Performed service on a thermal king/Goodman mix match system. Customer reports no response to any thermostat function. I found the low-voltage fuse to be popped. Indoor transformer was measured and tested. Transformer appears to be a bit weak but is not the primary issue. The outdoor Contactor was popping the fuse on the low-voltage part of the circuit. This was replaced today. The customer signed up for future maintenance is and is aware that with a system is old Electrical components can become weak. System is operational at end of appointment.