Repair a phone call, the company’s large screen monitoring has a screen with no display, in a black screen state.
First, let’s look at the characteristics of the Hikvision LCD display unit.
From the model, we can see that this is a Hikvision LCD display unit with a 46-inch screen. By comparing the indicators of this faulty screen with those of other normal screens, we find that the faulty screen’s AC power is normal, the red switch light is on, but the LED green indicator light of the faulty screen is not lit. It can be inferred that there may be a problem with the power supply or the mainboard.
The next step is to open the cover and measure with a multimeter to understand the relevant data.
From the above image, we can see that the LCD display unit is divided into three parts:1. The top part is the power board, which has three connectors. The bottom right corner of the image shows the AC power input terminal, the top left corner shows 24vDC, the bottom left shows +12vDC, +5VDC, 5VSB, and the power-on signal. This is very similar to a desktop computer’s power interface. The next step will be to analyze the circuit structure.2. The lower board is the CPU mainboard, IO processing interface, and the driving module to the LCD screen.3. The third part is the imaging function of the LCD screen.Understanding the above divisions helps to measure more precisely.First, measure the 24v output, which shows 24v, consistent with the circuit board markings.Next, measure the 12v output, which shows 11.8v, also without abnormalities.Then measure the 5v output, which is 3.44v, abnormal. Because (5.0-3.44)/5*100%=31.2%, this is significantly low. The low output could be due to the power supply itself or a load issue that pulls down the power supply output. Continue measuring below.
Then measure 5vSB, which is only 2.32v, indicating that this initial standby power is also incorrect.This is the PS-ON power signal, which should only be slightly low. Combined with the 24v and 12v having voltage output, it has not yet triggered protection shutdown. The fault is no screen display, black screen. It can be inferred that the 5v power supply is abnormal, causing the mainboard control processing or signal processing to be abnormal, hence the screen shows no display.Since this is a monitoring splicing screen, there are many similar screens to measure for comparison to confirm my judgment. There are two plans, first measure the corresponding parameters of the machine as follows:
This is the good machine corresponding measurement data.
From the image, it can be seen that the normal voltage should be 5.2v.Using the second plan, remove the power board from the good machine and replace it on the black screen machine, plug in the three connectors, power on, and the screen lights up, displaying monitoring images. This proves that the mainboard is normal, and the problem lies with the 5v power board.Sometimes, repairing things is like solving a case; one must have a clear thought process. The first step and the second step must be sequential. After the first step, based on the results, proceed to the second step, or skip the second step. Each step has its reasoning. Understanding this “way” is equivalent to knowing the cause, leading to understanding why it happened… Enough of that, let’s continue our inspection and repair.♥→This is the back of the power board, where the high voltage area is clearly separated from the low voltage area. The high voltage circuit consists of three ICs. ♥→This is the front of the circuit board. It consists of the filtering and anti-interference part, AC-DC rectification conversion part, PFC power factor correction circuit part, 5v standby power circuit part, and then the controllable output 12v and 24v circuit part (for push-pull inverter output circuit). The large magnetic ring is a high-frequency transformer, its secondary is rectified output, like the front stage AC-DC rectification, the heat sink material is sufficient, and the circuit board protection function is complete, with no uninserted components or empty slots. Four optocouplers are for controllable output and protection functions. The functions are complete.
♥→This partial image shows the 5v standby and 5v controllable output circuit. The surface-mounted power switch tube is used for 5v output control switching.Upon inspection, it was found that the 5vSB power supply also had low voltage without load. Considering the working environment of this machine and the common issues with power supplies, I decisively measured its filtering capacitor with a multimeter. Based on the appearance of the capacitor’s top, I removed it and replaced it with a good old capacitor, selecting a good brand, rated for 105 degrees, such as Rubycon or Nichicon capacitors.
Don’t ask me why?Ask and it will be time that explains everything. The harsh working environment determines all of this. The choices of most people prove this point; more importantly, the price reflects the corresponding quality. In fact, it is still the budget that determines the corresponding parameters.→♥Because analysis shows that, without the mainboard, similar to a desktop computer, to have output, it must have a power OK signal. I used an external 5v power supply, adjusted the output to 2.44v using a potentiometer, and connected it to the PS-ON signal terminal. At the same time, to determine if there is a controllable output voltage of 24v, I connected a 24v sound and light alarm buzzer to the 24v terminal; once powered on, the output will have sound and light prompts to indicate output status immediately.♥→With the power on, the flashing and buzzing of the buzzer also indicate that our repair is on the right track.
The Long March is just the first step; it must be tested continuously for 24 hours, 72 hours or more. If it passes the test, then we can say we found the problem; otherwise, there might be other faults or undetected bad components. From the photos, it can be seen that it has basically withstood the 72-hour test, which can be considered a success. After all, I used second-hand components, not new ones; sometimes, if a fault occurs again, it requires comprehensive analysis. During this time, I used an old component for replacement, but halfway through, it went black screen, and upon testing this component, it was intermittent.
But after removing the aluminum casing and retesting, it was normal again. As the saying goes → time will tell.
Well, I’ve said so much, it’s just to give everyone a thought process. Perhaps the same fault won’t be caused by the same component damage; only by grasping the “way” within can one ask oneself a few more whys, and I believe everyone will gain something, and their confusion will become clear. Friends, what do you think?! Next time we will continue our chat, ending our mutual exchange with music…
This is the PS-ON power signal, which should only be slightly low. Combined with the 24v and 12v having voltage output, it has not yet triggered protection shutdown. The fault is no screen display, black screen. It can be inferred that the 5v power supply is abnormal, causing the mainboard control processing or signal processing to be abnormal, hence the screen shows no display.
♥→This is the front of the circuit board. It consists of the filtering and anti-interference part, AC-DC rectification conversion part, PFC power factor correction circuit part, 5v standby power circuit part, and then the controllable output 12v and 24v circuit part (for push-pull inverter output circuit). The large magnetic ring is a high-frequency transformer, its secondary is rectified output, like the front stage AC-DC rectification, the heat sink material is sufficient, and the circuit board protection function is complete, with no uninserted components or empty slots. Four optocouplers are for controllable output and protection functions. The functions are complete.
