor.
It displays no power light or any sign of life at all.
I have confirmed that the power cord is supplying power (via multimeter) and that all the internal connections appear solid.
I have the monitor opened up and see no signs of scorching or other obvious damage, though I honestly have not done an exhaustive search with a flashlight and magnifier. I can confirm that I'm getting 120v at least into the main board, but that's as far as I've gone.
I'm fairly comfortable with poking around inside equipment of all sorts, but LCDs are a bit foreign to me. I know there's some pretty high voltages in here, so I'm being cautious.
Can anyone get me started in the right direction for checking this thing out?
A schematic or service manual would be awesome, but I suspect your first-hand knowledge will be more helpful anyway.
Thanks in advance,
Elmojo

[Elmojo]

Re: LG Flatron L17NT-A no power

Sorry for the redundany post, but I can't figure out how to edit my original post.
I thought a couple pictures might help.




Thanks,
Elmojo

[PlainBill]

Re: LG Flatron L17NT-A no power

Sorry for the redundany post, but I can't figure out how to edit my original post.
I thought a couple pictures might help.

Thanks,
Elmojo

A request - instead of embedding pictures, attach them by scrolling down to Additional Options and using 'Manage Attachments'. I mention this because it is a PITA to view an embedded picture while replying to a post.

1. Carefully measure the voltage across the large cap with the monitor plugged in. It should be about 1.4 times your nominal line voltage.
2. If this is correct, measure the output voltages of the power supply.
3. Attach a picture of the power supply / inverter board. It appears a couple of caps are bulging, but the small size of the picture makes it difficult to be sure.

PlainBill

[EGuevarae]

Re: LG Flatron L17NT-A no power

3. Attach a picture of the power supply / inverter board. It appears a couple of caps are bulging, but the small size of the picture makes it difficult to be sure.

PlainBill

Agree. Look like we have some bulged Samxons

[Elmojo]

Re: LG Flatron L17NT-A no power

HI guys,
Sorry it took me so long to reply.
I thought the board would notify me of replies, so I assumed no one had answered!
Oh well.
Anyway, thanks for the tips.

PlainBill: When you say, measure across the large cap, do you mean between the 2 pins on the large black cap in the bottom center of the orange board on the second photo?
As for the buldging caps, I assume you mean the greenish ones on the opposite side from the big black cap?
I didn't even notice those until you mentioned it!
I'll fix the photos also.
Thanks again for your past and future help!
Elmojo

EDIT: How can I edit my earlier posts to fix the photo screwup?
I only get an [edit] button for the last post!

[Elmojo]

Re: LG Flatron L17NT-A no power

Okay, I took some better photos.
I hope these aren't too big, I chopped them down a lot in Photoshop, but they are still pretty detailed.
I can clearly see at least 1 bulging cap. Anyone see any other suspicious areas?
Also, if I replace the bad caps, am I likely to have a working monitor or are there probably other issues in play?
@Bill: In your reply, you suggested that I measure the output voltage from the power supply. Where on the board should I take that measurement? I'm not too good at reading/tracing PCB routing.
Thanks again for any/all advice,
Elmojo

PS: Someone PLEASE fix the thread editing (or lack thereof) on this forum!

[Elmojo]

Re: LG Flatron L17NT-A no power

I measured across the pins of the big black cap and got basically 0. It briefly jumps up when first connected, then quickly flatlines at 0.
Each time I do it, I get a different "spike", but the highest I've seen so far is 63v.
Is this the expected behavior or have we possibly found another issue?

[PlainBill]

Re: LG Flatron L17NT-A no power

Okay, I took some better photos.
I hope these aren't too big, I chopped them down a lot in Photoshop, but they are still pretty detailed.
I can clearly see at least 1 bulging cap. Anyone see any other suspicious areas?
Also, if I replace the bad caps, am I likely to have a working monitor or are there probably other issues in play?
@Bill: In your reply, you suggested that I measure the output voltage from the power supply. Where on the board should I take that measurement? I'm not too good at reading/tracing PCB routing.
Thanks again for any/all advice,
Elmojo

PS: Someone PLEASE fix the thread editing (or lack thereof) on this forum!

I see 1 cap that is definitely bulging, and a couple more that are suspicious.
The bulging caps MAY be the cause of the 'no power' symptom, another cap may be responsible, or it may be on the logic board. Hang onto your hat, you are going to get a speedy tutorial in troubleshooting power supply problems.

A short summary - all LCD monitors I have encountered use a similar concept. The power LED is controlled by the video processor on the logic card. A problem on the logic card MAY result in what appears to be a totally dead monitor.

I prefer to use a 'divide and conquer' troubleshooting approach. Others use a 'walk through' approach; losers use a 'shotgun' approach (replace parts until you irreparably damage the board or it starts working).

The first step of my approach is to CAREFULLY measure the voltage across C101 with the power cord plugged in. This should read approximately 1.4 times your nominal line voltage. In the USA this would be about 165 VDC. If this is correct we know the outlet, power cord, fuse, and bridge rectifier are all good, and the main filter cap is probably good.

The next step is to switch to the 20VDC scale on your meter and hold the black lead on the screw labeled PG2 (near the 6 caps); then probe each pin on the 11 or 12 pin connector immeadiately below PG2. If any pin reads either 5V or 12V you probably have a working power supply. If you measure both 5V and 12V you definitely have a working power supply.

If NONE of the pins show a voltage of 5V or greater, you have a bad power supply. In that case, the likely suspects are C103 and C105.

At this point you have to make a decision. Do you recap the power supply at a cost of approximately $10, or do you give up.

There is nothing to 'fix' about the Edit button. You are only allowed to edit a post for the first 10 minutes after you post it. In the immortal words of Steve Jobs, 'That's not a bug, that's a feature'.

PlainBill

[PlainBill]

Re: LG Flatron L17NT-A no power

I measured across the pins of the big black cap and got basically 0. It briefly jumps up when first connected, then quickly flatlines at 0.
Each time I do it, I get a different "spike", but the highest I've seen so far is 63v.
Is this the expected behavior or have we possibly found another issue?

How did you measure this? That is very strange behavior. The meter should be set to the 200VDC or 1000VDC range and the monitor should be connected to AC power. If you were doing that, unplug the power cord, change the meter to the 200 Ohm scale and check the resistance of the fuse (it's the cylinder covered in heat shrink tubing next to the large blue box (capacitor).

PlainBill

[Elmojo]

Re: LG Flatron L17NT-A no power

DCV! Jeez, I'm an idiot!
Okay, now that I have basic multimeter operation 101 figured out, I have more details to report based on your suggestions.
C101=164V yay!
11-pin connector (CW201) = No power on any pin, other than the 2 on the end farthest from PG2, and they are both 0.14v.
C103 and 105 both "look" okay, but I'm cool with replacing them if that is likely to be the issue.
What about the obviously bulging cap (C206)? What subsystem is that part of?
I don't mind spending a little on this monitor. After all, it was free.
There is a practical limit where we give it and send it to the great desktop in the sky.
For now, let's pursue the "fix it" option.
Speaking of fixing, that 10 minute timer on the edit button is retarded. It makes the threads unnecessarily complicated and cluttered. I could have appended most of our conversation to the first post and kept this much cleaner.
Oh well, I don't mind if no one else does.
One other question: After unplugging the AC power, is there a safe place to short out to drain the power from the caps so I don't fry something while poking around later? How long do these caps hold a charge after losing AC power?
Thanks,
Elmojo

[PlainBill]

Re: LG Flatron L17NT-A no power

OK, you appear to have a dead power supply. There are a couple of tests we can do, but I will need the part number of the SMPS controller, U101.

Ordinarily the large cap will be discharged when the monitor is unplugged. This doesn't happen if the power supply isn't working, though. The best way to discharge it is to take short across the leads with a 100 ohm resistor for a couple of minutes. Just be sure to remove the resistor before plugging the monitor back in.

One problem with allowing unlimited editing is some people have the habit of going back and adding information to earlier posts. While it's nice to be able to remove mistakes, it would be a pain to have to reread every post in the thread to see what has changed.

PlainBill

[Elmojo]

Re: LG Flatron L17NT-A no power

U101 is:
FAN7601
VF52
Thanks!

[PlainBill]

Re: LG Flatron L17NT-A no power

U101 is:
FAN7601
VF52
Thanks!

That was easy. The datasheet is here and the application note is here.

Pin 1 is the startup voltage source; pin 7 is Vcc (operating voltage source). Check the voltage on pin 1 of U101 by setting the DMM to the 200VDC range and holding the red probe on pin 1 and the black probe on pin 5. I would expect you see 164 volts (or a large fraction of that voltage).

By the symptoms, either there is a shorted diode in the secondary of the transformer or C103 or C105 is bad. Based on experience, I would suspect the caps. It's time you made a list of all caps on the power supply. Include manufacturer, series, value, voltage, diameter, and height.

PlainBill

[Elmojo]

Re: LG Flatron L17NT-A no power

Man are you quick!
I'll check those voltages when I get home and get you a list of the cap info.
How do I know which caps are part of the PSU?
Elmojo

[PlainBill]

Re: LG Flatron L17NT-A no power

Man are you quick!
I'll check those voltages when I get home and get you a list of the cap info.
How do I know which caps are part of the PSU?
Elmojo

Look at the very first picture you posted. They're the 6 electrolytic caps at the left end of the power supply and the two small electrolytic caps near U101.

PlainBill

[Wizard]

Re: LG Flatron L17NT-A no power

Replace those capacitors FIRST then troubleshoot if new caps doesn't fix the problem.

Cheers, Wizard

[Elmojo]

Re: LG Flatron L17NT-A no power

So 8 caps total to start, right?
Where's the best place to get those?
I assume the host of this site sells caps, right?

[PlainBill]

Re: LG Flatron L17NT-A no power

So 8 caps total to start, right?
Where's the best place to get those?
I assume the host of this site sells caps, right?

Yes, he does, but he is more oriented towards motherboards.

My preferred replacement caps are Panasonic FM or FC series from Digikey. Digikey offers first class mail as a shipping option. It puts the parts in your hands in two days for under $3.00 for shipping. Mouser is another good supplier, but you will pay over $8 for UPS ground shipping, IIRC.

PlainBill

[Elmojo]

Re: LG Flatron L17NT-A no power

Cool, I'll check that out.
So what info do I need off the caps to order the correct replacements?
I see the uf rating and some other data.
For example: GF(M) V5A 1000uf 25V
There appear to be 3 different caps in the goup of 6, plus the other 2 smaller ones.
Should I list them all here and let you ID them or will it be obvious when I get to digikey?
Okay, I went to digikey and tried to search, but couldn't find anything to match my specs. I'll post my list when I get home and see if you can do better.
Thanks,
Elmojo

[Elmojo]

Re: LG Flatron L17NT-A no power

Okay, so I finally got around to getting up the list of my caps.
I'm not too familiar with the markings, so I'll include everything.
There are 8 total, as you mentioned.
They are all Samxon, all 105°C6, and all GF(M) unless otherwise noted:
(2) 1000μf, 25v, V5A, 10mmx20mm
(2) 1000μf, 16v, C6A, 10mmx20mm
(1) 680μf, 25v, V4A, 10mmx20mm
(1) 470μf, 25v, V4A, 10mmx15mm
c103: 33μf, 50v, V5A, GF(M), 7mmx12mm
c105: 0.47μf, 50v, V5A, KM(M), 5mmx10mm

Does that give you what you need?
I'm going to try looking them up again now, but I wasn't having much success last night, so your help would be appreciated.
Thanks!
Elmojo