desktop monitor "dead" - Page 1
Forum section: electronics / beginners

author: jalves
author_rank: Contributor
timestamp: February 27, 2021, 11:23:52 pm
content: Hello all.I am trying to return to a hobby I had as a young boy, live steer me other way.I am buying some stuff to "build" my electronics lab.First project?Retrieve an ASUS VP248 from "dead state"The lcd monitor don't power on, the power supply seems to be ok, it delivers 18,56V to the controller board as soon as the power cord is connected.But pressing the power button do nothing.My question to the forum is this:How do pressing the power button do the startup of the monitor?Probing the test point near the connector that came from the on/off pcb in the front of the monitor, as soon as the power cord is connected, it shows 3.356V, pressing the power on button, this voltage decrease to zero as long the button is pressed.How do this signal is worked in order power up the monitor?Is the controller board that do the wake up of the monitor?The controller board has always 18,56V in its terminals, thus have the power to power up.Or the power on signal is transmitted some how to the power supply board and is the PS board signals the processor of the controller board to power up the LCD?Google so far only return replacing caps and pc power supplies...Thanks in advance.

author: sam_sam_sam
author_rank: Contributor
timestamp: February 27, 2021, 11:45:57 pm
content: This is the first place to start “ replacing caps and pc power supplies...”Post some pictures of the switching power supply board ( front and back ) and let see what you have and go from there

author: jalves
author_rank: Contributor
timestamp: February 28, 2021, 12:21:14 am
content: Here they are, the pics.In the power supply board, almost had an hazard with a bit of solder wire in the bench, guess where?Nothing seems to suffer from the event, except the spooky time I had.

author: wraper
author_rank: Unknown
timestamp: February 28, 2021, 12:33:21 am
content: Quote from: sam_sam_sam on February 27, 2021, 11:45:57 pmIt's not. Suggesting replacing capacitors without considering symptoms is counterproductive. And becomes less and less relevant with years passing since capacitors become less of an issue. It's mostly relevant to monitors made 10 years ago and earlier.My suggestion is measuring voltages after DC/DC converters on scaler PCB.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: February 28, 2021, 12:37:49 am
content: Bottom left corner of photo 3 looks like your problem. Looks like scorching and broken solder joints.

author: wraper
author_rank: Unknown
timestamp: February 28, 2021, 12:40:11 am
content: Quote from: JustMeHere on February 28, 2021, 12:37:49 amI see none of that. Just a hole with copper around it covered with solder specks. Completely common. EDIT: or maybe you mean 4th photo. Looks suspicious. However if there was a failure (rectifier), there should be no output voltage from PSU.

author: wraper
author_rank: Unknown
timestamp: February 28, 2021, 12:49:48 am
content: Measure resistance of switches on button board. It seems this model is prone to developing switch fault with them becoming slightly conductive (in 10s of KOhm range). This may prevent power button from working if other button is detected as pressed.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: February 28, 2021, 01:13:37 am
content: Yeah, it's 4.

author: jalves
author_rank: Contributor
timestamp: February 28, 2021, 08:27:30 am
content: CAn you pin point in #4 photo where is the suspicious point?I had access to a working and equal monitor and measure the voltages at the power supply board terminals:Where this monitor has 18.56V the working monitor had 19.01V, could this slightly difference indicate problems in the power supply board?Also the working monitor in the on state and with image in the screen had the 18.56V in the terminals.What are your thoughts about this?

author: Manul
author_rank: Super Contributor
timestamp: February 28, 2021, 11:49:54 am
content: If 19V is the only output and it is there, I would assume the power supply to be working. No need to be picky about small variantions or capacitors. Although half volt difference might be a clue, that it is actually on, just showing no picture. Does it have a standby LED showing that it is actually in standby?If you do not mind, you may cut the 19V wire and hook the multimeter to measure current after powering, after pushing the button and so on. Might be useful, might be not.But first you should trace the power button signal on a board and find where it goes. Might be very useful.Also check if no local power rails are shorted on the controller board. Next step - check voltage of local rails while running.

author: wraper
author_rank: Unknown
timestamp: February 28, 2021, 12:00:19 pm
content: If you have another monitor, swap scaler board and see if it works.

author: jalves
author_rank: Contributor
timestamp: February 28, 2021, 04:25:08 pm
content: The main problem is that pressing the power on button don't do nothing visible.The other monitor is at work (this faulty display is also from work), it was bought for the service at 2018, so it has 2 years on site.Tomorrow I may open the good one.When I test the good working monitor, I did some measurements.Between power supply board and the controller board, there is a 6 wire cable with this sequence:Red, empty, red, black, black white and brownBoth Black wire is DC groundBoth Red wire as soon as the power AC is connected, stay at 18.56V (working monitor 19.01V)White wire and brown wire voltages in the voltmeter are varying but in the mV scale (auto), pressing the power button nothing change.BUT, in the working monitor the situation is different:Both red wire as soon as power is connect, the voltage is 19.01VWhite and brown wires voltage oscillating in the mV scale.Pressing the power button the led lit white and the brown wire voltage ramp up to 3.3V but the white wire stays oscillating in the mV scale.The monitor search for video signal and not finding it, suspend.The led change to amberBrown wire voltage become mV oscillating, white continues with its voltage oscillating.The last test I did, connect the video from the computer and with image in the LCD the voltages become:Brown wire: 1.054VWhite wire: 3.170VRed wire (both): 18.75VCan someone explain, in simple terms, how after pressing the power button, the LCD power up?The signal from pressing the power button (The test point voltage is 3.356V, pressing the power button the voltage become almost zero, 0.1mV) is transmitted to the controller main cpu (RTD2525AR) ?Sorry for this long post...

author: kripton2035
author_rank: Super Contributor
timestamp: February 28, 2021, 05:14:07 pm
content: I would first exchange the power supply module between the two monitors to see if it works with it.this way you can narrow the search to the power supply or the main board.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: February 28, 2021, 07:38:48 pm
content: Quote from: jalves on February 28, 2021, 08:27:30 amThe two pins in the bottom left corner look like the solder is cracked in the photo.

author: amyk
author_rank: Super Contributor
timestamp: February 28, 2021, 10:02:37 pm
content: RTD2525 datasheet is available, you can look for more details in it.

author: jalves
author_rank: Contributor
timestamp: March 01, 2021, 06:11:31 pm
content: Good news and bad news.Try the controller board in the other monitor and it didn't work.Couldn't try the power board because I fry it yesterday.So, it seems the problem was in the controller board.Couldn't find the proper RTD2525 datasheet.This chip has 32 pin per side and what I could find has 16pin per side (RTD2525LRH)

author: John Red
author_rank: Newbie
timestamp: July 29, 2021, 01:10:50 pm
content: I have exact the same issue.Were you able to solve it?