saec 407/23 headshell compatability

I've never seen your arm/ceramic head.

Curiously, this photo shows a separate soldered wire from edge of headshell fitting to screw of the lifter. Never saw that before either.

Have you tried a wooden headshell?

Probably not the problem, just to know: Ortofon cartridge wires are slightly different diameters at each end

Ortofon SH-4 headshell and Ortofon cartridges’ terminal PINs diameter is 1.2 mm, whilst Ortofon headshells’ PINs diameter is normally only 1.0 mm.

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My Blackbird Tonearm, using RCA,s (4 wires) have a junction box converting tonearm wires left/left ground and right/right ground to the RCA jacks. Notice, the mini din connector is only 4 wires (not 5 pin din).

AND there is a separate ground wire from the tonearm base to the junction box’s ground terminal. That’s like the 5th wire of a 5 pin din connector. Perhaps you can make a separate ground jumper and try grounding the arm.

Take a close look at the SAEC headshell that does not hum. What is different about it compared to the 3 that do hum? Pay particular attention to the orientation of the contacts points where headshell joins arm wand.