Confirm if your T7 uses an ATmega324PA or ATmega644. You must download the firmware specific to your chip model.
This usually happens if the contrast settings in the firmware don't match your LCD. You may need to edit the config.h file and recompile. lcr t7 firmware link
After flashing, the device will require a full calibration (Short the 1-2-3 test points) to ensure accuracy. Troubleshooting Common Issues Confirm if your T7 uses an ATmega324PA or ATmega644