Unplug you machine from the power socket. Unplug the connector from CN4. Unplug the connectors from the top of the heater relay. Set your meter to a low ohms range (200 ohms ?) Connect one of the meter probes (We will call this probe A) to CN4 If you can get to the connections to the inductor L1 connect the other meter probe (We will call this probe B) to each of the two wires in turn and note each resistance reading. Now connect probe B to D20 negative then D22 positive and note each reading. Disconnect probe A from CN4 and connect it to the connector on the top of the heater relay that had the wire to the noise filter connected to it. If you can get to the wires on PTC1 connect probe B to each in turn and note the readings. Connect probe B to D21 negative and D23 positive in turn and note the readings. Post the results of these tests. I think using an angle grinder to remove the circuit board is a bit drastic. Can you post some well focused pictures (You may need to use manual focus on your camera if it has that option.) of how the circuit board is mounted in the plastic housing and how the housing is mounted in the machine. I may be able to see a way to remove it without using an angle grinder.
Les.
Les.