the first photo shows some white that doesn't look kosher. the brown stuff near the solder pads is, I'm guessing, Flux residue.
The other photos where there's lots of brown stuff under the capacitors, is glue.
That barrel socket definitely has seen better days, makes one wonder what the inside contacts look like.
Anyway, plug it in, turn it on, and measure the voltages.
Right to repair is fundamental. You own it, you're allowed to fix it.
How To Ask Questions The Smart Way
Do not ask Why !
https://www.vogonswiki.com/index.php/Serial_port