The most common cause for this I’ve found is something sticking in the arms. Either the idler gear on its shaft (this is fixed in the 4.1 upgrade), or the belt spool itself sticking.
Here’s a technique I’ve been trying to diagnose which arm may be problematic.
Do the usual to unmount the M4
- Extend all the belts
- Go into config, and drop the retraction force down to 600 and “Save” it.
- Now try retracting.
- If any belt fully retracts (not just partially), write the retraction force on a post-it note and stick the note to that arm’s motor. i.e. keep a record of the retraction force on the arm itself.
- For any belts that have not fully retracted, go back to step 2 and up the retraction force by 100. Keep repeating this process until all arms have fully retracted.
Any arm that needs a retraction force of over 2000 needs to be disassembled and cleaned, paying special attention to the idler gear and the belt spool.
When putting the arms back into the stack, try to have them in order from lowest retraction force to highest (here’s where those post-it notes come in), with the lowest being closest to the sled.