How to replace a heatbed thermistor (MINI/MINI+)

Relevant for:

MINI MINI+

Last updated 7 months ago

Difficulty

Moderate

Steps

Available languages

Step 1 Introduction

⬢This guide will take you through the replacement of the heatbed thermistor on the Original Prusa MINI and MINI+.

Some parts might slightly differ. However, it does not affect the procedure.

⬢If the heatbed thermistor is damaged, you will see the MINTEMP error message on the display when you preheat the printer.

⬢Heatbed thermistor replacement set is available in our eshop shop.prusa3d.com

Note that you have to be logged in to have access to the spare parts section.

Step 2 Tools necessary for this guide

⬢For the following steps, please prepare:

⬢2.5mm Allen key

⬢TX10 Torx key

Step 3 Opening the box with the electronics

Turn the printer off and unplug it from the socket!

⬢Release and remove the M3 screw on the box with the electronics.

⬢Remove the printed cable cover.

⬢Lift the second electronics cover slightly. Before you remove it completely, pull it first towards the aluminium extrusion to release both pins from the holes.

Step 4 Disconnecting the heatbed

⬢Disconnect the heatbed cable.

⬢Disconnect the heatbed thermistor.

Step 5 Removing the heatbed

⬢Remove the steel sheet.

⬢Release and remove 9 screws on the heatbed by using a Torx TX10 key.

⬢Remove the heatbed from the printer.

Step 6 Disassembly the heatbed

⬢Put the heatbed on the clean surface, far from metal parts (screws, Allen keys, ...). There are magnets on the bottom side of the heatbed.

⬢Release and remove the M3x12 screw from the MINI-heatbed-cable-cover-top.

⬢Remove the MINI-heatbed-cable-cover-top.

⬢Remove the MINI-heatbed-cable-cover-bottom.

⬢Remove the textile sleeve all the way from the heatbed cable bundle.

Step 7 Removing the thermistor

⬢Turn the heatbed upside down.

⬢Remove the (yellow) Kapton tape.

⬢Remove the (silver) aluminium tape.

⬢Remove the thermistor from the heatbed and cable bundle.

Keep the thermistor, for now, we will need to shape the new one according to the old one.

Step 8 Cleaning the surface

⬢Use IPA and a paper towel to remove glue residue from the thermistor area.

Step 9 New thermistor - parts preparation

⬢For the following steps, please prepare:

⬢New thermistor (1x)

⬢New aluminium tape (1x)

⬢New polyimide/Kapton tape (1x)

Since we have started shipping the universal size of polyimide/Kapton tape for use on multiple Prusa printers, it is necessary to cut the tape to the required size for your printer.

⬢Please, cut the tape in square shape 7 x 7 cm (2.76 x 2.76 inch).

Step 10 New thermistor installation

IMPORTANT: we need to adjust the shape of the new thermistor to match the old one.

⬢The new thermistor has a pair of straight wires.

⬢The old thermistor is shaped in an "oval".

⬢Guide the thermistor cable through the gap between heatbed cables.

⬢Lay the new and properly shaped thermistor on the heatbed and fix it temporarily using pliers or any other tool.

⬢Make sure the tip of the thermistor is in the circle. If you miss this spot, the printer will read incorrect temperature values.

Step 11 Fixing the thermistor in place

⬢Take the silver aluminium tape and carefully peel off the protective film.

⬢Glue the tape to the heatbed, BUT ONLY in the center of the thermistor (inside the oval shape). We need to double-check the correct position.

⬢Peel or bend the tape slightly, to reveal the tip of the thermistor.

⬢Ensure again, that the tip is right above the circle. If you miss this spot, the printer will read incorrect values.

⬢As soon as the correct position is ensured glue the entire tape to the board.

Step 12 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!!!

Be careful not to stick the tape over the screw holes.

⬢The Kapton tape has a bigger area than the silver tape. Make sure the silver tape is overlapped from all sides.

⬢Peel off about 0.5 cm (0.2 inch) strip of the Kapton tape and glue it to the heatbed, like in the picture. Ensure the tape is properly connected.

⬢Continue with gluing the tape and stop for a second above the area with the thermistor. Pay great attention to glue the tape everywhere.

⬢Finish gluing the tape to the heatbed. Double-check the entire surface is properly connected.

Step 13 Checking the heatbed cable connection

⬢Make sure the heatbed cable connectors are tightened and do not move freely.

Loose screws on the connectors can fatally damage the electronics.

Step 14 Reassembling the heatbed

⬢Turn the heatbed upside down.

⬢Place the MINI-heatbed-cable-cover-bottom under the connectors like in the picture.

⬢Wrap the textile sleeve on the heatbed cable bundle all the way to connectors on the heatbed.

⬢Make sure the connectors fit properly into the cover.

⬢Take the MINI-heatbed-cable-cover-top and slide it over the screw heads.

⬢Secure it with the M3x12 screw.

Step 15 Heatbed cable guiding

⬢Make sure the textile sleeve is properly inserted in the covers.

⬢Make sure there is almost no gap between both covers.

Step 16 Mounting the heatbed

⬢Move the Y-carriage all the way to you.

⬢Place the heatbed on the Y-carriage.

⬢Align all 9 holes on the heatbed with the heatbed spacers.

⬢Insert nine M3x4b screws to the holes. DON'T fully tighten the screws.

⬢After all screws are in place, tighten them in the following order:

⬢Center screw

⬢First four screws (edges)

⬢Last four screws (corners)

Step 17 Connecting the heatbed cables

⬢Connect the heatbed thermistor cable in the lower connector on the Buddy board.

⬢Connect the heatbed cable in the lower green connector on the Buddy board.

Step 18 Covering the electronics

⬢Before covering the electronics, make sure the square nut is correctly positioned in the printed part. The nut must not fall out! This can cause fatal damage to the electronics.

⬢Insert the cover back in, make sure it is properly seated in the slot. Note: on an older design, there were holes instead of slots, the assembly procedure is the same.

⬢Place the second cover on the top and arrange the cables:

⬢Extruder bundle, ensure the textile sleeve is partially in. Also, it must be tilted away from the printer.

⬢Heatbed bundle, ensure the textile sleeve is partially inside the box.

⬢Filament sensor cable (optional), ensure that the textile sleeve wrapped around the cables is partially inside the box.

⬢Now, tighten the second cover. Check that no cable is pinched.