Trouble shooting amw LCD Monitor but cannot identify parts

Thread Starter

cstroh

Joined Jan 16, 2019
94
I have an F199B amw LCD monitor that lost the normal display and went to only a yellowish/white screen. I cannot find data sheets on some of the parts. For instance, one part that looks like a surface mount transistor has a marking of RKM and checking with a diode checker get only a diode indication between two points and nothing with the third point. Thought at first it was a bad transistor but found another one on the circuit board with the same marking and the same behavior. If this is not a transistor, what might it be?
 

twohats

Joined Oct 28, 2015
294
Have you done a search on the internet, including YouTube? " F199B amw LCD monitor problems".
Transistor, maybe it's a 'fet or diode.
Keep us updated please.
Good luck............
 

Thread Starter

cstroh

Joined Jan 16, 2019
94
Thanks for your response. It is probably an FET but I am not sure yet. This whole project started when all of a sudden the monitor did not display the computer screen but had what looked like an exploding ink plot. Now I cannot even get the yellow on light to come on. Getting component data has been extremely difficult. I will post again if and when I make some progress. I am learning a lot about LCD monitors but not yet enough to make progress. I have replaced all electrolytic capacitors but there must be something else that has gone bad.
 

Thread Starter

cstroh

Joined Jan 16, 2019
94
I cannot get the unit to startup. Can anyone tell me how I make sure the controlling IC NT68F633L and NT69521AEF are still alive and well? The various voltages are present, 12 volts, 3.3 volts etc. but the unit just will not startup. I watched the voltages on the eeprom and there seems to be activity but I cannot tell where and the hang-up is.
 

Thread Starter

cstroh

Joined Jan 16, 2019
94
In the attached is a schematic of the Power board for this unit. Can anyone explain how everything is passed forward through the transformer at top middle of schematic after the AC was transformed to DC?
 

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Ya’akov

Joined Jan 27, 2019
6,043
In the attached is a schematic of the Power board for this unit. Can anyone explain how everything is passed forward through the transformer at top middle of schematic after the AC was transformed to DC?
It is an SMPS, a switch mode power supply. The rectified voltage is being used to power IC001, which is this one or a close cousin. It also provides power for the MOSFET on pin 8 which creates a high frequency AC for the transformer.

So the answer is the power is converted to DC then back to much higher frequency AC, then on the other side of the transformer to DC again at the desired voltages.
 

Thread Starter

cstroh

Joined Jan 16, 2019
94
The previous transmitted circuit schematic is not exactly what I have. Attached is a hand print of how my circuit differs from the previous schematic at the downstream side of the transformer. The circuit provides the 12 volt signal but I have been unsuccessful in trying to show how. Can an oscilloscope be used to see the voltage at the transformer? So far I get nothing. The transformer pins on the downstream side of the transformer are indeed tied together in two groups which also mystifies me.

Mod. Opened pdf image.
ScreenHunter 18.png
 

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Thread Starter

cstroh

Joined Jan 16, 2019
94
Attaching a photo of the arrangement of the transformer output pins. As can be seen, three of the pins on the left are tied together and go to opposing diodes in previous sketch and 2 of the pins on the right are tied together and go to circuit ground. Just how does something like this work?
 

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