Marantz CD63 & CD67 mods list

I solved my ground loop problem at last! It was so obvious in the end. My two clock PSUs share secondaries on the same transformer, but they were wired in opposite directions to each other. I had been hunting around the servo and output supplies, but the problem was right in front of my nose the entire time!
 
It's not a switching design, but nonetheless not suited for use in CD players.

These types of circuits can deliver just a small amount of current, the series capacitor acts as an impedance and blocks most part of the mains voltage and must be very large for larger currents, which is not very practical. It behaves like a big series resistor, but without the heat dissipation. It can be used for a relay or some LEDs.

And then there's the safety-issue: without a transformer, the circuit is directly connected to the mains grid. Potentially lethal!