1. Check if the trigger circuit signal is reaching the SCR.
2. The PLC displays a signal; normally, both sets of trigger signals should be functioning correctly.
3. One instance of no current output might be caused by a switch (controlling the alternating conduction of the two SCRs) after the trigger signal. Check if it's working properly.
4. Check if the SCR is damaged (replace the trigger signal wire).
If the solder joints are still yellowing and there is arcing at the contact surface, try increasing the gas pressure.