It is easy "vintage" scheme on discrete components. 20 ohm resistor (most left) is internal resistance of the Capacitor Under Test (ESR tester stimulas: Generic 555 Astable Generator).
The sensitivity and linearity will increase, if we apply the Schottky diodes (or better the old germanium diodes).
Scale of microampermeter is nonlinear as in dc ohmmeter:
ESR %
0 100
1 91
5 51
10 24
20 8
To compare scales see photo of original ESR-tester (http://ludens.cl/Electron/esr/esr.html).
