@Phaedrux
The documentation of the Smart Effector says:
The version 2 and later Smart Effectors perform an additional self-test at power up to check that the strain gauge output is within the expected range. This test may fail during the first few seconds after power up as the voltages settle, so it is repeated until it passes. Therefore, if your sensor flashes 6 or 7 times once or twice before it flashes twice, this is normal. However, if it continues to flash 6 or more times, the sensor is faulty:
7 flashes - strain gauge output out of tolerance (too high)
It is an old sensor, I bought it with a second hand non working delta printer. Don't know its life history or age.
I measured the voltage drop across each resistor and found one that had much more across it than all the others. When I tried to redo the solder joints the resistor split in two parts. Replaced with a new smaller (1206) 100 Ohm resistor. This will not be strain sensitive. Replacing another resistor in the same way.
The sensor works now, probably with reduced sensitivity, Flashes twice at startup.
I have no photos since I didn't know about the crack until I resoldered the resistor.
The crack was not visible under amagnifying glass.
If it happens again I think I will try to glue six real strain sensors to the board as a better fix.