The G‑1687‑1 is a single‑phase SCR control board used on Lincoln Electric welders such as the SP150 and SP105 models to regulate arc current via an SCR driver circuit. Common failure modes include burned or failed SCRs, MOSFETs, electrolytic capacitors, and associated gate drivers; fuses may blow due to overcurrent or shorted outputs, and wiring harnesses can develop corrosion or broken contacts that interrupt the control loop. For a first‑check diagnosis, verify the main power supply voltage and continuity of the input wiring, inspect the board for visible signs of component damage (burn marks, bulging caps), and confirm that the SCR driver and MOSFETs are receiving proper gate signals by measuring the drive voltages with an oscilloscope or multimeter. If these checks are normal, review any error codes displayed on the welder’s control panel and test the output current regulation loop for correct operation.[oss-gpt-20b]