Okay, so I need to figure out how to repair a dead boot on an S918B U7 device using USB in debug mode. Let me start by breaking down the problem. The device isn't booting, which could be due to a corrupted bootloader, firmware issues, or hardware problems. Since the user mentioned using USB and debug mode, maybe they want to reflash the firmware or recover the bootloader via a computer connection.

First, I should recall the standard procedures for Samsung devices, as S918B might be a Samsung model, possibly the Galaxy S23 Ultra (since S918B is a model number for that). If it's a Samsung, tools like Odin are commonly used for firmware flashing. But if the device is completely dead (no download mode, recovery mode), then the process might be more complicated.

The term "DEBUG" here probably refers to accessing some sort of debug interface, maybe through USB. Maybe the device has a Qualcomm chipset, which would allow for EDL (Emergency Download Mode) or similar modes. But Samsung typically uses their own download mode. Alternatively, if the bootloader is dead, maybe using a combination of key presses to force it into download mode.

Steps I should consider:

1. Check if the device can enter Download Mode (Odin Mode) or Recovery Mode. For Samsung, that's usually Volume Down + Power + Bixby or Volume Up + Power + Bixby. If it can enter Download Mode, then flashing firmware via Odin would be straightforward.

2. If it can't enter Download Mode, maybe the battery is dead. Try charging it for a while. If it's a hardware issue like a dead battery, that's a different problem.

3. If the device is completely unresponsive, maybe using a USB jig to force it into Download Mode. A USB jig is a small device that plugs into the USB port and triggers the download mode by simulating a specific resistance.

4. If the bootloader is corrupted, using EDL mode with tools like QFIL (Qualcomm Flash Image Loader) might be necessary, but Samsung devices usually don't support EDL publicly; it might require authorized accounts or specific firehose files.

5. Alternatively, using a combination of key presses while connecting to USB to trigger some debug interface. Maybe using ADB (Android Debug Bridge), but if the device isn't booting, ADB might not be accessible unless in recovery or download mode.

6. Another possibility is using Samsung's official repair tool like Smart Switch's Emergency Recovery, which can reinstall firmware if the device is recognized in a certain state.

7. If the device is detected as a Qualcomm HS-USB QDLoader 9008 port in Device Manager, that indicates EDL mode, which requires specific tools and files to flash.

8. For Samsung, maybe using a combination of Odin with a specific firmware version, ensuring the correct model number and region (CSC) are selected. Using the HOME_CSC might preserve data, but if it's a dead boot, using regular CSC to wipe might be necessary.

9. Also, checking if the USB cable and port are working properly. Sometimes a faulty cable can prevent communication between the device and computer.

10. If all else fails, taking it to a service center might be necessary, especially if it's a hardware issue like a faulty motherboard.

Wait, but the user mentioned "DEBUG" mode. Maybe they are referring to enabling USB debugging in the developer options. But if the device isn't booting, that's not accessible. Unless they can get into recovery mode and enable ADB from there. Some custom recoveries allow ADB access even if the system isn't booting.

Alternatively, if the bootloader is unlocked, maybe fastboot mode can be used. However, Samsung devices typically use Odin instead of fastboot. But some models might have fastboot, but I'm not sure about the S918B.

Another angle: the user might have a U7 variant, which could be a specific carrier model. That might affect the firmware choice. Using the correct firmware for the exact model is crucial.

So, putting together a step-by-step guide: start with checking if the device can enter Download Mode. If yes, use Odin to flash stock firmware. If not, try a USB jig. If still no response, check for EDL mode detection on the computer. If detected in EDL, use QFIL with appropriate files (though Samsung might not make these available publicly). If not, try charging the device, check hardware connections, or seek professional repair.

Also, mentioning the importance of using the correct firmware version and tools, as using wrong files can brick the device further. Including warnings about data loss and advising backup if possible.

Need to verify the exact key combinations for Download Mode on the S918B. For recent Samsung devices, it's usually Volume Up + Volume Down connected to USB, but I need to confirm. Maybe Volume Down + Power + Bixby (if the device has a Bixby button). Alternatively, Volume Up + Power when connecting to USB.

Also, considering that the device might be in a boot loop, but since it's a dead boot, maybe no signs of life. So, key combinations and USB jig are primary methods.

If the user is in a repair scenario, maybe they have access to a box tool like Octoplus or Z3X, which can handle Samsung dead boot repairs by writing bootloaders directly. But those are paid tools and require specific hardware.

In summary, the steps would involve trying to enter Download Mode, using Odin with correct firmware, checking for EDL mode, using a USB jig, and as last resort, professional service. Also, ensuring drivers are installed on the PC, using original USB cables, etc.