Anycubic i3 MEGA not heating past 140 ºC
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.
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.
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.
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.
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.
If you cannot do that yourself, ask someone to do it for you.
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.
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.
If it is not the MOSFET, power supply or the heater, then I have no idea.
You should be able to trace one of these loops:
power supply -&gt; (fuse -&gt;) heater -&gt; n-channel MOSFET -&gt; power supply
power supply -&gt; (fuse -&gt;) p-channel MOSFET -&gt; heater -&gt; power supply
Most likely will be the first loop.
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).
Hope that helps and good luck.