The Maestro Echoplex is a legendary vintage tape echo effect unit that shaped the sound of rock and roll from the 1960s through the 1980's.
You're looking at the EP-3: Introduced in 1970, this transistorized (solid-state) model featured a famous Field Effect Transistor (FET) preamp and a sliding lever to change delay time.
This piece has been part of the secret sauce for, Jimmy Page, Eddie Van Halen, Brian May, Eric Johnson & more.
This unit has been tested and is working just fine.