Inbound power is the problem. And as you mentioned, a new board won't help here unless your power supply is dragging the inbound power down.

Measure DC voltage directly across the low voltage cap in the base. Two measurements - #1 with J1 disconnected at power supply, #2 with J1 connected.
Significant change? Also measure AC voltage there with J1 connected.

If that cap is original (yours looks original with another cap taped to the side) - definitely time to replace.
For this, use a 10000uF to 12000uF, 25V screw terminal cap.
As an example, I use this in my Ice Fever (80A) but you should be able to locate these in Italy:
https://www.greatplainselectronics.com/proddetail.asp?prod=CCG-12000uF-25V

If it was me - I would stick with the name brand caps such as the Elko 12000uF (5th link).
That one might fit within the VR3A capacitor clamp that you have shown in the photo below the old 6800uF radial cap.

Ed

No -- too old. That one has a date code from the 44th week of 1978.
No need for screw terminal caps, I just find them easier to use in this location.

That power supply still needs the base capacitor that OP which is what he is looking for.

With the exception of the dimensions.. Marco really needs to update the descriptions on that page. His description is verbatim from what was on my website (he used to sell my boards back when I was still making boards). He highlights features from my board that this boards does not have.