Welcome to the SeeMeCNC Community

Rostock MAX V2 has been printing fine for last several months - extruder heats to 235 deg C for ABS, bed was working at 90 deg C. Today the bed wouldn't heat up at all, so pulled out the RAMBO board to see if any fuses blown. Attached is a pic of the main power connector. Note the melted plastic - that can't be good.

What's the next step? Replace the PSU? Replace the RAMBO board (ouch)? Can I replace the power connector myself?

No, that's NOT good. This connector and the way power is divided from one port to the other is a real PITA and asking for trouble. It works however fine if you make it a habit to check the screws every now and then. Personally I would lay off the twisted ATX threads to a bigger screwterminal or
junction box and go to the Rambo connector from there. I never had to change that though because I upgraded to an industrial PSU and a Duet. If you go looking for it, this forum has a lot of picture like yours

The power connector is replaceable. I believe it's this one. You should get ferrules and a crimper, and install them on the ends of the wires. Do not solder or tin the wires.

You can replace the plug (the burnt part), but I'm sure that the socket was damage as well. I would replace both and you should be good to go.

And I gave away my Rambo board after my upgrade to my Duet, so I don't know offhand what this terminal feeds. But I would have to guess that this is the heated board connection. If that's the case (or the hotend for that matter), use a stand alone heated bed controller and just disconnect your wires from that terminal.

These are the ones that I bought, I would avoid the relay types since DC is death on relays. Go with the MOSFET types.

https://www.ebay.com/itm/3D-Printer-Mos ... 2749.l2649