Electronics
Always use analog multimeter to test this component
Select the x1 scale
Take the probes to measure the SCR
Put the black probe on anode and the red probe on cathode. To test the shot, close a contact between anode and gate by clicking the trigger. The multimeter should show a low resistance.
No reading should appear with inverted probes.
Putting the black probe on gate and the red probe on cathode, the reading should show a low resistance.
In other positions should mark infinite.
SCR measurement
Shoot
TRIAC in good condition
Burned TRIAC
TRIAC in short-circuit
Formulas
Phototransistor