Where to find detailed info about vlf metal detectors

[wawinfo]

Hi friends. A few months ago I attempted to bulid a metal detector and after many googling and reading articles finally found and choose attached circuit to bulid. I built that and I made very accurate sreach coil and when try to run it I faced many problems. I have a little workshop for making and repairing electronics so I have enough test equipment and experience for troubleshooting that. I want to chek every stage signal and output. But couldn't find proper detailed information for this task. Can anyone guides me to find realy good references for understanding every block here to be able to troubleshooting the circuit really accurate? Like exact block diagram and every block inputs and outputs and theory of operation.