This was originally going to be a request for help, but in the effort of collecting information for my issue, I was able to find a solution.
I got a misterfpga.co.uk mt32-pi hat, a Raspberry Pi 3 Model A+ board, and a Geekworm enclosure. I prepared my SD card, both via guiformat in Windows and (when that didn't work) using the
mt32pi_installer.sh script. But with each preparation method, every time I tried to turn it on, the OLED panel didn't turn on and show the boot screen. Instead the Green ACT LED blinked in a 4 long, 7 short pattern, indicating with "Power Failure Type B".
I was at wits-end trying to find a solution by scouring all the support posts here. But in preparing my post I stumbled on this thread indicating that new RPi 3 board revisions need new firmware, which is not present in some OS distros.
Because I have a Revision 2 Model A+ PCB, I needed firmware as recent as April 2023 as indicated on the PCN. Since the latest release was prepared in March 2023, my firmware was out of date. So, I updated the firmware from the latest release, following the instructions here: https://pip.raspberrypi.com/categories/ ... rmware.pdf.
For mt32-pi, this meant downloading https://github.com/raspberrypi/firmware ... stable.zip, and copying files from the latest firmware's
boot directory over the corresponding files in the root directory of the SD card:
Once I did that (as well as the other manual steps of adding the ROMs and SoundFonts), the unit booted with the OLED panel showing the splash screen just as expected once plugged into my User port.
Unfortunately, I can't add an issue to the GitHub to requesting a firmware update in a new release, but anyone else with a new Raspberry Pi 3 board revision can also manually update their firmware as a workaround for the time being.