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Repairs – Change the Hotbed Thermistor – I3 Mega S



This machine has long been problematic so when this came up I used it as an opportunity to show you guys how to change the thermistor of my hot bed.

Transcript

Hello everybody and welcome to another repair video. In today’s video I’ll be showing you how I was able to replace the thermistor on my 3D printer and finally fix the ongoing issues with my printer. Now a quick disclaimer that this video wasn’t sponsored by Anycubic and to do this modification at your own risk.

So before anyone asks, I’ve been having issues with this machine for a very long time. Unlike my first I3 Mega, this one has been prone to stopping randomly, and although I’ve replaced quite a few components and updated the firmware the issues have persisted. While most of the issues were from prints failing to complete on this occasion things were quite different. The print bed temperatures were fluctuating quite dramatically which was preventing the print from starting. I had replaced the motherboard in the past, updated the firmware, replaced the SD card reader and re-checked the wiring, so I was fairly certain that these weren’t the issues. So I unscrewed the build plate and began double-checking the resistance with a multimeter.

The very first thing I noticed was that the thermistor was completely black which could be normal however I still decided to double-check. So the resistance rating for most of these thermistors is normally 100k, but I was reading some strange numbers. Also, you’ll want to make sure that the print bed is fully cooled down prior to trying to take your measurements since this seems to affect the readouts. I also noticed that the readings weren’t the same once re-soldered together so that’ll be something you want to keep in mind.

While soldering, I noticed that the old solder wasn’t coming off easily even when adding flux. I suspected that the heat bed was helping to defuse the heat, so I preheated the surface, but that wasn’t enough. I ended up adding some fresh solder to the connections along with flux which then allowed me to use solder wick to remove the material from the connection points. Once I removed the old thermistor I noticed that there was a small hole where the thermistor was located. With the black coating it’s difficult to see so here’s a small diagram to show you what it looked like.

I did need to shape the wires slightly to make sure that they would align correctly however everything soldered together nicely. After testing it the resistance seemed better than before, but I wasn’t entirely sure that it had worked since i wasn’t getting the same results as when I tested the thermistor prior to soldering it into place. I decided to put the machine back together and do a final test.

When everything was put back together I began running the test print. I had outfitted this machine with a 0.8 nozzle since it was originally used for printing large functional parts. The first print didn’t look good in the beginning however the temperatures quickly stabilized and stopped fluctuating irregularly after the first print. All following prints had stable temperatures. This also got me thinking about what might be the issue with this machine. When I began hitting the reset bottom for the machine all issues stopped when starting a new print. So I began to experiment with the G-code a bit and after trying out the G999 command and a couple of others I decided to compare it with another machines settings. This is where it discovered that there was some extra lines of code and decided to copy and past the end G-code from my CR10-V3 to see if this made a difference. Sure enough, after over a year of being problematic the problem finally seems to be fixed. Or was it? Unfortunately once I started a larger print, I began running into the same issues as before although at least my print bed temperatures were correct this time. I will be posting the G-code in the description below just in case it does end up working for you.

Having seen a lot of issues with the wiring hubs of this machine I will begin trying to rewire it in an effort to get the machine working properly. Make sure to keep an eye out for the follow-up video when I undergo this process since I will be including a price breakdown as well.

So was this repair worth it? I’d say yes considering that the thermistors are fairly inexpensive now days especially the one’s which are compatible with the Anycubic printer. In case some of you were wondering, I bought the NTC 3950 100k version at the time from Amazon however if you’re worried about compatibility issues and don’t want to risk having to change your firmware settings you may which to order it from the main Anycubic website. After over a year of having a problematic machine, I finally seem to have one that works on par with my expectations.