[SOLVED] I'm looking for LG LMK420YM K Series repair guide or schematics (free) - Phones & Tablets Forum

Hi,

I'm hoping you can help me out with my LG LMK420YM 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.

@Tahir

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 LMK420YM 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://www.ifixit.com/Answers/View/151058/Won't+slide+to+unlock
Take a look at comment #1046
Also, this : https://xdaforums.com/t/q-what-kernels-supoort-data-formated-as-ext4.1132403/.
You can also check this video starting from minute 9:

The LG LMK420YM 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 LMK420YM 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 LMK420YM 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 LMK420YM 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/264116/How+to+Recover+Lost+iTunes+Library+after+Hard+Drive+Crash

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

These features often deliberately slow down or pause charging to extend battery lifespan. Apply new adhesive strips (recommended for a secure seal and to restore some water resistance). In conclusion, identifying smartphone accelerometer issues involves systematically testing software functionalities and then diving into raw sensor data analysis using diagnostic apps. After installing the new lens cover, allow sufficient time for the adhesive to cure. Dedicated thermal sensor ICs (often referred to as thermistor controllers or digital temperature sensors) are specialized chips designed to accurately measure and report temperature data to the phone's operating system and power management unit. Severe Corrosion: May require ultrasonic cleaning or more aggressive chemical treatment, which is typically done by professional board repair specialists. Reduced Risk of Short Circuits and Accidental Damage: Working in tight, confined spaces with soldering tools increases the risk of accidental shorts or damage to adjacent, functional components. Once the solder around the anchor pins (mechanical attachment) begins to melt, gently lift the old connector straight up with tweezers. An unresponsive touchscreen is arguably one of the most frustrating issues a smartphone user can encounter. Troubleshooting smartphone fingerprint sensor malfunctions begins with the least invasive software solutions: restarting, cleaning, and re-registering fingerprints. Swelling: A swollen battery is already under stress and is even more prone to damage. It can reveal internal breaks in traces or vias without destroying the board. Isopropyl Alcohol (IPA) (99%+): For cleaning flux residue and the motherboard. Their small size and integration within the tightly packed chassis require careful handling during repair. Temperature (if applicable): Some vacuum sealing machines incorporate a heating element. This comprehensive guide will walk you through the various aspects of analyzing screen flickering, from identifying symptoms and common causes to implementing detailed diagnostic steps and potential solutions. This reduced top-side heat application is vital for protecting small, sensitive components located close to the large IC. Few moments in smartphone repair are as frustrating as encountering a stripped or rusted screw. Photo Degradation: Images taken with the camera will likely show imperfections such as blurry spots, hazy areas, glare, starburst effects around light sources, and visible crack lines. Aesthetics and Fit: It ensures the speaker grille or speaker module sits flush and securely in its designated place. Carefully disconnect the screen's flex cables from the logic board and set the old screen aside safely (or dispose of it if it's completely beyond repair). Solder paste (a mixture of tiny solder spheres and flux) is applied evenly over the stencil, filling each hole. A full firmware reflash (e.g., via Odin or Fastboot) also overwrites system and data partitions, effectively erasing everything. Rapid Battery Drain: If the phone powers on but the short is on a non-critical rail or is intermittent, the device may drain its battery extremely quickly due to constant excessive current draw. If these work, it confirms the system is responsive, pointing more towards a hardware button issue. Damaged Antenna Flex Cable: If the antenna is part of a flexible PCB, inspect it for rips, tears, or creases, especially near hinges or tight bends. Ensure the haptic engine (taptic engine on iPhones) is connected and functioning, and that the home button assembly is seated correctly and not impeded. Not Charging or Intermittent Charging: The phone might not register a charger, charge extremely slowly, or stop charging unexpectedly. Locate the NFC Coil: Refer to a model-specific repair guide or teardown video to pinpoint the exact location of your phone's NFC coil. Flash: Follow the specific instructions for your flashing tool (e.g., load files into Odin, or use Fastboot commands).

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