Partially Solved Bed temperature reading stuck at 100°C

Hey there. My 3D printer (Robo R1+) reports 105°C present on the heated bed, as it is on room temperature (in both Cura and MatterControl), but everything else still works (motors, lights, extruder temperature readings...). When I disconnect the bed from the printer, the slicer reads 103°C, and when I use an 81K resistor and put it on the connector for the bed (on the corresponding thermistor sensing wires), the reading goes up to 120°C, going back down to 103°C when removed.

I measured the resistance of the bed thermistor at room temperature and it was 91K, I also measured the voltage coming from the sensing wires and measured 2.734V DC. I flashed the official firmware into the printer and the issue persisted.
I decided to unscrew the bottom panel and inspect the internal components, I found some dust, but most importantly, I saw that a pin that comes from the 12V DC power supply and into the RAMPS board, was black and a little burnt (it is a pin labeled as "+ (11A)"). I didn't find any other issues at a surface level, I checked continuity on the sensing wires and lightly sprayed everything with electronic parts cleaner.

Upon reconnecting, the issue still persisted, so I believe there might be more damage than what I could see (since I didn't take out the boards or anything). What do y'all recommend me to do at this point? Just straight up buy another RAMPS board or printer?

 

I have taken the RAMPS and the Arduino board completely off the printer for a better visual inspection, aside from the overheated pin, nothing else seemed to be damaged. I labeled all the connections and disconnected them off the board, so I can take it to one of my electronics teachers to debug it. I tested the output of the power supply unit and they read 12.08V DC, the nominal.

I will keep the thread updated, and I will also take any suggestions or advice the community may have.

 

Upon some off-printer testing, I could determine that the circuit responsible for sensing the bed thermistor is damaged in some way.
I used some leads and a breadboard to measure the voltages that the thermistor reader circuit was using, taking the extruder one as a reference (The T0 pair of connectors belonging to the extruder thermistor, and T1 to the heated bed thermistor).

With an open circuit, the extruder one (T0) measured exactly 5V, while the bed one (T1) measured 2.734V. A voltage drop or a bad connection was preventing the full 5V from even reaching the terminals where it is supposed to be.
There are quite a few resistors around the connectors, each measured the correct resistance value according to what was displayed on them.
There are also capacitors, but I'm unable to reliable measure their value using my multimeter.

I used the continuity function on my multimeter to determine where the traces and pins went, one again using the components around T0 to know what to expect on T1. Every single trace lead somewhere and there was no resistance on any trace, for both T0 and T1. I used references to ground and +5VDC as well, no resistance on those traces either.

I might be hitting a dead end there, what I'll try to do is to reflow the solder on T1 and maybe on some of the SMD components surrounding it, otherwise I don't have more ideas.

 

It's been a while. A few months ago I bought the generic RAMPS 1.4 board, and went ahead with the board conversion steps, only to find that the bad reading was still there.

So I determined that the Arduino itself was the failing component. I got my hands onto another Arduino MEGA today, and once I flashed the correct software into it and installed it into my printer, the reading was finally giving me proper values.

However, I was not able to properly connect the endstops to the new RAMPS board. Every wiring diagram and picture I've seen has different configurations, and none of them worked for me, except for the X axis only. Every new configuration I've tried makes the printer behave in a different way when I home the Y and Z axes. Right now I have a X00Y0Z configuration (the zeros being empty headers) and homing the Y axis makes the bed move just a little before stopping, even though it is nowhere close the limit switch. You can move the bed by spamming the button, and when it does get to the switch, it just makes the motor skip. As for the Z axis, it sometimes does not respond to the homing input, and when it does, it completely ignores de endstop, and I have to shut the main switch off to prevent it from damaging the bed.

Other configurations give off more bizzare results, such as the bed travelling to the oppossite direction of the switch when homing.
Is there a proper diagram I can follow to hook these up correctly? Thanks!

 

Hey, just curious—did you manage to fix the heated bed issue? Would love to hear how it turned out!