Monitor Samsung 172N problem

[renatosc]

Hello. I'm new in the forum and I would like to ask your help.

I've got a Samsung 172n monitor at a thrift store and I'm trying to fix it. The symptom is this - I turn it on the power LED turns on, keeps on a little bit, and turns off. There is no signal of any activity whatsoever. The power board looks ok, there is no signs of overheating or burned parts. The electrolytic capacitors were not bulging, but I changed them all anyway. That made no difference. I checked the voltages at the cable that goes to the main board and the 13v and 5v are there and are steady. So, I got to the conclusion that the power source part of the board is ok and the problem is in the inverter part. Then I checked the voltages on the inverter main IC and everything is almost at zero v. I've changed the IC and 2 transistors in the line that provides the DC to the IC, as well as a diode. That made no difference either.

The part number of the power board is BN44-00092A. Does any of you have the schematics? l've searched a lot for it, but I could not find it on the internet so far. What I found is the schematics of the model BN44-00082A, which is somewhat similar, but not totally. So, without the right schematics I am trying some things, but I'm not totally sure about what I'm doing.

Another doubt I have is this - what is the voltage I would have to expect on the on/off pin? That comes from the main board and seems to be almost zero as well. Does that indicate the problem may be in the main PCB?

I understand a little bit of electronics, but I'm not familiar with this type of power source.

Any ideas? Help would be appreciated..

[selldoor]

Re: Monitor Samsung 172N problem

Whilst you have given a good resume of the problem in order to help we could do with picture of the boards so we know what we are dealing with.
A couple of checks - check that you have replaced the caps the correct way round.
I think on the power board there is a green torpedo fuse- meter on ohms 200 touch the probes together - should be 0.0 or near to. Then with NO power to the board put one probe on each end of the fuse - should be the same as touching the probes together.
Post result.
Welcome to the forum-
We can help a lot more if you please post good clear pictures of the whole chassis, and then pictures of each board, front and back and close up of connectors, (max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

Examples of what is needed
https://www.badcaps.net/forum/attach...1&d=1290283049

https://www.badcaps.net/forum/attach...7&d=1280167246

It will be useful if you can say a bit about yourself - skills tools etc.

[renatosc]

Re: Monitor Samsung 172N problem

OK. About me - I have studied and learned about electronics since I was a kid, but I don't have a strong theoretical basis or formal electronics studies. I work with computers and I understand quite a lot about hardware and software (I'm not a programmer or something like that, it's just that the computer is my main work tool). I have already fixed a lot of electronic devices along life, but what I really like doing is the restoration of old tube sets. So, I can say that electronics is a hobby for me (although it really helps me in other areas of my life.)

Now, returning to the board - I forgot to mention - I had already checked that green fuse and it tests ok, as well as the 2 transformers that do not seem to be shorted, but I don't have a ring test to be 100% certain. If I understand correctly, that type of power source needs a kind o feedback signal to keep working and I thought that maybe it is not receiving it, but how do I test that?

If you have any articles to refer for me to read on how that works, maybe that will be useful.

Thanks for any help.

[Pentium4]

Re: Monitor Samsung 172N problem

It kind of looks like that cap between the transformer and the primary switcher is bulging a little bit, sort of hard to tell from that angle though.

[renatosc]

Re: Monitor Samsung 172N problem

Thanks for the reply, but not really, I've changed all the caps.

[renatosc]

Re: Monitor Samsung 172N problem

An update - By accident, while checking some voltages on the main board, I've shorted what seems to be the so-called ADj_BL line and the On/off line, what caused the lamps to turn on . That means that the inverter is working after all and that I have not messed thing up with my attempted fixes... That makes me believe that the problem in reality is in the main PCB as, according to what I read today, it's the Micom chip on the main board that controls the on/off and the inverter sending the right signal...

[selldoor]

Re: Monitor Samsung 172N problem

Ok- the picture of the main board is not clear enough for troubleshooting and
(This is not aimed just at you - I think I will add it to my standard request for pictures) I cant understand why anyone would post a picture of a board
one side then turn it upside down and TURN it to post the other side.
When you dont have the board it makes it difficult flicking between pictures to see where something on the bottom is on the top. Rant over.

The reason I was looking is that there are four or five big solder joints which I think must be the transformer - can you reflow them

Please can you list the voltages you got on the connector between the power supply and the main board. Take them with everything connected.

pin1 12v 11.99v
pin2 pson 3.22v and so on.


When doing the pson test when plugged in but not switched on
Test again when switched on and do a test with the probes in position whist switching on.

When it is all on and connected but no back lights have it connected to a known working pc with a vivid picture. Can you see a very dark image of the picture on screen. perhaps shining a flashlight at an angle will help.

What caps did you use for the retest make/series

[rogfanther]

Re: Monitor Samsung 172N problem

An update - By accident, while checking some voltages on the main board, I've shorted what seems to be the so-called ADj_BL line and the On/off line, what caused the lamps to turn on . That means that the inverter is working after all and that I have not messed thing up with my attempted fixes... That makes me believe that the problem in reality is in the main PCB as, according to what I read today, it's the Micom chip on the main board that controls the on/off and the inverter sending the right signal...

"Micom" is the slang they use in that brazilian forum for the microcontroller. The brand of the chip is MStar, and sometimes it is the culprit, but mostly not.

Measure also the voltages in the regulator chips in the main board ( marked 1117-something ) . Although the picture is out of focus, the solder in them seem a little brown from the heat.

[renatosc]

Re: Monitor Samsung 172N problem

Selldoor, I'll try to make myself clear.

Answering to your question, the voltage that is rated as 13 V is 14.15V and the one rated 5V is 5.3 volts on my board (both steady). The writing on the board cites 3.3 volts, but that does not exist in reality, as the two pins are not connected to anything. There is also a pin called "adj" and the voltage varies from 0.07 when the set tries to turn on 0.03 when it shuts down. The "on/off" is 0.01 volt.

As I said in the previous post, by accident I managed to turn the lamps on, so my conclusion is that the inverter and lamps are OK, what is missing is the right signal to activate them.

So, things are pointing to a problem on the main board. I have been trying to measure the voltages there and things are odd. It seems that the IC that provides 5v to the 3,3 volt regulator sends 5V for a moment and then shuts down. So there is no 3,3 volts for the main chip. There is no 5 volts either where the main board should feed the panel board (not even when I turn the set on).

Anyway, I've found a guy online who was selling a used main board for this model and it was not so expensive. So I bought it and hope that solves the problem if the main board is in reality the problem. Meanwhile I will keep digging to see if I find something conclusive.

I attached some scans of the board, divided as I understand them and a PDF file showing the pins that connect the power board to the main PCB. I'll try to post other pics the way you asked.

[budm]

Re: Monitor Samsung 172N problem

The ADJ pin is for the backlights Dimming control, it will varies from 0~5V, the 'ON/OFF' is for the backligts ON/OFF control, >3VDC = backlights ON, <1VDC = OFF. Both signals are generated by the Main board from the Micro controller.
It sounds like the Power-invertr board is OK. You can easily verify that by disconnect the connection between the power board and the Main board, the power supply board is always on, it does not go off, it will be running the minute you AC applied to the board. Use a 1K resistor between 'ON/OFF' pin and the 5V pin, that should turn on the backlights.
Notes on functioning monitor: The backlights and the power LED will turn off if you do not have the monitor hooked up to a running PC.
I think you may have problem on the main board.

One of these service manual should match your chassis:
http://elektrotanya.com/?q=showresul...oria=&kat2=all

[renatosc]

Re: Monitor Samsung 172N problem

Budm, thanks a lot. That is the type of essential hint. It seems that the inverter is indeed working here and I'm going to try doing the 1k resistor connection to be sure. I got a good used main board, so when it comes I will post the results. One question though. you said the monitor should turn off if no data cable is attached, but in normal functioning conditions wouldn't it display the 'no connection' warning (or something like that)?

[budm]

Re: Monitor Samsung 172N problem

Some monitor will display 'No signal' or 'Check VGA cable' for a few minutes then it will go into standby mode, the power LED may turn from Green to Amber. Some may not display anything on the screen, it will just go into standby after 10 seconds or minutes if it does not detect video signal from PC, they do this to save energy.
Some of the SAMSUNG has problem with corrupted EEPROM.