Monoprice Select Mini V2 Heat Bed Re-wire and Thermistor Replacement

My MPSM V2 started to have an issue heating up the bed after about 15 hours of printing and I had not done the bed re-wire yet so it was time. The symptom was the the bed would heat sometimes but not always. I assumed that it was a faulty heating wire or cracked solder joint etc. I came prepared to also re-wire/fix the thermistor as well since I had read that the SMD is likely to eventually fail as well.

However, after chopping up the v2 bed insulation and making rather of mess of it, I found of the v2 thermistor is a glass bead type! Maybe others knew this but I hadn't seen it yet, hence this instructable. Probably it would have been smarter to re-wire the heat bed, test and THEN the thermistor if it was needed but I'm never patient and did this whole thing at one.

I purchased my MPSM V2 direct from Monoprice in February 2018

This isn't a guide or how too, there are great ones already. This is meant to document what I did to replace the thermistor for the V2.

I followed this guide:

https://www.pingle.org/2017/10/08/rewiring-the-hea...

Note that on my V2, I had no choice but to remove the bottom panel, there was a zip tie inside to clip right at the case on the inside top and the only access was with the bottom panel off.

Tools / parts:

- Wire bracket: I printed this: https://www.thingiverse.com/thing:2710870 I'm using it now as you'll see but may print the centre brace version in the future

- New side panel: https://www.thingiverse.com/thing:2451362

- 18 AWG silicon wires (what I used)

- Thermistor, (brand Happisland) NTC 3950 100K (what I used) - this worked out of the box as far as I can see. On mine, the tiny black embossed arrow on the connector was the negative (black) wire

- Kapton tape

- Foil tape

- Heat shrink tubing

- Cable sleeving (PET tubing, spiral wrap etc. What I used)

- New zip ties for brace, wires, etc

- Usual tools: wire cutters, soldering iron, de-solder braid, helping hands, needle nose pliers, allen key for bed, screewdrivers

Step 1: Expose Existing Thermistor

The old thermistor is at the centre of the bed, I used a utility knife to carefully cut/ expose, CAREFULLY

It was soldered to the board, and sealed with a white silicon compound.

If I had known it was a glass bed I never would have started with this step, I would have re-wire the heat bed wires first, tested, then moved to this if had actually been needed.

Step 2: New Thermistor and Wires

I added a thermistor against the bed and with some heat sink grease, then enclosed it in kapton tape. I made sure the bed was as clean as possible first. Since I used the wires that came pre-attached to the themistor, I just ran them down the centre of the heat bed. I also soldered the new 18 AWG silicon wires as well

After I replaced the cut insulation and sealed it back up with foil tape.

Not sure how this will hold up, it's working for now. I wonder if a better installation would have been to solder the thermistor director to the bed like this: http://www.nf6x.net/2016/07/monoprice-select-mini-...

It definitely would have allowed more control on getting the thermistor to reliably contact the base of the board. As it stands now, using a IR heat probe, the surface of my heat bed is ~ 4 to 5 deg C warmer than the setting on the MPSM, which I attribute to poor contact between the board and thermistor.