This unusual circuit from a Popular Electronics of the 90s uses LED indicators, operating as a bridge. The components used are common. It is based on the tuning of the signal by a PLL.
This circuit is a from publication 1980, but still can be mounted with BC548 transistors for Q1...
This circuit for testing zener diodes by lifting their curve on an oscilloscope was presented in...
This high quality circuit with complementary symmetry was found in an Electronics World of 1966....