Recently my heated build platform stopped reading a sane temperature and was reporting a constant temperature of 255 Celsius. The problem was the thermistor was shorting where it was attached to the board. I think they could probably re-do the HBP board design with the thermistor mount contacts larger and rotated 90 degrees to reduce the chances of this happening. I re-did the joint and put down lots of kapton tape for insulation. Everything seems fine now.
update : temperature reading is more noisy and spiky than it was before. I'm not sure whats up, but at least its working.
debug process looked something like this :
- disassembled, all connections are visibly intact
- signal cable is intact
poking around with multimeter ( power, signal cable disconnected )
- "sig" line appears to be contiguous with ground
- gnd, 5V intact
- [sig.gnd] to 5V : 4.6 Kohm resistance
with signal cable connected
- gnd,5V do in fact have 5V between them
- signal line is resting at gnd
assumption is that the signal line should probably not be shorted to ground like this