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Yesterday I received a donation of #Commodore hardware from a childhood friend, including this lightly used #Plus4 in its original box, with perfectly preserved manuals.
In spite of its excellent conditions, this #Plus4 does not power up at all 
The AC fuse in the PSU and the DC one on the motherboard aren't blown, so my next suspect is the "wedge" power-supply.
These PSUs made by #Commodore in the '80s are known to fail often. Sometimes, they even output higher voltages on the +5V line which powers ALL the chips on the board! 
But this isn't the case: the power LED is steady off, and I measured near 0 volts on VCC pins.
Last night I carefully opened the #Commodore wedge PSU with flat screwdrivers, but the interior is filled with a potting resin which is stuck to the box.
I measured +5V after the regulator, but it drops as soon as I connect the #Plus4 board.
If the board was shorted, the fuse would have blown. So I guess the regulator is unable to source more than a few milliamps.
I couldn't make further progress without tools. I'm literally working from my mother's kitchen! 
@codewiz yeah I guess the regulator is borked. Ditch the old PSU, buy a round DIN socket for the Plus/4 and use a modern C64 PSU. Sad that the PSUs are all potted. Otherwise one could easily repair them.
@root42 Even these C64 -> Plus4 adapters cost a lot, like 15 EUR + shipping.
Ok ok, I guess spending 2 hours repairing a PSU would cost more, but... isn't it more fun? 
@codewiz if the PSU weren‘t potted: go for it! That’s why I love the original 1541. You can repair the whole PSU section easily.