Variable regulator with crowbar protection (CB21189E)

This circuit was found in a Popular Electronics from the 90's. The power transistors must be equipped with a heat sink.

Megaphone NEC300A (CB18423E)
This circuit is from a NEC megaphone made in Japan in the 60s. The circuit has 4 W of power and...
Optical Explorer (CB14990E)
This circuit uses a photodiode or a phototransistor as a sensor. When pointed at a light source it...
FET Field Strength Meter (CB19194E)
This circuit covers 400 kHz to 40 MHz in three bands according to the selected coil. The double...