marantz cd63 digital out problem

I have re-housed my CD63 in a wooden case along with a Monica2 NOS DAC.

I have taken the spdif signal for the Monica module direct from the board (having removed the PCB mounted socket. I have taken the positive signal from the rear peg (fitted by myself) and the return from the front one. I have not used the other rear connection, which appears to be designed to be grounded to the chassis, but I have attached a lead to it for use if needed.

I cannot get a lock on my DAC (which works OK from an alternative spdif signal). Can someone start me off on my debugging route please? Maybe it is a chassis grounding issue, or have I got my connections mixed up.

TIA.

Photo of digi connection:
 

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