MMU2>MMU3 Upgrade Idler stepper not homing
Today I observed several folks reporting on Facebook they have been experiencing error message on MK3 screen about Idler not homing.
Background: When Printer is initialised, it starts MMU. MMU initialised by operating Idler Stepper and Selectors stepper, by sending them to home position on each end-stop. This causes an inductive pulse to be sent back from stepper to Trinamics driver, which is sensed by the driver chip as the home position. (Those steppers are "dumb" not having other means to indicate their position.). This generally works fine, however several things can cause it to malfunction.
1.
Firstly its driven by 5 volt rail, and thus affected by the "low 5V rail" issues that has been addressed by the new PD Board and earlier similar fixes.
But secondly, the connectors for the steppers are tin plated female molex pins inside the black housings. Those tn plated pins can become oxidised particularly by humid conditions. What is causes is higher resistance at the point of mating the pin with gold plated male pin on the MMU Conrol Board. These are two dissimilar metals, and corrosion can occur due that simple fact.
2.
So, the ultimate cure would be to change the female pins for types having resistance to oxidation corrosion, eg. Gold Flashed or plated variety. This, it seems, is beyond Prusa, and so SFAIAA, they have not upgraded them, not even in new MMU3 units (requires confirmation). So an alternative cure is to apply a chemical contact cleaner that will remove the oxide layer. Such a product is called "Deoxit D" that is readily available at electronics suppliers. It has a companion product "Deoxit G" which is a protectant, making the cure last longer.
Prusa have already addressed the first issue above, by providing the PD Board for MMU3. But they have not addressed the second issue, probably think its of no consequence. But We already see instances of this issue occurring as folks dust off their MMU2 sitting on the shelf and start to use it again, now there are new firmware and hardware improvements available. This issue could easily be addressed by:
a) include a piece about applying contact cleaner in the build instructions for MMU2>MMU3 upgrade.
b) Include a reference to this issue in any documentation (yet to be deployed) for decode of the various new error messages that now appear on Mk3 screen (and presumably Mk4 screen as well).
I hate to say I told you so, but I told you (Prusa) so, in my document that was written and sent to PR in September and October 2022. Evidently, the part about improving the 5V supply was read and understood, but the part about using contact cleaner was not.
Here is a link to it, in case you lost it already: https://github.com/bryn51/Prusa.git