How to Disassemble a Samsung 720n Monitor
Nothing is eternal. Especially the technique. Especially electronic. Especially modern. Chinese Sooner or later, as a rule, after the end of the warranty period, your favorite electronic “devaysina” ceases its normal life cycle and miracles begin.
It does not turn on, it does not turn off, it does not work, it turns on and works, but it doesn’t. And sometimes with fireworks it flies into components and, much to the displeasure of the owner and firefighters, arranges a fire show. However, the latter happens incomparably less often than quietly does not work as it should.
A lot of LCD monitors, including Samsung, have visited my repair repair table. Today I will tell you how to independently repair the backlight of the Samsung monitor from the SyncMaster series with diagonals 15, 17, 19″. The backlight inverter circuitry is the same for all diagonals, the difference in the ratings of some elements. Having understood the principle of actions, you can restore any monitor.
As experimental will be used 17″ Samsung SyncMaster 740N
First you need to disassemble the monitor. It will require a flat, solid surface of approximately 0.8 square meters. m. and a soft backing the size of a monitor screen. We put the monitor with the screen on the substrate and stand for ourselves, while the base of the stand hangs from the edge and does not interfere with manipulations with the rear of the monitor.
On different models, the option to mount the stand legs may vary. Typically, these are a few screws that need to be unscrewed, after which the stand is disconnected from the case. Our subject has a stand with the function of rotating the screen 90 degrees. Fastens with two screws at the top:
and two metal grips below:
We unscrew the screws and, tilting the base of the stand, we disengage the grips.
We set the stand aside and turn the monitor upside down. The housing of the LCD monitor consists of two halves on the latches, which must be divided. There are several disassembly options. Pry off one of the halves with a flat-blade screwdriver and, moving along the perimeter of the case, snap off all the fasteners. Press with a scalpel or metal ruler into the joint between the two halves, pulling the tabs of the ears and, again, moving around the perimeter. All these methods are fraught with the appearance of burrs on the plastic of the case with inaccurate movement. It is recommended to use special “openers” made of plastic, similar in shape to a pick. I prefer the opposite for Samsung monitors. Its essence is that the separation of the halves begins on the side of the screen and only with your hands.
To do this, I put the thumbs of both hands on the end of the back half of any long side (usually on top),
and with index and middle I pick up the plastic of the front half. screen frame.
The important point is not to put pressure on the matrix. The force is applied to the end of the housing and to the frame.
I squeeze my fingers and pull the frame towards myself and a little up.
As a result, the central latch is released. Similarly with the opposite long side, and then with the short ones. The hardest part is the corner. It is necessary to place the monitor at an angle to itself and simultaneously squeeze both sides and pull the corner of the frame toward you and up.
In the presence of a plastic “opener”, you can insert it into the formed gap and force it toward the corner of the monitor, snapping off the remaining latches.
As soon as the first corner yields, the rest will accept their fate and succumb much more easily. The frame can be slightly pulled away from the screen, making the remaining latches easier to disconnect. From the side of the control buttons there is a loop, it is advisable not to damage it.
The frame has been snapped off! We put it smoothly and neatly on the screen and, taking the ends of the monitor, turn it face down on the substrate. The matrix fits into its grooves in the frame (remember the button cable), and the back cover remains on top, which is no longer attached to anything. Raise it and remove it to the side. And no burrs or broken latches!
Expand the matrix in the frame bottom to itself. On the left is a metal plate covering the backlight connectors:
We press on the edge indicated by the big red arrow, and a little upwards release the hook indicated by the small red circle with the arrow. Then we press on the edge indicated by the green arrow, releasing the hook indicated by the small green circle with the arrow. Then we shift and raise the plate, releasing the hooks indicated by large circles. The plate is dismantled. Put it aside.
The backlight connectors were opened.
In this monitor, they are fixed with black plastic frames, removing which you can pull out the plugs from the connectors on the board. It is advisable not to pull on the wires, the connectors are quite tight, it is better to pry the plug with a thin flat screwdriver and pull it out, taking it by the plastic.
We take out all four plugs from the connectors marked with red circles. The backlight is disconnected.
The position of the connectors is not critical, the upper and lower ones cannot be mixed up due to short wires, and they are indifferent to each other in pairs. Turning the plug over does not work out because of its shape; it is possible to insert in only one position. If it doesn’t climb, you have to turn it over. For peace of mind, you can mark the initial position of the connectors with a marker. It will not be worse.
It is worth noting that on modern monitors, each plug is equipped with its own latch-lock, which must be pressed and only then removed the plug from the connector.
Go to the right side of the monitor. On the side there is a connector for connecting a cable from the control buttons.
Pressing on the latch indicated by the red arrow, pull the plug in the direction of the green arrow. You can pull on the wires, but carefully, without fanaticism. Next, we peel off one side of the adhesive tape, release the ribbon cable and swing it down along the second green arrow.
At the top of the monitor was the last connector on the matrix interface cable. While pushing simultaneously from both sides on the latches marked with red arrows, pull the plug down in the direction of the green arrow.
And now we take all this iron construction and raise it above the matrix. Turn over and put on the table. The matrix can be moved away from sin far away.
We look at the picture. On the left is the power supply and backlight inverter board, and on the right is the control board. We are interested in the left, which is more.
We unscrew the three screws indicated by the red arrows and disconnect the power / control cable connector by pressing on the latch and pulling the plug towards the green arrow. We take out the board from the iron casing and turn it over.
From them! The “sinful trinity” of electrolytic capacitors is circled in red, because of which, in fact, everything turned out. You can see with the naked eye. they are swollen, which indicates a loss of capacity. And the brown dried-up electrolyte residues leave no doubt. on the board are aluminum caps, which used to be capacitors. Drink and change! They cost 820 microfarads x 25 V, I usually change to 1000 microfarads x 25 V. Modern capacitors with the same dimensions usually have a larger capacitance.
The board on the side of the tracks is covered with a protective translucent plate attached in three places with plastic clips. Squeezing the antennae of the clips, push them out.
Armed with a soldering iron and suction of solder (preferably), we solder the “dutiks” and solder new ones.
Remember the polarity! On electrolytic capacitors, one of the terminals is indicated by a strip with a minus sign. The second, unlabeled, is a plus. On the board, the negative terminal of the capacitor is usually marked with a wide strip-arc or half the circle is painted over from the minus side. You can remember the position of the minus of “ditiks” and solder new capacitors in the same way.
Some tricky electronics manufacturers sometimes mark the positive output on the board (so that they hiccup prematurely), and out of habit, you can solder new capacitors on the contrary, which they really do not like. They begin to bask, inflate and explode, scattering scraps of paper all over the device and filling everything around them with electrolyte. Enchanting, but then to launder to whom?
In the simplest case, when the monitor was turned on for a short time, it turned on the image and then the screen turned black, although the power indicator glowed merrily blue, replacing the capacitors with serviceable ones, we get a working monitor.
If there is no image at all, and after replacing the capacitors, it does not appear, and the indicator cheerfully shines blue, as if it were necessary. our exciting repair continues.
We turn our attention to a small green part similar to a resistor located next to capacitors:
In the picture she is surrounded by a small oval. This is a 3.15 A fuse. We turn the board upside down and look for the designation F301.
We pick up the multimeter in the “dial” mode or measure the resistances and put the probes on the fuse leads circled in a red circle. If the fuse is intact, the multimeter will beep and show a resistance of several ohms. If silence and infinite resistance. the fuse is blown.
It can burn out due to the increased current consumption of the inverter, caused by the loss of capacitance of the capacitors, or by burning the key / keys of the voltage converter. In the first case, it is enough to replace the capacitors and the fuse, in the second it is necessary to change the keys as well.
In general, I recommend that immediately after seeing the “dutik”, check the fuse and in case of breakage. check the keys.
As keys, the manufacturer used two assemblies of field-effect transistors of p- and n-type AF4502C in a SOIC8 package.
They are located next to the PWM chip and are labeled U202, U203. Indicated by red arrows:
To test the key closure, turn on the multimeter in the test mode of semiconductors or “ringing” and put one of the probes on any of the lower pins in the figure, indicated by two large green arrows. All conclusions of both keys on this side are interconnected. The second probe alternately touch the terminals on the opposite side of the key casing, indicated by small green arrows. The multimeter in any position of the second probe should not show resistance less than 100 ohms, only if briefly until the capacitors present in the key circuits are charged. Similarly, we check all the conclusions of the second key. If in any position of the probes the multimeter protects or shows a resistance of less than 100 ohms, the key is broken. Have to drink it. In the presence of an air soldering station. a matter of a couple of minutes. In case of its absence, dancing with improvised tools and devices is guaranteed.
You can simultaneously use two soldering irons on both sides.
You can grind a soldering iron tip out of copper so that all legs are heated immediately.
You can heat each leg with a soldering iron and lift it with a blade (thin wire) above the board.
You can bite off the legs from the body with pliers, and then solder one at a time with a soldering iron and remove with tweezers, because the key is no longer needed.
Yes, there’s a lot more that can be done, the main thing is not to overheat the board and not damage neighboring elements. The task is further complicated by the fact that the cases are glued to the board (circled in green),
and the conclusions on one side are sealed to the bus with a heat sink in the form of a radiator plate (circled in red) on the opposite side of the board, i.e. it’s hard to warm up.
It is necessary to change both keys at once, even if only one is broken. This is recommended in all forums, so I do. Key closure happens in about 10% of cases. The rest is limited to capacitors and fuses.
Here, when disassembling another monitor, it was immediately clear that the key was Khan:
The protective plate melted over the case of one assembly because of its heating, it was this key that turned out to be broken, and when evaporated it fell apart into pieces:
Once in my practice there was a case when the key was not broken, but one of the transistors was cut off. Unfortunately, it doesn’t work to check for a cliff without evaporating.
Well, if you’re lucky and the keys are safe. solder new capacitors
With short circuit, the wire burns down, checked, and more from it is not necessary.
The monitor assembly process is performed in the reverse order. It is advisable to connect the power cable before closing the back cover and make sure that the monitor is operational. the moving test image is on the screen:
After final assembly, let the monitor run for several hours.