[SOLVED] I'm looking for LG KF757 Secret repair guide or schematics (free) - Phones & Tablets Forum

Hi,

I'm hoping you can help me out with my LG KF757 Secret. 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.

@Junior

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 KF757 Secret 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/cm10-1-4-2-2-unofficial-jellybeer-v4-18-hybrid-beergang-may-14.2280419/
Take a look at comment #230
Also, this : https://xdaforums.com/t/swap-out-microdrive.332082/.
You can also check this video starting from minute 8:

The LG KF757 Secret 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 KF757 Secret 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 KF757 Secret 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 KF757 Secret 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://xdaforums.com/t/image-galaxy-s4-product-image-revealed-real-or-fake.2112273/

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

Camera: Testing front and rear cameras, flash, focus, video recording. Over time, it learns your battery's actual capacity by measuring the charge put into it versus the percentage gained. Power Off and Disconnect Battery: If the preliminary test was successful, power off the phone and disconnect the battery again before proceeding with full reassembly. A technician can properly assess the damage, ensure correct reassembly, and replace any damaged components or the frame itself. Manual Image Files: As a low-tech alternative, you can download or create simple solid-color image files (e.g., a pure red JPG, a pure green JPG, etc.) and view them full-screen in your phone's gallery app. This sustained pressure is crucial for the adhesive to bond effectively and create a complete, watertight seal. Insert SIM Card: Insert a known good SIM card into the newly installed reader. While some cases might necessitate professional repair or component replacement, many can be resolved with simple DIY troubleshooting and cleaning. Bridging a broken trace is one of the most satisfying repairs in micro-soldering. It's usually a small, coin-shaped component or a linear resonant actuator (LRA), often situated in the top corner near the power/volume buttons, or occasionally at the bottom of the device. Replacing the primary communication modem IC, often referred to as the baseband IC or cellular modem, is one of the most challenging and intricate repairs on a smartphone motherboard. Vacuum Chamber: For professional setups, a vacuum chamber can rapidly draw out moisture. Careful transfer of all internal components , logic boards, cameras, vibrator motors, speakers, and antennas , from the old frame to the new one is essential. Methodically check other major power rails (e.g., those supplying backlight, CPU, storage) if the main line isn't shorted. This is a delicate process of applying tiny solder balls to the chip's pads. Professional Repairs: Even professional repair services can sometimes exhibit poor quality control, resulting in rushed jobs with missing or misaligned screws. Modern smartphone motherboards are incredibly dense, compact, and feature multiple internal layers where traces (electrical pathways) are routed. This often manifests as intermittent charging or the cable not fully inserting. The Process of Safely Transferring Data from an Old Screen to a New Smartphone Screen Missing Components: Tiny resistors, capacitors, or inductors around the RF IC can easily be knocked off. Functionality Test: After the adhesive has cured and you've removed the clamps/pressure, perform a full functionality test (as described in Topic 2) to ensure everything works as expected. AGPS data download: These apps often have an option to "download A-GPS data" or "reset A-GPS data." A-GPS (Assisted GPS) uses cellular network data to quickly get an initial satellite fix. Carrier Aggregation (CA): Modern networks use CA to combine multiple bands for higher speeds. Force Restart: Attempt a hard reset (specific button combinations vary by phone model). While occasional crashes might be due to a one-off glitch, frequent and random crashes across multiple applications indicate a deeper underlying problem, ranging from software conflicts to hardware malfunctions. Causes: Drops, prying components with excessive force, poorly executed soldering (too much heat, prying hot components), or liquid ingress. Parts Swapping: Genuine parts can be salvaged from damaged devices, complicating the ecosystem. Ensure the phone isn't being charged in a hot environment or used extensively in direct sunlight. A short or excessive draw on these rails points to the FEM or its surrounding components. If it only happens on one network, the problem might be with that specific router or network environment.

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