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How to replace a Heatbed Thermistor (MK4 / MK3.9 / MK3.5)

How to replace a Heatbed Thermistor (MK4 / MK3.9 / MK3.5)

How to replace a Heatbed Thermistor (MK4 / MK3.9 / MK3.5)
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Last updated 4 months ago
How to replace a Heatbed Thermistor (MK4 / MK3.9 / MK3.5)
 Difficulty
Moderate
 Steps
20
 Available languages
CSPLDEESITFR
Introduction
Introduction
This guide will take you through the Heatbed thermistor replacement on your Original Prusa MK4 or MK3.9.
All necessary parts are available in our eshop prusa3d.com.
Note that you have to be logged in to have access to the spare parts section.
Removing the cables
Removing the cables
Removing the cables
Removing the cables
Remove the heatbed-cable-holder on the back of the xBuddy box, held by two M3x6 screws.
Remove the heatbed terminal screw holding the black cable.
Remove the heatbed terminal screw holding the red cable.
Undo the heatbed thermistor cable.
Note there is a safety latch on the connector, which must be pressed in order to be able to disconnect the plug.
Pull the heatbed cable bundle out of the xBuddy box.
Removing the heatbed
Removing the heatbed
Removing the heatbed
Removing the heatbed
Remove the eight M3x4bT screws on the heatbed.
Remove the M3x14bT screw holding the middle of the heatbed.
There is a spacer in the middle of the Y-carriage part, below the heatbed. Save it for later use.
Cable bundle disassembly
Cable bundle disassembly
Cable bundle disassembly
Cable bundle disassembly
Remove the two M3x10 screws holding the cable-cover-top part.
Remove the textile sleeve from the cable bundle.
Remove the M3x10 screw holding the cable-cover-bottom part.
The thermistor cable goes through the cable-cover-bottom part.
Thermistor removal
Thermistor removal
Thermistor removal
From the bottom side of the heatbed, peel off the yellow Kapton tape and silver aluminum tape.
Remove the heatbed thermistor cable from the heatbed.
New heatbed thermistor: parts preparation
New heatbed thermistor: parts preparation
For the following steps, please prepare:
MK4 Heatbed thermistor (1x)
Kapton tape (1x)
Aluminum tape (1x)
Pay attention to the instructions! The "yellow" Kapton tape tends to curl up and you won't be able to restore its initial shape!!
Preparing the heatbed and thermistor
Preparing the heatbed and thermistor
Preparing the heatbed and thermistor
Preparing the heatbed and thermistor
After you have removed the thermistor and the tapes, clean the entire board to remove any grease. You can leave the glue, which was under the aluminum tape.
Place the thermistor onto the heatbed. The tip of the thermistor must be in the circle.
If you miss this spot, the printer will read incorrect temperature values.
Note which side the sensor is attached to the heatbed.
Take the silver aluminium tape and carefully peel off the protective film.
Glue the tape to the heatbed, BUT ONLY in the centre of the thermistor (inside the circle shape). We need to double-check the correct position.
Peel or bend the tape slightly, to reveal the tip of the thermistor.
Covering the thermistor
Covering the thermistor
Maintain the thermistor position and cover it with aluminum tape. Ensure that the whole thermistor sensor is covered with the tape.
Fixing the thermistor in place
Fixing the thermistor in place
Fixing the thermistor in place
Fixing the thermistor in place
Now it is time to apply the final "yellow" Kapton tape. DON'T PEEL OFF the entire tape, it will curl up!!!
Peel off about 1 cm (1/2 inch) strip of the Kapton tape and stick it to the heatbed. Make sure the tape adheres properly.
Make sure the Kapton tape does not overlap any screw holes in the heatbed.
The Kapton tape has a bigger area than the silver tape. Make sure the silver tape is overlapped from all sides.
Keep applying the tape. Make sure it doesnt curl too much in the thermistor area.
Check that the entire adhesive surface is properly attached to the heatbed.
Assembling the heatbed-cable-cover: nylon filament
Assembling the heatbed-cable-cover: nylon filament
Assembling the heatbed-cable-cover: nylon filament
Assembling the heatbed-cable-cover: nylon filament
Guide the heatbed thermistor cable through the heatbed-cable-cover-bottom.

Make sure the nylon filament goes through the opening in the heatbed-cable-cover-bottom. Don't let the nylon filament stick out too much on the other side. It should not protrude more than 2 millimeters.

When inserting the nylon filament, ensure that the filament does not damage the thermistor cables under the printed part.
Orient the curve of the filament as shown in the third picture.
Assembling the heatbed-cable-cover-bottom
Assembling the heatbed-cable-cover-bottom
Assembling the heatbed-cable-cover-bottom
Slide the cable-cover-bottom under the heatbed cable connectors (M3nN nuts). See the correct orientation in the picture.
Secure the cover with the M3x10 screw from the top. Tighten the screw firmly.
Assembling the heatbed-cable-cover-top
Assembling the heatbed-cable-cover-top
Assembling the heatbed-cable-cover-top
Assembling the heatbed-cable-cover-top
Wrap the end of the cable bundle together with the nylon filament in the textile sleeve. Slide the sleeve as far towards the heatbed as possible.
Attach the heatbed-cable-cover-top onto the junction and secure it with two M3x10 screws.
On the bottom side, leave a slack on the thermistor cable for one finger to be pushed through.
Make sure there is not a large gap between the covers.
Wraping the textile sleeve
Wraping the textile sleeve
Finish wrapping the cable bundle in the textile sleeve. Twist the sleeve around the cables but do not twist cables inside.
Attaching the heatbed
Attaching the heatbed
Attaching the heatbed
Attaching the heatbed
Place the spacer onto the Y-carriage and align it with the hole in the center.
Put the heatbed on the Y-carriage and secure it by the M3x14bT going through the spacer. Do not fully tighten the screw yet.
Insert the M3x4bT screws into the remaining holes in the heatbed. Do not fully tighten the screws yet.
Tightening the heatbed
Tightening the heatbed
After all screws are in place, tighten them in the following sequence:
Center screw
First four screws (edges)
Last four screws (corners)
Tighten the screws firmly and carefully.
Guiding the heatbed cables
Guiding the heatbed cables
Guiding the heatbed cables
Guiding the heatbed cables
Guide the heatbed cables through the square opening on the back of the xBuddy Box.
Push the filament through the marked hole below the square opening.
Place the black heatbed cable onto the left terminal and secure it with the terminal screw.
Place the red heatbed cable on the right terminal and secure it with the terminal screw.
Connect the heatbed thermistor cable to the xBuddy board.
Covering the heatbed cables
Covering the heatbed cables
Attach the heatbed-cable-holder to the xBuddy box. The cable bundle must be pointing up. Secure it using two M3x6 screws. Tighten the screws firmly.
Final check
Final check
Final check
Final check
Connect the printer to the electricity and turn it on.
To check if everything works correctly, go to Preheat menu.
Select one of the preheat options such as PLA.
On the bottom of the screen, you should be able to see the temperature readings increase as the heatbed heats up.
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