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Model 732NW Samsung LCD Monitor with hidden fuse blown
This is my own monitor. This morning when I tried to start my job, it simply didn´t work. The screen remained unlit, although the On/Off button was turn on solid. When there is a problem with the main board – responsible for treatment of the signals – the screen presents itself normally lit, but with no images. So, it became evident that the problem is with the SMPS power supply.
This model of monitor does not have the DVI connector – just the analog RGB one, interconnected to the computer through the classic cable with D-sub 15 pin connectors at the two ends. Even with the cable disconnected, the internal circuits in the signal processing PCB would generate the fluctuating message, certifying that the monitor is operating correctly. This was not happening. Even so, in order to be sure and to perform a complete work, I decided to check the status of the cable. A pin-to-pin continuity testing was accomplished with the multimeter in the Ohms mode with buzzer feature, showing the cable was sound.
As I got another cable, it was used for another test connecting the computer to the monitor. No result: the problem stubbornly persisted. Undoubtedly, the problem was not with the connecting cable.
Before opening the device, I´ve performed a search on the Internet. A number of articles and forums were found, describing the maintenance of LCD monitors, some of them approaching exactly this Samsung 732NW model. Not all of them are in English – some are in Portuguese and Spanish. Cited below is a partial list of the references found:
With the knowledge acquired from these sites, I´ve felt myself able to start the maintenance work.
Opening the monitor: first the front frame, very easy to draw.
and then with the disengagement of the parts. Below is the internal content:
This is the best picture I could take. The short wiring to the different parts did not allowed to open the set completely.
The circuit board with the SMPS circuit and the inverter board was drawn off and taken to the bench. Below is a picture of the components side, got from http://www.fecomponentes.com.br/produtos/placas/Placa_LCD/pl120.jpg
Two details to emphasize on the down part of the picture: First one, the small green component below the main yellow capacitor and, second, the white large line: The green component will be referred to later – it is the bandit of the story. The white line divides the areas concerning to primary and secondary sides of the SMPS. In the solder side there is a corresponding black line:
The secondary side generates two voltages: +5 Vdc for the signal processing and +13.5 Vdc for the inverter that drives the CCFL screen backlight lamps. Depending on the manufacturer, the inverter is built in a separate circuit board. In this monitor Samsung opted to build it in the same circuit board of the SMPS, together with the secondary side.
A visual check showed the circuit board was OK. All the electrolytic capacitor were recently replaced, so I didn´t worry about them, knowing by sure they were good and didn´t show any signs of bulging. They were not checked ESR because I don’t have an ESR tester.
The input fuse and the rectifier bridge were checked OK. No short-circuit in the switching IC (high resistance between Drain pin and ground). The IC used is FSDM0465RB, very common in monitor power supplies. It is a complete IC including inside the FET switching transistor named SenseFET.
It was now time to voltage testing. A power cord was inserted and the SMPS powered. The voltage test was developed in the sequence: a) The voltage on the main capacitor measured 171,8 Vdc (127 * 1,41); b) the +5 output measured 5.1 Vdc and c) the +13 output measured 13.5 Vdc. This latter voltage is for the CCFL lamps converter, ranging normally between 12 and 19 V. In this device the Samsung designer opted to use 13.5 V. The converter generates an AC high voltage (between 300 and 1200 V, depending on the design) that power the CCFL lamps used for display backlight. I decide not to include the pictures of these measurements, as I consider it unnecessary.
All OK up to this point. It only remains the converter, powered by the +13.5 Vdc secondary voltage. A visual inspection didn´t show any problem. The surprise came when I tried to measure the supply voltage on the converter circuit: absolutely nothing! How come, if the power supply is delivering the voltage? There came the surprise: there is a second fuse between the +13.5 Vdc power supply output and the converter circuit. This fuse is that green one already shown in the picture of the components side above. Although installed in the circuit board, this 2.5 A fuse is not represented in the service manual schematic diagram. Downside for Samsung!
A quick measurement showed that the fuse was open. Luckily I had another in my junk box, although not in the same color: it was blue. The ampère rating marked on it revealed it had a 2.0 A rating. There came the doubt: If I solder this fuse and perform a test, it can blow. This moment I took into consideration that fuse was the only one I had available. For security and cautionary reasons I decided to do the test using an ordinary separate 3 A glass fuse in a fuse holder. The two wires from the fuse holder were temporarily soldered to the corresponding points in the circuit board. In case the fuse blows during the test, I had a stock of them, which is a guarantee to continue the troubleshooting work, without sacrificing the only (and most difficult thing to find) definitive fuse.
The SMPS board was powered again. The external fuse didn´t blow, a good sign. The voltage applied to the converter circuit could now be measured:
I turned off the board, installed the definitive blue-colored 2.0 A fuse in place and powered again.
No blowing, despite the fuse used was 2.0 rated, instead of the original 2.5 A. The +13.5 Vdc was still being applied to the converter circuit. A measurement on the lines to the CCFL lamps revealed no voltage. I think this only works with a command from the signal processing board, disconnected in this test (remember that up to this point the SMPS board was tested separately on the bench, without being connected neither to the signal processing board nor to the CCFL lamps. Once all interconnected, it will work normally).
The monitor was then assembled and powered on, still without the signal cable. The floating box notifying that the cable was not connected was shown on the screen:
This means two things: the CCFL backlight lamps were finally working and the signal processing way was OK. In general terms: the monitor was repaired.
When the cable was finally connected to my computer, it worked normally.
The blowing of the fuse was perhaps fortuitous, with no apparent reason, as it was found that the CCFL lamps are now working correctly. I´ve read somewhere that this fuse blows sometimes with no evident reason. I think that is due to inrush current in the moment the CCFL lamps are energized. If it happens again, I´ll implement a radical solution: install a fixed permanent fuse holder in the rear cover and bring its wires to the point in the circuit board. In case of a future blowing, the fuse can be quickly changed from outside without opening the case.
This article was prepared for you by Henrique Jorge Guimarães Ulbrich from Curitiba, Brazil. Retired electronics technician. Loves electronics, telecommunications, cars and grandchildren.
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