This filter, which rejects frequencies around 1 kHz, was found in the August 1993 Electronics Now magazine. The fine tuning is done at R3, and the source must be symmetrical.
This remote control receiver via the power grid appears elsewhere on this site from other sources....
This General Electric circuit is from the 80's. The PUTs used in this meter are not common...
This circuit was found in a documentation from the 50's. The transistor is made of germanium and...