Acer K242HL LCD Monitor Power Supply/Backlight issue

[nathanel23]

Hi all,

a few weeks ago my girlfriends's monitor started to act up and suddenly turned off. After a few more tries to turn it back on, it stopped working completely. Not even stand-by LED was turning on any more. It is an Acer K242HL with LED backlight from around 2015 and otherwise was still in very good condition, so I decided to take a look and see if it is possible to salvage it. I took it apart to inspect the circuit boards, but didn't see any obvious burnt areas or bulging caps. Searching for schematics on various sites didn't turn up any results. However, I found various online shops selling used stocks of the power supply board (Type 4H.22V02.A10) for only a few bucks. So my thought was instead of wasting much time to figure out what's wrong, it would be worth a try to just replace the power supply board and see what happens.

When the replacement board arrived I figured that Acer is using the same board in different model series with somewhat different component ratings based on power consumption. As none of the shops explicitly offered the board variant for the K242HL, I decided to see what the differences are exactly and also made some hi-res pictures of the SMD parts:
- 33uF (replacement) instead of 68uF cap (K242HL model) on the primary side
- Only one (replacement) instead of two 330uF caps (K242HL model), plus some changes on the SMD resistor values on the back side for the backlight circuitry
- One 1500uF/10V (replacement) vs two 680uF/16V type caps (K242HL model) for the logic board supply part

Unfortunately, the part numbers on the ICs and MOSFETs on the back side are extremely washed out and impossible to decipher on the original board, and also only barely to make out on the replacement. Not sure if this is the result of a deliberate attempt to etch the markings off in the factory.

As there was not too much of a difference, I decided to give it a try and ordered the missing/different parts and mounted them on the replacement board to make it match the original one. After some basic verification/comparison with a multimeter I installed the modified replacement board in the monitor and powered it up. Good news is the Power/Standby LED is turning on again and I can confirm the video processing board operating properly. When I shine a flashlight against the screen I can faintly see the expected startup logo/picture. Only the backlight is not turning on.

After a bit of further research and tracing out the backlight section of the power supply I figured out the following:
- The driver chip is an AP3064 (Datasheet: https://www.diodes.com/assets/Datasheets/AP3064.pdf)
- The actual circuit is the same as in the example given in the datasheet, except for (again) different values for caps/resistors, which I already figured out when comparing the original and replacement boards
- Hooking up an oscilloscope to various pins on the driver chip reveals that the output voltage to the LEDs is rising up to ~50V within just a few milliseconds, which in turn makes the voltage divider on the OVP (overvoltage protection) pin go above the 2.0V level, forcing an immediate shutdown of the driver chip.

Now I'm somewhat unsure how to proceed from there. Here are my thoughts:
- The LEDs in the panel might be damaged, either when the old power supply died, or as a result of some issue with the replacement board. How to test this?
- Maybe the AP3064 on the replacement board is broken and does not allow the voltage to drain properly through the current regulator channels. Also, how to verify this?
- It looks like there could be a somewhat different type of MOSFET in the same packaging on the original board. Unfortunately markings are not readable, the replacement board has an 18T10AGH.

Any ideas what I could try next?