Heatbed not heating - very strange situation  

Heatbed not heating - very strange situation

So I am a very satisfied PRUSA customer using PRUSA printers more than 5 years with no major issues up to now 🙂

However, today I encountered a problem that got me puzzled. I wanted to print a model, so I preheated the printer for PET while slicing the model and exporting it to the SD card. Suddenly, as heating was half way through, a "thermal anomaly" error came up and the buzzer went nuts.

So I went to investigate and quickly noticed, that the heatbed is not heating. The red LED on the heatbed is also not lighting up. I did the following checks:

1. I verified that the thermistor is working correctly (it shows the room temperature accurately and also works if I heat the bed using a hot air blower

2. The heatbed itself also works. It shows the correct resistance (3,4 Ohm) and also if disconnected from the electronics board and connected directly to an external PSU it heats as it should.

3. The board supplies 24V to the heatbed terminals with the heatbed disconnected. The voltage check in the printer menu also shows 24V.

4. All fuses are ok

Now the strange problem: If I connect the (otherwise working) heatbed to the electronics board and select Preheat from the menu, there is no voltage on the heatbed terminals. The hotend heats up correctly and doesnt seem to be concerned.

I also tried reflashing the firmware but it didn't help. What could be the issue?
- the heatbed works
- the electronics board supplies 24V to the heatbed terminals with the heatbed disconnected
- with the heatbed connected, there is no voltage and it doesn't heat up, which triggers the thermal anomaly error.