An electromagnetic hammer rapper having forward and retract coils for controllably causing a centrally disposed armature to impact an impact transfer pin to cause the transfer pin to knock-off slag and other encrustations from heat transfer tubes or walls of heat exchanger components, and for causing the armature to be propelled to a retracted position. An electrical circuit for controlling energization of the forward and retract coils is also provided and includes a manually actuatable switch for initiating an impact between the armature and the transfer pin. An auto sequencer/manual selector circuit controls the forward and retract coils of a plurality of electromagnetic solenoids. A phase detection and triggering circuit having an operator adjustable phase control controls the duration for which the forward coil of each electromagnetic solenoid is energized. An independent logic circuit associated with each electromagnetic solenoid is also included for further helping to control the energization of the forward and retract coils. An SCR and a triac each associated with a specific logic circuit and responsive to the logic circuit are included for controlling energization of the forward and retract coils of each electromagnetic solenoid.