Did the onboard power hardware change between 3B and 3B+ ( and between 3B+ revisions)?
If you OS is old (i.e. dates from the time of the 3B) it may have an old bootloader which can't cope with any potential change.
Grab a spare SD card, install Bookworm, see if the 3B+ boots. If it does it's a software issue.
Assuming it boots, back up the files in the boot partition of the older SD card then copy the firmware files from the new one to the old one (bootcode.bin, start*, fixup*, etc but not the kernels, overlays, cmdline.txt, config.txt) and retest.
If you OS is old (i.e. dates from the time of the 3B) it may have an old bootloader which can't cope with any potential change.
Grab a spare SD card, install Bookworm, see if the 3B+ boots. If it does it's a software issue.
Assuming it boots, back up the files in the boot partition of the older SD card then copy the firmware files from the new one to the old one (bootcode.bin, start*, fixup*, etc but not the kernels, overlays, cmdline.txt, config.txt) and retest.
Statistics: Posted by thagrol — Mon Jan 20, 2025 12:37 am