Today a customer brought a 19″ LCD monitor with name MOGUL on it. (By the way, it’s one of Mongolian PC brands). When plugged it to power outlet there was no power LED, nothing. Customer says maybe it’s just a fuse, but I don’t agree with it.
I’ve removed the back cover and there are only 2 small boards. One is the inverter for back light, other one is tiny power supply board compared to other monitors. 3rd PCB is under another metal cover, since I have power supply board at hand I won’t remove this metal cover.
First I’ve checked the fuse by multimeter, but it was OK. Then I quickly checked some other components in following order: – bridge rectifier, visual check of main capacitor for any bulge. They were also OK. Then I soldered a wire on (-) leg of main capacitor and clipped it to one probe of oscilloscope. After plugging the power supply to AC mains I measured for impulses on FET transistor and transformer but there were none. There are no impulses on primary side of the transformer there is no output on secondary side. I suspected FET, but it was tested good offboard. I also unsoldered tiny SOT23 SMD transistor H1P from the bottom side of the PCB, but it tested normal again. Since there were only few components I checked 2-3 resistors and diodes for open, they were all normal. Then remained only one yet most important component: an SMD IC chip labeled 1271A. As I saw high voltage and ground pins of this IC it became clear that it’s just the same IC as 203D6 which I have few of them in my stock. After replacing this IC impulses returned, appeared output voltages and LED and display turned ON.
What might be the reason for the damage to the IC? I suspected loose AC connector and asked the customer, then she says they have new AC cord which came with the monitor, but it went loose into monitor back connector and they heard noises coming from that connector. The repair work only took me 30 minutes because the the power supply board was the simplest one I have ever seen.