This S-meter circuit was found in a 1976 publication. The circuit is used to measure the signal strength received by a station. The transistor can be of any type and the instrument is 20 to 50 uA full scale. The diodes can be 1N4148 or equivalent. The circuit is used in FIs of 455 kHz and even 10.7 MHz, in which case the capacitor values must be changed.