Samsung SyncMaster 720N - power LED blinks

[dvorav]

Hello everyone,

I have got broken Samsng SynsMaster 720N. There is no picture when turned on. But there is also power led is blinking.
All caps seem fine, they are Suscon's (I don't know the quality - yet). Can bad caps do the blinking? Or is it probably something else? I have checked solder joints as well - all little bit oxidised, but I don't think it is the main source of the problem.
I'll be glad to see your opinion.

dvorav

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

Further testing showed, that monitor can't be even turned off - it is turned on permanently.

[PlainBill]

Re: Samsung SyncMaster 720N - power LED blinks

Have you checked the power supply output voltage? The blinking power LED may indicate a fault condition.

PlainBill

[Bobdee]

Re: Samsung SyncMaster 720N - power LED blinks

I have same symptoms on LG Flatron, after about 1 minute of flashing on/off, my bridge rectifier is cooking,but the odd thing is when I check it , it checks out OK, but I have heard of faulty ones doing just that, might be worth checking yours out

[EGuevarae]

Re: Samsung SyncMaster 720N - power LED blinks

Further testing showed, that monitor can't be even turned off - it is turned on permanently.

I now have that problem with one monitor. The fault (in my case) is the video board. Also, I had that problem with a Proview 568, and it ended up being the caps at the video board. The voltages out of the PSU are stable?

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

I have measured the PSU. The main capacitor has voltage around 100V against ground, it was stable. I haven't measured output voltage to the CCFL because I'm not sure about the voltage - 2000V is too high for my equipment to measure. Once the CCFL went on for short time, I suppose, that they are good. During the measurement LED stopped blinking for a while. There was such a high buzzing noise. Then it stopped. LED wasn't shining any more since then. But the buzzing noise continued to appear every 2 minutes or so.
Any ideas?

Thanks in advance.

[Scenic]

Re: Samsung SyncMaster 720N - power LED blinks

about 100V on the primary cap seems "a bit" too low...
seems like something's shorted out or something...

any components getting hot...?

i'd suggest to replace all caps at first. Su'scon isn't what i'd call reliable.. :S

[EGuevarae]

Re: Samsung SyncMaster 720N - power LED blinks

I have measured the PSU. The main capacitor has voltage around 100V against ground, it was stable. I haven't measured output voltage to the CCFL because I'm not sure about the voltage - 2000V is too high for my equipment to measure. Once the CCFL went on for short time, I suppose, that they are good. During the measurement LED stopped blinking for a while. There was such a high buzzing noise. Then it stopped. LED wasn't shining any more since then. But the buzzing noise continued to appear every 2 minutes or so.
Any ideas?

Thanks in advance.

When I suggested to check if the voltages were stable, I was referring to the voltages in this output:


Your voltages are stable?

• Between 5 & 6 (GND & BLON) there must be voltage with the monitor ON and with signal (VGA cable connected). That is the line that turns on the lamps.Normally 5v.
• Between 6 & 1 (or 6 & 2) should be 5v also.

And one more thing: 100 v in the big guy seems too low to me.

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

When I suggested to check if the voltages were stable, I was referring to the voltages in this output:

Your voltages are stable?

• Between 5 & 6 (GND & BLON) there must be voltage with the monitor ON and with signal (VGA cable connected). That is the line that turns on the lamps.Normally 5v.
• Between 6 & 1 (or 6 & 2) should be 5v also.

And one more thing: 100 v in the big guy seems too low to me.

Fitst of all, thanks for contributing.

1) When VGA cable connected, I was possible to turn it on (power LED was shining). Then I measured 3.21V between 5 & 6. After while the LED started blinking. I measured the 5 & 6 again obtaining 0.02V (almost zero). That's the problem.
Voltage measured between 6 & 1 was 5.08 - I suppose that is ok.

2) I was confused how to measure that big cap. I measured AC between legs and ground (now I know it wasn't proper) resulting in around 130V. But then I measured voltage between legs - 346V or so. It seems better.

And the conclusion is - bad signal board? Or maybe several caps on signal board are bad?

[PlainBill]

Re: Samsung SyncMaster 720N - power LED blinks

Check the voltage on the big cap across the legs of the cap. You can't get a meaningful reading to ground. If you're running the monitor on 120 VAC, the voltage across the cap should be 165 - 170 VDC.

Don't bother trying to read the voltage out of the inverter. It's a high frequency, high voltage signal. It takes a special meter to get an accurate reading.

EDIT. OK, you already did that. 340V is about right if you're running it at 240VAC. I assume you're in Europe where the use REAL electricity, not the wimpy stuff we have here in the USA.

PlainBill

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

Well - I got it now (too slow ) but if output voltage from PSU isn't stable - it must be the PSU. But where is the source of that problem?

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

Check the voltage on the big cap across the legs of the cap. You can't get a meaningful reading to ground. If you're running the monitor on 120 VAC, the voltage across the cap should be 165 - 170 VDC.

Don't bother trying to read the voltage out of the inverter. It's a high frequency, high voltage signal. It takes a special meter to get an accurate reading.

Thanks for remark.

I have realised I wasn't right. I'm running that monitor on 230V AC so reading 346V or so is meaningfull.

Edit: to: PlainBill Exactly Europe.

[PlainBill]

Re: Samsung SyncMaster 720N - power LED blinks

Well - I got it now (too slow ) but if output voltage from PSU isn't stable - it must be the PSU. But where is the source of that problem?

A quick lesson. The power supply converts line voltage to DC, in this case it outputs 5VDC for the logic board and probably 12VDC for the inverter (which happens to be on the same board). The logic board feeds two signals back to the inverter - On/Off and Brightness. When the power LED starts blinking the logic board also instructs the inverter to turn off. That's why pin 5 dropped from 3.21V (On) to 0V (Off).

One question - was the VGA cable connected to a WORKING computer - one with an active output?

PlainBill

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

to: Scenic There aren't any components radiating extraordinary heat. But the caps may be bad. I'll try to exchange them.

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

A quick lesson. The power supply converts line voltage to DC, in this case it outputs 5VDC for the logic board and probably 12VDC for the inverter (which happens to be on the same board). The logic board feeds two signals back to the inverter - On/Off and Brightness. When the power LED starts blinking the logic board also instructs the inverter to turn off. That's why pin 5 dropped from 3.21V (On) to 0V (Off).

One question - was the VGA cable connected to a WORKING computer - one with an active output?

PlainBill

Thanks for explanation.

If I understand it right, PSU giving 3.21V. Thus signal board evaluates the PSU signal as faulty and then turns the PSU and itself off.

I'm testing it on my PC from where I'm actually writing. VGA was connected to running secondary monitor - I used it as a source.

[PlainBill]

Re: Samsung SyncMaster 720N - power LED blinks

Thanks for explanation.

If I understand it right, PSU giving 3.21V. Thus signal board evaluates the PSU signal as faulty and then turns the PSU and itself off.

No, the signal is going in the opposite direction.

Pins 1 & 2 are 5V from the PSU to the signal (logic) board 5.08V is right for this.
Pins 3 & 6 are Ground
Pins 4 & 5 control the inverter (the power supply for the backlights). 4 is brightness, 5 is On / Off

I'm testing it on my PC from where I'm actually writing. VGA was connected to running secondary monitor - I used it as a source.

That would indicate the problem is on the signal board. For some reason it's not recognizing the VGA signal.

PlainBill

[EGuevarae]

Re: Samsung SyncMaster 720N - power LED blinks

No, the signal is going in the opposite direction.

Pins 1 & 2 are 5V from the PSU to the signal (logic) board 5.08V is right for this.
Pins 3 & 6 are Ground
Pins 4 & 5 control the inverter (the power supply for the backlights). 4 is brightness, 5 is On / Off



That would indicate the problem is on the signal board. For some reason it's not recognizing the VGA signal.

PlainBill

Exactly.
Think of it like this :
The PSU has the inverter on the same board, but it does not know when to turn the lamps on or off. It pumps out 5v (5.08v in your case) to the Video/Signal/Main board for it's components to work, and internally, via jumpers, delivers between 12 and 14v to the inverter part of the board.
The VBoard process the signal input, and if there's valid signal, it send a signal (5v normally) back to the inverter to signal it that the Back Light should be turned ON (that's why normally this line is called BLON, BL_ON, or, in your case, ON/OFF). The line called BRI (Brightness) is used to control the intensity of the lamps when you modify the bright setting. It is variable.
If you remove the signal, the VBoard removes the signal from the BL_ON line and the inverter shuts off. This is under normal conditions. If the VBoard has any fault, and the V on the BLON line cuts or varies, the lamps would be turned off.
That's why I (tried to) told you in post #5. Now, with the readings you posted, it makes more sense. Nw we must give proper attention to the signal board. Post a picture and info on the caps present there, please, so we can continue the trip to the other side of the monitor.

[dvorav]

Re: Samsung SyncMaster 720N - power LED blinks

Thanks for patience with me. I have great regard for it.

I have taken some pictures of the Video Board. All caps are Su'scons. Although they are in good visual state, they may be faulty. Complete list and placement of the caps is attached.

Is there chance, that the problem is in faulty caps?

[EGuevarae]

Re: Samsung SyncMaster 720N - power LED blinks

Thanks for patience with me. I have great regard for it.

I have taken some pictures of the Video Board. All caps are Su'scons. Although they are in good visual state, they may be faulty. Complete list and placement of the caps is attached.

Is there chance, that the problem is in faulty caps?

Well, in my case with the 568, they were G-Luxon, and I ended up changing all of them. Take a look at this thread

[h4rdw4re]

Re: Samsung SyncMaster 720N - power LED blinks

Thanks for patience with me. I have great regard for it.

I have taken some pictures of the Video Board. All caps are Su'scons. Although they are in good visual state, they may be faulty. Complete list and placement of the caps is attached.

Is there chance, that the problem is in faulty caps?

Hi,
same situation here.
Power led slow blinking and won't turn on.
All capacitor look good, but I suspect to Video board.
Connect ON/OFF pin to 5V and backlight turn on, but blinking just like pwer led does, 5V voltage are stable, but on video board that voltage change its value from 5V to few mV, it seems that some kind of short circuit is there...
Now I'm trying to find what cause that situation...still not done replace capacitor on video board.
Any sugestions?
Do I go to the right direction (suspectiong on video board, nor power board)?
Sorry for my poor English.
Regards to all.

Ups, just saw that this post is more than 3 years old!
Sorry for that.