Schematics of extension board Builder3D printer

I recently acquired a defective older Builder3D-dual feed 3D printer.
As far as I can determine, the add-on extension board is defective, since when I mount it
on the main board, the power supply which should be 5V at several points that I measure, drops to a waaay to low voltage.

What I'm looking for is the schematics of thisThe main board is extension board.
(Check the image attached.. the extension board is at the bottom right, the HDMI connectors is on that board)


The main board is a PrintrBoard REV D of which I have all info, just the extension board info I'm missing...

Anyone have any info/experience with this?