We found this circuit in a 1976 publication. The transistors can be BC547 or 2N2222. Crystals up to a few tens of Megahertz can be tested. The LED can be of any type. The capacitors must all be ceramic.
In the figure we have a simple way to obtain (almost) sinusoidal signals from an integrated...
This circuit is from an old Mullard transistor manual. The diodes are made of germanium and the...
Found in a Popular Electronics from the 90's, this circuit makes use of an integrated from the...