[SOLVED] I'm looking for VIETTEL V8412 repair guide or schematics (free) - Phones & Tablets Forum

Hi,

I'm hoping you can help me out with my VIETTEL V8412. It's started acting up, and I'm looking for a detailed service manual with boardviews and schematics to properly diagnose and repair it. I need to take precise voltage measurements around the board, so having the right documentation would be very helpful.

Thanks in advance for your help.

Selected Answer

I actually found that service manual on a tech Discord server a while back. A really helpful member there shared a direct link to their blog and I've saved it. I'm happy to pass it along here. Hopefully, these boardviews and schematics help you fix your phone, just like they got me through my repair. Looks like we have the same model.

@Mateo

Absolute legend! That's exactly the info I was searching for. This is going to save me so much time probing in the dark. Seriously, thanks a ton for sharing the link!

Hi there,

I also have the VIETTEL V8412 and just downloaded the manual you shared. I'm pretty new to board-level phone repair, and this is a bit intimidating with all the tiny test points and the schematics. Could you point me in the right direction on how to start troubleshooting this ? Any advice on the first few things I should check would be a massive help.

Thanks so much for your time

General advices: start by checking the voltage at the battery connector on the board. With a known-good battery connected, you should see a steady voltage between 3.7V and 4.2V. After that, a great next step is to check the main power management IC (PMIC) for shorts. Using your multimeter in diode mode, check for shorts on the large input capacitors surrounding the PMIC.

Here are a few useful references for troubleshooting your device:
https://www.ifixit.com/Answers/View/807089/My+phone+screen+get+pink+colored+it+spread+rapidly+over+the+screen
Take a look at comment #1440
Also, this : https://xdaforums.com/t/q-cant-see-google-maps-in-the-market-after-install-miui.921444/.
You can also check this video starting from minute 4:

The VIETTEL V8412 service manual and boardviews from the link above were exactly what I've been searching for. I couldn't find a complete, free copy anywhere else. Seriously, thank you for sharing this you're a lifesaver!

Hi everyone, I'm working on a VIETTEL V8412 with a no power issue and need some guidance with my measurements.
I'm detecting 3.3V on the VREG_MAIN line (pin 1 of the PMIC), which looks good, but I'm getting 0V on the VDD_CPU line (pin 8) where the schematics indicate I should see about 0.9V.
Since this is a core voltage for the application processor, could this missing rail be why the device shows no signs of life?
What's the best procedure to diagnose this further? Should I check for shorts on the CPU rail first, or look at the PMIC's enable signals?
I've already verified the main 3.3V and 1.8V power rails are present and stable.

My VIETTEL V8412 was working perfectly until yesterday when it suddenly went completely dead. Now it won't respond to the power button, doesn't vibrate, and shows no signs of life even when connected to a charger. I'm worried there might be a serious issue.

I have a decent multimeter, a basic soldering iron, and a healthy dose of patience. While I've successfully replaced iPhone batteries and charging ports, this will be my first attempt at actual diagnosis. The sheer density of BGA chips and microscopic components is honestly a bit overwhelming.

I'm particularly curious about the alcohol trick I've seen online where you apply isopropyl to the board and look for evaporation hotspots to locate shorts. Is this actually a reliable method for beginners, or are there better approaches I should try first with just a multimeter?

I learned this lesson the hard way last month with mine, it was declared "dead" by two different shops. The phone showed absolutely no signs of life - no charging indicator, no vibration, nothing. Before diving into complex board work, I decided to try one more basic test: wireless charging.

To my complete surprise, it actually heated up on the charging pad! This single discovery completely changed my diagnostic path. It turned out the issue wasn't with the main board or processor, but with the notoriously fragile USB-C port that had failed completely. A $15 replacement part and some careful soldering brought it back to life.

The moral? Always exhaust every external testing method before opening the device. Test wireless charging if available, try different charging methods, and don't assume the worst case scenario. Sometimes the most "dead" devices have the simplest solutions hiding in plain sight.

I suspect my issue might be related to that cheap, third-party fast charger I used at the airport last week... Now the device gets extremely hot during charging, the screen flickers at low brightness, and sometimes it randomly shuts down at 30% battery. Could this have damaged the power management IC or battery calibration?

If your VIETTEL V8412 starts acting up, random reboots, fast battery drain, or connectivity issues, there are several diagnostic steps you can take before assuming the worst:

• Check your charging habits: Using poor-quality chargers or wireless pads can gradually damage your battery and charging circuit, leading to unpredictable behavior.
• Inspect the physical components: A slightly damaged charging port, worn battery, or even accumulated pocket lint can cause issues that seem like major hardware failures.
• Monitor temperature patterns: If your phone gets unusually hot during specific tasks (like camera use or gaming), it could point to a failing component rather than a software issue.
• Use diagnostic tools wisely: Ampere for battery health, phone diagnostic codes (*#0*# on many models), and a thermal camera can reveal problems without opening the device.
• Know when to stop: If you see liquid damage indicators tripped or smell burnt electronics, it's time to consult a professional before causing irreversible damage.

Also visit this link it may help : https://www.ifixit.com/Answers/View/851640/How+do+I+replace+the+Keyboard

Here's what I discovered on forums and technical databases:

Update OS: Ensure the operating system is fully updated to the latest available version. Identify the Target: Locate the specific shielding can you need to remove. The component immediately stops working, leading to a dead phone or a specific function failing (e.g., Wi-Fi module, charging IC). Efficient Troubleshooting: Instead of relying solely on visual inspection or component removal, TPs enable non-invasive electrical checks. A fault in these circuits, or in the related Power Management IC (PMIC), can erroneously detect a charger connection. ESD Bags: Always have a supply of metallic shielding anti-static bags to store sensitive components when they are not actively being worked on. If the phone charges at a normal speed in Safe Mode, a third-party app is the cause. Slowdown During Intensive Tasks: Tasks that usually run smoothly (e.g., video editing, large file processing) become extremely slow or crash. Corrosion/Damage: Look for any signs of corrosion (green, white residue), bent pins, or physical damage to the connector on both the battery flex and the logic board. This step is critical; any residue will prevent the new coating from bonding properly. Then, press and hold a specific button combination (e.g., Power + Volume Up, or Power + Volume Down, or Power + Home + Volume Up for older Samsung). This usually points to a physical issue: the fingerprint sensor flex cable is not properly connected, is damaged, or the sensor module itself is faulty. It measures angular velocity , the rate of rotation around the X, Y, and Z axes , allowing the phone to understand its orientation and how it's moving in three-dimensional space. It transforms what would otherwise be a junked device into a functional one, making it an invaluable asset in the arsenal of advanced smartphone repair.4. The Process of Board Separation (Layered PCB) for Complex Smartphone Repair This cuts power to the entire device, preventing accidental shorts while you work on other components. Repeat this process several times, using a fresh surface of the putty each time, until you see no more debris being pulled out. Apply Flux to Motherboard: Apply a fresh, thin layer of flux to the clean pads on the motherboard where the PMIC will sit. Ensure your mouth is not directly pressed against the mic hole, but rather speaking towards it. Dispense a tiny, controlled amount onto a clean, non-absorbent surface (e.g., a piece of glass, ceramic tile). Apply heat carefully and evenly, then use a suction cup and opening picks to gently pry open the phone. Working Around the Perimeter: Slide the plastic pick around the perimeter of the phone, slowly cutting through the adhesive. Does the phone see the network but refuse to connect? Does it fail immediately after entering the password, or does it attempt to connect for a while before failing? Is it happening on one specific network or all networks? This context provides crucial clues. The goal is to remove the motherboard from the chassis, as the display driver IC is typically located on the motherboard itself, often near the display connector. Try making calls to different numbers, especially landlines, to rule out network interference. Among these, the Time-of-Flight (ToF) sensor has emerged as a crucial component, especially for advanced photography, augmented reality (AR), and even some forms of facial recognition. Faulty or failing components, such as a shorted capacitor, a struggling power management integrated circuit (PMIC), or an overloaded processor, can also generate abnormal heat. The most commonly inspected external connectors are the charging port (USB-C, Lightning, Micro-USB) and, less frequently now, the 3.5mm headphone jack. Anti-static Mat and Wrist Strap: To protect sensitive electronic components from static discharge. Tweezers: Crucial for handling tiny screws and small, delicate connectors.

@Vinicius, Not sure what you're trying to say but ok :)