The red led repeatedly blinks 4 long 4 short and the Pi doesn't boot. Tested with various power adapters.
When powered up without an SD card the red led is solid. I'm using an SD card that works fine with a RPi 2 and 3B.
What could be the problem?
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RED LED is controlled by "STATUS_LED_R" signal. Only solid knowledge available to public is that RED LED is a power indicator and turns off when RPi supply voltage goes bellow 5V with some tolerance. Even with "reduced schematic" it really does not matter from where this STATUS signal comes from, what is important is its absence indicating power problem with your SD CONNECTOR, long time known "problem" - they do go bad! If your SD works elsewhere , and your RPi stays on without it - what else is there which is common? Try different SD to verify my guess.