Ok. That's a design flaw, since it may allow 5V to appear on the SPI pins on the GPIO header. The GPIO pins accept a maximum of 3.3V. I'm not saying it is the cause, but it's something you should avoid.VDD (Voltage at the Drain) = 5V
You also asked about the power supply. In general, Pi problems are always due to the power supply (except when they are not). In order to eliminate this as a cause I recommend acquiring one official Raspberry Pi 4 PSU and powering all units one at a time with an HDMI monitor but no SD card and no additional hardware attached.
You already tried this with your existing PSU, which you say let the v1.4 Pis show their diagnostic screen, but the v1.5 Pis did not.
Statistics: Posted by ame — Wed Jun 18, 2025 1:25 am