Non Contact Voltage Detector

We have a qty of non contact Voltage Detectors pen type. The Supplier does not exists anymore and we need to be able to repairs them and in many cases, provide calibration. A few thousands of these are not finished and requires final quality control, then closing the case (glued case). We do not have documentation. We need to draw the Schematic from the working samples (will provide by courrier) Schematic can be hand drawn on pencil or any cad format. We need to Build three hand reference boards to refer to when doing repairs and calibration or when doing testing (referenced to schematic and PCB for easy fault finding). (NO PCB layout Required) There are less than 30 parts on the small PCB. From the schematic, to describe the working operation of the circuit and refer top the PCB and reference PCBs. Make a fault-finding procedures to ensure we can make the products working when they are not. Describe the possible calibration and Repair Procedures