Swapping the inputs should reverse the direction of drift if the problem is the channel switch and leaves the Y amplifier closer to its normal operating point.
My 7904 (B103172) has the single channel switch IC U625. I had to remove the A7 board to swap the IC and strictly speaking I do not know if the problem was the part, its pins, or having re-seated the four Peltola connectors on the back of the A7 board. One day I should swap the original back in to check.
Roger
