[SOLVED] I'm looking for YICHEER C668 repair guide or schematics (free) - Phones & Tablets Forum

Hi,

I'm hoping you can help me out with my YICHEER C668. 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.

@Tania

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 YICHEER C668 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/calling-id.430775/
Take a look at comment #920
Also, this : https://xdaforums.com/t/q-echoerom-v14-1-optimization-proposals.2629985/.
You can also check this video starting from minute 8:

The YICHEER C668 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 YICHEER C668 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 YICHEER C668 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 YICHEER C668 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/828293/Where+is+the+middlepin+connected+to+the+mainboard

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

While Optical Image Stabilization (OIS) in cameras utilizes gyroscope data, an OIS problem can also stem from a faulty OIS motor, so it's not a definitive standalone symptom but can contribute. Furthermore, sharp edges from cracked plastic can be uncomfortable or even dangerous to handle. Smartphone battery drain is one of the most common and frustrating problems users face. Solder the Anchor Points/Side Pins: Once the data pins are done, use your soldering iron to securely solder all the larger anchor legs and any other remaining pins on the side of the port to the board. Samsung: Power off, then hold `Volume Up + Bixby Button + Power Button` (for newer models) or `Volume Up + Home Button + Power Button` (for older models). Anti-static Mat and Wrist Strap: To protect sensitive components from ESD. The connectors themselves (ZIF, FPC, or press-fit types) can become loose due to: Component Size: Modern baseband chips and their surrounding components are incredibly small, making precision work under a microscope mandatory. If these are bent, damaged, or not reinstalled, components they are meant to hold can become loose. It moves beyond component swapping into the realm of data manipulation at the chip level. iOS (Activation Lock): For iPhones and iPads, ensure you sign out of your Apple ID and turn off "Find My iPhone." Go to Settings > [Your Name] > Sign Out. The screenshot captures the digital output sent to the display, so if the lines aren't there, the problem lies with the display panel itself or its physical connection, not the data being sent to it. Another software diagnostic technique is booting the phone into Safe Mode (available on most Android devices). Over time, the magnet embedded in the cover can weaken, shift, or even fall out entirely. A damaged bootloader from a failed rollback attempt can lead to permanent bricking. During reassembly, ensure all screws are properly tightened (without over-tightening) and that any metal brackets or shields are correctly seated to make their intended ground connections. Fails to Connect: The phone sees networks but fails to authenticate or obtain an IP address, constantly trying to connect. In many designs, it's part of a larger flex assembly that might also include the earpiece speaker, ambient light sensor, proximity sensor, and sometimes the flood illuminator/dot projector for facial recognition. Voltage at Connector: With the phone powered on (if possible) or plugged in, measure the voltage at the power button's connector on the PCB. Repairing a broken volume flex cable is a rewarding task for the DIY enthusiast, restoring full functionality to your device. Distorted or Muffled Audio: Sound quality is poor, crackly, or staticky. Avoid pure black or pure white backgrounds initially, as these can sometimes mask subtle ghosting. Flux is used in almost every soldering operation on a smartphone logic board: By carefully evaluating symptoms and systematically eliminating potential causes, one can accurately diagnose the problem and determine the most appropriate course of action.4. Remove Phone Case and Old Screen Protector: Ensure the screen is completely bare. It's almost always integrated into a flex cable assembly that also includes the front camera, earpiece speaker, and proximity sensor. Inspecting these springs correctly can help diagnose connectivity issues and prevent further damage during SIM card management. A faulty cable (especially cheap or worn ones) or a weak power adapter can cause charging issues. If it fails, it can cause the lens assembly to "float" loosely, leading to blur, focus hunting, and sometimes a characteristic buzzing or rattling noise from the camera module. Its exact location varies by model but is often found at the bottom of the phone, sometimes integrated into the speaker housing or held by a small bracket and screws.

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