This is exactly the same issue as discussed two years ago (thus new thread as suggested), title was "MK3 no output" from Oct 22, 2020 (forum doesn't allow url BBCode) - https://forum.icomp.de/index.php?thread/1433-mk3-no-output/
Setup:
ReAmiga 1.5, NTSC with s-video mod, Kickstart 3.1, Original PSU from A500, Indi flashed with latest firmware, signature 53464450
There is no output from Mk3, config tool reports hardware not found. However, if flashtool is run with parameter that causes it to read flash id, it comes back to live and works perfectly fine, surviving reboots, until the machine is powered down. Config tool also recognizes Indi. Same happens if "flashtool flash firmware" is called and just cancelled before the actual flashing starts.
My workaround is to just add "flashtool info" to startup sequence, this wakes Indi up. This Mk3 works fine on my other 1d4 PAL board, I have another Mk3 that behaves exactly the same. Mk2CR works fine on this ReAmiga board out of the box.
I don't need any support; this is just a heads up to investigate, that simply reading flash id causes it to work, in the old thread Mr.Toast I think was flashing it with rescue disk every time.