HMS 0700-4000-0002-0002: Filament Buffer Position Signal
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HMS 0700-4000-0002-0002 means the filament buffer position signal is in error and the position sensor may be malfunctioning. The buffer stores slack so retraction does not drag on the spool, and the machine needs to know how much slack is in it.
A position signal is an analogue value. When it goes wrong it usually goes wrong smoothly, which is why this one is worth measuring rather than guessing at.
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Understand what a position signal looks like
A potentiometer assortment gives you the same kind of part on the bench, so you can see how a position reading should sweep. It also lets you substitute a known value into a circuit while testing.
Watch the signal move
A proper oscilloscope probe set, switchable between 1X and 10X, is what lets you see a slowly changing voltage without loading it. A cheap probe on a high-impedance signal changes the thing it is measuring.
Refit the board without stressing it
Nylon standoffs hold a small board at the right height without the shorts a metal screw against a track will cause. They are the part most often lost and most often substituted badly.
Then move the buffer by hand
With the AMS open and off, move the buffer through its travel and watch the reported value. A reading that jumps or sticks at one point is the sensor; a smooth sweep means the fault is upstream.
How HMS 0700-4000-0002-0002 differs from a motor fault
A motor with no signal is HMS 0700-1000-0002-0004. This code is a sensor reporting the wrong position rather than nothing moving.
What the buffer is protecting
It absorbs the pull that otherwise overloads a slot, which is HMS 0700-6000-0002-0001. Layout is in the AMS hub guide and the system in printers with an AMS.
HMS 0700-4000-0002-0002 FAQ
What does the buffer actually do?
Stores slack so retraction does not fight the spool.
Can I print with it faulty?
Feeding gets unreliable, especially with retraction-heavy prints.
Is the sensor mechanical or optical?
It reports position either way. The test is the same: move it slowly.
Could it be the cable?
Yes, and an intermittent reading points that way.
Signals that fail to arrive have their own family. HMS 0500-0400-0001-0002 means the printer failed to report its print state, and the job itself carries on unaffected.
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