Playstation 2

Troubleshooting

I got a HDMI adapter, and I don't get a signal, just a black screen

In the PS2's own console settings, switch the display output mode from RGB to Composite (the other option). Internally, these adapters use the composite output to get a HD signal out of the console, and the RGB output is not capable of that. Once you change this setting, assuming you're currently connected with component cables (yellow/white/red) to get into the settings menu, your TV will turn black immediately; this is fine, once you restart the console with the HDMI adapter connected, you'll get signal over HDMI.

When running OPL (Open-PS2-Loader), the screen keeps flickering or turning on and off

In the PS2's own console settings, turn off optical display output. This seems to happen specifically when using composite input (eg. when using an HDMI adapter for the PS2, which internally uses composite). The problem should go away. This won't work if you're actually using the optical output, of course.

OPL is not detecting anything on my SD card (using an MX4SIO adapter)

It seems that OPL, at least with some cards, requires 32KB cluster size for ExFAT-formatted systems. If you have a card bigger than 32GB, the default cluster size is probably bigger; and your filesystem is almost certainly ExFAT.  Reformat it as ExFAT with a 32KB cluster size and it should work. On Windows, Rufus can apparently be used for this.

Reformatting will delete any data currently on the disk.

Be careful that you set the cluster size, and not the sector size. Changing the sector size can brick your card in some cases, and it will not fix the issue.