BrocNeal,
OK I guess I can assume the replacement charger board did not fix the problem. Your system may have only one SCR or maybe two SCRs wired in parallel. A SCR is a 3 terminal device, an input terminal (anode), an output terminal (cathode) and a gate terminal. When connected to shoreline power, measure the DC voltage to ground on all three SCR terminals. The output terminal should be the same as your house battery voltage, 12.48 volts per a previous measurement you made. The voltage at the input terminal should be I believe around 18 volts, the same as the output voltage from the transformer module you previously measured. The voltage on the gate terminal will tell the story. If it is more than 1.0 volt higher than the SCR’s output terminal, then that or both SCRs are toast.
