The Aprilaire 4740 is the correct replacement for the following humidifiers:
The Aprilaire 4740 Humidifier Fan Relay is a replacement relay that powers up the circulator fan in the latest Aprilaire 700 model, 700A and 700M humidifiers since 2008. Newer versions of the 700 model series uses an external transformer to power the control circuit, along with the Aprilaire Fan Relay. Older versions used part number 4238 that contained a small circuit board with a low voltage transformer. 120 volts could cause serious injury from an electrical shock, so be sure to unplug the power to your humidifier before servicing.
The tools you will need for installing your fan relay are a Phillips screwdriver and a pair of needle-nose pliers. Installation instructions: