Mars 2 Pro no joy :(

[cliffyk]

It sounds to me as though the ball on the platen mount is not round, or has one or more flat spots--it should not move when the locking screws are snugged up as the ball rides in a split sleeve upon which locking screws exert force.

Here's what it all looks like with the sleeve still in it's socket:



Here the sleeve has been removed:



It is good practice to round off the end of the locking screws (remove the projecting ridges left over from when the treads were formed):



To disassemble it:

• loosen the lock screws;
  
• carefully loosen the small socket set screw that holds it all together (it will fly apart when the set screw is just partially removed);
  
• fully remove the set screw to remove the split sleeve;
  

Clean everything to remove old lubricant and debris, then apply a VERY light coat of high pressure grease¹ (SynLube's PTFE is good) to the ball and end's of the lock screws; and assemble in reverse order.

When tightening the lock screws remember you are not replacing a wheel on your car, just a bit "snug" is all it takes.

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¹ - This is counter-intuitive, but trust me, the grease prevents the sleeve and ball from "walking" as the assembly is locked up.