That 4-pin device in the box labeled
L217 is an optocoupler. The pins on the top in that photo are its input, cathode on the left and anode on the right. The cathode is tied to the system ground plane and if i’m following these traces right, the anode is connected in series with R36 and the microcontroller pin (
Leave the board otherwise disconnected from anything but USB and +24V power. Hook your meter in voltage mode up to the screw terminals labeled, “Laser Diode Out” - these are the screw terminals just below the 4-terminal screw connector. Run a test cut from Lightburn and check to see if you get voltage on those terminals when the laser is supposed to fire. If so, good - we’ll move on to the next step.
With the meter still in voltage mode, connect one end to a system ground point (the upper-left pin of the optocoupler should work - if that’s too difficult to probe use the ground pin from the power connector or the shield from the USB connector), then run your test cut again with the other side of the meter connected to the following points and report back if the meter does not read voltage on any of them:
Optocoupler upper-right pin
These should all read roughly 3.3V if the “laser” is set to 100% power for your test cut.