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: