Looking at the OVP line, it would be fine for a while, then start to rise, and eventually jump up and down by several tenths of a volt. Running a hair dryer on the board would trigger the fault in a couple of seconds. The board heats up in operation, and the via begins to increase in resistance, and make sudden jumps to much greater resistance.
It's near the switcher MOSFET, diode, and inductor. The ground side of the resistor connects to ground through a via to the ground plane on the underside of the board. The divider has two resistors, 300k from the LED drive voltage side, and 8.22k to ground.
#ACER S271HL MANUAL DRIVER#
It is driven by a simple voltage divider, delivering around 1.23V at the highest allowable LED driver voltage (configured to 46.07V in this case).
There is an over-voltage protection pin (OVP) to detect this. If the voltage drop becomes too great, it will need to raise the LED drive voltage too high. It regulates the overall LED voltage based on the string with the largest voltage drop, because that one will be closest to ground and otherwise it wouldn't be able to bring about enough of a drop. The strings are common anode, and the ground returns from each go to open-drain drivers. It regulates the current to each string and it must detect these conditions to avoid putting excessive current. One thing it checks for is an LED string shorting, or going open. The LED driver does many safety checks and shuts things down if any fail, in order to avoid damage/fire. Given that sometimes the strings would go individually dark, not all at once, if it were due a problem with the LEDS then all of them would have to be bad, either due to a defect, problem with common lines, or damage done to all due to problem. I didn't want to use much current and damage them, and this test gave good evidence that they were working, since they gave such similar readings. This uses an M270HGE-L10 panel, specified at 38.4V typical string voltage at 100mA. Through the 1K resistor this comes to 4.8mA, so their voltage drop will be a little less than normal. Each gave a 31.2V drop, very close to each other, and each lit with similar brightness (dim since we're running very little current through). I checked the LED strings by using a 36V supply through a 1K resistor to power each string separately, and measuring the voltage drop across the string. Inverter components checked out: diode, inductor mosfet. Inside the case, voltages were good: input 19V, 5V and 3.3V on the signal board, 19V on the LED driver board, and when the LEDs were working, 39V going to the LED common. Turning off and on would sometimes get the lights for a minute or so more, though they lasted less time and sometimes it just blinked on for a moment. Flashlight showed image, so it was just the backlight failing. Eventually or immediately the last light would go dark. The symptoms: turned on fine, but after a few minutes or less one or more backlight segments would go dark, leaving the monitor with only part of the bottom edge illuminated, as if with spotlights on a wall at night. The datasheet for the MP3394 LED controller was of great help in understanding operation. First I found that there's very little information about problems, repair, or available parts for this model. I saw this broken Acer S271HL bid monitor on craigslist for $30 and figured I'd give repairing it a shot. Fix is to run extra ground wire to that point.
Via for ground to resistor divider of voltage feedback would heat up from nearby inverter components, and ground voltage would jump several tenths of a volt, triggering overvoltage and disabling LED strings until all were off. Tl dr: LED driver board's vias were faulty and would gain tens of ohms when board heated up.
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