After a hiatus of a couple of years, I’m finally starting to get back into 3D printing. One of my Printrboards got messed up when some wires on my hot end shorted. The hot end temperature was no longer reading correctly. Since my other Printrboards all work correctly, I knew that the problem was not a bad thermistor or wiring. Instead of throwing out the Printrboard, I decided to try to fix it. The first step was to have a look @ the Printrboard schematic. Here’s what the temperature sensing circuit looks like:
I got out my ohmmeter, and R9 was OK, but E-THERM to GND was reading as a dead short, so I assumed that C10 was bad. This was a good opportunity to play with my AOYUE INT 2702 hot air rework station, which I’d never used. I removed C10, and much to my chagrin, the reading from the hot end ADC pin was stuck at 1024. Furthermore, shorting E-THERM to ground was causing my Printrboard to reboot! This led me to conclude that something was fried inside the AT90USB1286 on the ADC pin connected to E-THERM (PF1_ADC1). Conveniently, 2 other ADC pins, ADC2 and ADC3, are broken out into an expansion header on the Printrboard. I first soldered C10 back into place. The trace connecting ADC1 to E-THERM was inaccessible, so I couldn’t cut it. Instead, I disconnected it by lifting the pin on the MCU off the PCB. Next, I connected a piece of 40AWG wire wrap wire between the A2 header pin and the E-THERM trace.
Success, the ADC2 pin is working perfectly! The only caveat is that I have to remember to run modified firmware when using this board, reassigning the hot end thermistor pin from ADC1 to ADC2. In Marlin firmware, it’s as simple as finding the Printrboard section of pins.h, and reassigning TEMP_0_PIN from 1 to 2.