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:
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
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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.
Step 3: Re-wire Mostly Complete
The new wires were soldered to the existing connectors, cable sleeve added and everything attached
Here is the current status. I need to print the side panel but the heat bed is back to working