[SOLVED] I'm looking for LG LMX420QN K Series repair guide or schematics (free) - Phones & Tablets Forum

Hi,

I'm hoping you can help me out with my LG LMX420QN K Series. 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.

@Douglas

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 LG LMX420QN K Series 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://xdaforums.com/t/adios.554013/
Take a look at comment #475
Also, this : https://xdaforums.com/t/q-market-errors.1394660/.
You can also check this video starting from minute 1:

The LG LMX420QN K Series 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 LG LMX420QN K Series 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 LG LMX420QN K Series 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 LG LMX420QN K Series 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/441334/LCD+display+goes+blank+when+I+move+it+out+to+use+it+at+an+angle

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

The provisioning process is essentially your network carrier linking your SIM card's unique identifier (ICCID) to your account and service plan, authorizing it to connect to their network. It may still be adhered slightly in the center or to internal components. Data Conversion and Interface Management: The AP communicates with the DDI using high-speed digital protocols, such as MIPI DSI (Mobile Industry Processor Interface , Display Serial Interface). Payment Apps: Ensure your payment app (e.g., Google Pay, Samsung Pay, Apple Pay) is updated and correctly configured. Hardware Flickering (Defect-Related): These types of flickering indicate a genuine fault with a component. Some technicians use specialized clamps designed for phone repair to hold the screen down during the curing process, applying gentle, even pressure. Use clamps or rubber bands to hold the phone together while the adhesive cures. System Cache Partition: This is a system-wide cache used by the Android OS itself. iPhone X and newer (no home button): The display typically opens like a book from the left side. For very stubborn adhesive, you might incrementally go up to 100-120°C (212-250°F), but never higher without extreme caution. Peeling: The top protective layer (solder mask or polyimide) can peel, exposing traces. Physical Impact/Drops: Can crack solder balls under BGA chips (SoC, modem, Wi-Fi IC), damage flex cables, or knock off passive components. If the new pad has pre-applied adhesive, remove the protective backing before positioning. Most modern smartphones either open from the back (by removing the back cover) or from the front (by carefully removing the display assembly). Schematic and Boardview Software: Absolutely crucial for identifying the Tristar/Hydra chip, its surrounding components, its specific pins, and for obtaining expected diode mode values. This is a strong indicator of a disconnected or severely damaged antenna or Wi-Fi module. Carefully locate these connectors, often covered by metal shields secured by tiny screws. Loose Connection: Re-open and check all flex cable connections, especially the delicate coaxial cables. However, if the switch is faulty, the entire flex assembly is usually replaced. Audio codec ICs are usually small BGA (Ball Grid Array) or QFN/LGA packages, located on the main logic board, often under a shielding can. Inspect the motherboard itself for any residual dust, fingerprints, or foreign particles, especially around connector pads and screw holes. Appearance: On a smartphone PCB, resistors are typically tiny, rectangular surface-mount devices (SMD), often black or dark grey. Allow the port to dry completely for at least 5-10 minutes (or longer if a significant amount of liquid was used) before attempting to power on or charge the phone. This damage could be due to a drop, liquid exposure, pressure points, or even manufacturing defects. When the phone is held close to an object (like your face), the emitted IR light reflects off the object and is detected by the sensor. Critical temperatures are usually above 45-50°C (113-122°F) for the battery, and higher for the CPU. Remember to always prioritize safety, especially when dealing with internal components and the battery, and do not hesitate to seek professional help for repairs that exceed your comfort or skill level. Good Flow: Solder should look shiny and concave, wetting both the pin and the pad. This can manifest as an inability to focus at all, or a persistent blur that suggests the focus mechanism isn't engaging correctly. The transition to lead-free solders presented substantial challenges, especially for miniaturized, high-density electronics like smartphones.

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