Out of the blue my A6 printer displays a ERR MAXTEMP BED error and shows about 100 degrees on the bed even when just powered on.

A quick thermistor check shows that it's working properly : about 80 kOhm at room temperature, similar to other units (didn't bother to really check the specs).

Swapping the bed thermistor for the head thermistor connector does not change the temperature readings : the one connected to the bed input gets 100 degrees, while the other one works as expected, proving that the connector and thermosensor are ok.

It has to be in the board. How do I fix it?

  • $\begingroup$ the question clearly established the error is the board: the sensors in the bed were fine and the board refused to acknowledge so. Firmware also was fine because it triggered the maxtemp that is expected in case of a broken connection $\endgroup$
    – Trish
    Oct 4 '19 at 8:31

I'm posting the answer here hoping that will help anyone that encounters a similar issue. A post on another site indicated that these boards are known to be problematic.

After some digging, I came up with the schematics of this part of the mainboard :

enter image description here

I'm not an electronics genius, but clearly it's not the capacitor; so that leaves the pull-up resistor or the microcontroller.

Since the printer is already unusable, no harm in trying to replace the resistor, which is located just next to the connectors; for the bed it was the second from left, but if you encounter the same issue with another input, you may need to locate the proper one.

Make sure you have the proper tools and knowledge for removing and soldering SMD components; in my case it was a 4k7 resistor in 0805 footprint.

Bottomline : replacing a less-than-one-cent resistor saves a month of waiting for the delivery of a $25 board.

  • $\begingroup$ For note: The C43 is the capacitor belonging to the defunct R37 resistor that was replaced. Search for the labeling. $\endgroup$
    – Trish
    Oct 4 '19 at 8:24

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