<p><strong>[SOLVED]</strong> on [EDIT 3]</p>
<p>I just finished changing my i3 Mega's PTFE tube and brass nozzle, reassembled it, always taking care not to damage the heating element and thermistor wires.
Then I check the temperature and it was cooling down, despite the set temperature being higher (210 ºC). I turned it off, let it cool down and turned it back on. I set it again to 210 ºC and it only reaches about 135 ºC-140 ºC, not going over it.
So I change the heating element and thermistor for fresh new ones (original ones, that came with the printer) and the same behavior takes place: not heating up beyond 140 ºC.</p>
<p>Any ideas on why this is happening or how to fix it? I checked the connectors and they all seem to be fine. Cooling fans run normally as well.</p>
<p>Printer: Anycubic i3 MEGA. All stock parts, except the new nozzle (standard brass nozzle) and the PTFE tube (a blue one with 1.9 mm internal diameter). I'm setting the temperatures through the printer's interface, as I have always done.</p>
<p><strong>[EDIT]</strong></p>
<p>I did check some stuff with the multimeter and what I got is that:</p>
<ul>
<li>The cables leading to the print head are fine;</li>
<li>The heating element's voltage is correct and the MOSFET seems to be
working fine both by checking the voltage and from the LED that
lights up when it is sending current to the HE;</li>
<li>The resistance of the HE is correct as well;</li>
</ul>
<p>Form that, I am guessing the problem is the current fed to the HE. Later today I will try to heat both the nozzle and the bed to the (safe) max temperatures and see if the bed heating is problematic too. If so, the problem should be related to current.</p>
<p>If anyone has any other ideas, they are appreciated! :)</p>
<p><strong>[EDIT2]</strong></p>
<p>Heating up the print bed and the hotend at the same time didn't affect the bed at all. It heated up at the same pace as usual. Since the cables are fine, it shouldn't be a problem related to the power supply.</p>
<p>So I'm gessing the problem is the motherboard (Trigorilla 1.1). It didn't seem to have any burned components at a glance but I'm gonna check with a multimeter.</p>
<p><strong>[EDIT3]</strong></p>
<p>Turns out I messed up the heatbreaker, so the heatsink was cooling the hotend way too much... This is one way of learning I need to be careful with the heatbreaker, I guess. I'll leave a solution here for the newbies like me that end up in a similar situation.</p>

[{'answer_id': 14609, 'author': 'TurgonTheKingOfGondolin', 'author_id': 16997, 'author_profile': 'https://3dprinting.stackexchange.com/users/16997', 'pm_score': 1, 'selected': False, 'text': '<p>I recently had similar problem with my Prusa i3 MK2.5. It was not able to maintain the set tempereature. I measured the heater catridge (which was new) and its resistance was correct (about 4 ohms). So I changed the hotend MOSFET on the stock board with no result. So I changed the heater catridge for the older one and the issue was gone.</p>\n<p>Since you have changed the heater, it is unlikely that you have multiple bad ones. Thus the only thing that remains is the MOSFET and the power supply. Make sure your power supply is stable under load and can deliver enough current.</p>\n<p>More likely it is the MOSFET. It behaves like a resistor in the path for the current. If it is swithced off, it is like a large resistor (units or tens of megaohms) in series therefore no or very little current can flow. If it is switched on (units or tens of milliohms) the resistance is very small allowing the current to flow through the heater. If a MOSFET is bad, when it is switched on it can have higher resistance (units of ohms) and thus limiting the current and creating a voltage divider. Which you can measure.</p>\n<p>You have to get to the bare wires that lead to the heater. Turn on the heater. Place your voltmeter lead on one wire and the other lead on the otehr wire. The voltemeter should show voltage close to your power supply voltage. If it is showing less, the MOSFET is bad and needs replacing.</p>\n<p>For that you will need soldering tools and skills. You probably have an SMD MOSFET soldered directly to the board. I suggest removing it and replacing it with a THT MOSFET that you would place separately from the board with its own heatsink. When on the board, the board acts as a heatsink.</p>\n<p>If you cannot do that yourself, ask someone to do it for you.</p>\n<p>Before replacing the MOSFET, find whether it is a n-channel or p-channel MOSFET. It will most likely be an n-channel MOSFET but make sure it is.</p>\n<p>If you replace it with THT MOSFET I would suggest these two: IRFZ44N (n-channel) or IRF4905 (p-channel). They are general purpose MOSFETs and should work well enough for drop-in replacement.</p>\n<p>If it is not the MOSFET, power supply or the heater, then I have no idea.</p>\n<p>You should be able to trace one of these loops:\npower supply -&gt; (fuse -&gt;) heater -&gt; n-channel MOSFET -&gt; power supply\npower supply -&gt; (fuse -&gt;) p-channel MOSFET -&gt; heater -&gt; power supply</p>\n<p>Most likely will be the first loop.</p>\n<p>NOTE: Checking the heater resistance might help but does not have to. At room temperature the resistance might be within limits (as with mine example) and when heated up the resistance increases with temperature and thus limiting the current. You would have to measure the resistance when heated up and disconnected from all circuits. (Heat up -&gt; disconnect heater -&gt; measure -&gt; cooldown).</p>\n<p>Hope that helps and good luck.</p>\n'}, {'answer_id': 14667, 'author': 'Lurosset', 'author_id': 24994, 'author_profile': 'https://3dprinting.stackexchange.com/users/24994', 'pm_score': 1, 'selected': True, 'text': '<p>There is a part of the extrusion ensemble called heatbreaker, responsible for sepparating the hot- and cold-ends. The problem is that I screwed the heatbreak up into the cold-end heatsinks and the heated block was now touching it. This made it cool off way too hard.</p>\n<p>The solution was switching to a new, undamaged cold+hotend.</p>\n'}]
['https://3dprinting.stackexchange.com/questions/14607', 'https://3dprinting.stackexchange.com', 'https://3dprinting.stackexchange.com/users/24994/']