Alright regarding the photo’s and comments, this is most likely caused by a faulty power supply.
It’s an external power supply and replacing this is very easy (as the PS2 Slim does not need to be disassembled).
The power block you have there is not the original one. I recommend to replace it!
Part: https://www.ebay.com/itm/176206189545?itmmeta=01J2CJ5JBMWD3KD1B6XXGKNR95&hash=item2906b513e9:g:bEcAAOSwzg1lsqkB&itmprp=enc%3AAQAJAAAA8DxfSH8HZaYPqF6Z6bGb6hY37D5BZsUA8wqI9txR8th5KtIaKnJVZiLdqwh2TXy2N1Uf7Cx86mSqRp3cqKb8idIktMgYYocgx4B2Ld2SLhIqnJVZESe8R5vQxL3ObqVlH2%2BIaAiq2X0%2BTPvj%2Bks5%2BKvfWz9yauiTQvzETkWi9tM4vKOpbdVWmEKbfnr4jqv0qlQSiwLaqm8pAh4HrAuEHxzo7BSGGFDq0Tu2gtID5EQz1ocbW0IEIrjnrwWDvU7Mo%2Fz8gwuyS%2B7asU4pzTH0Q9d77dXxIqBMMmVwp5kLtRBP8q0JiSVj3awSUc5t49ZUHw%3D%3D%7Ctkp%3ABFBM8KWWkpNk
Just swap the power block and you should be all good again!
Good luck and let us know how it goes!