Single junction transistor working performance quick test circuit

As shown in the figure is a quick test circuit for the working performance of a single-section transistor. The single-junction transistor VBT under test (set as BT33), R3, and C2 form a relaxation oscillator. When VBT is on, the transistor VT1 obtains the bias current through the bias resistor R2 (at this time, S1 is open and S2 is closed), and the light-emitting diode H1 emits light. The +10V power supply charges C2 through R3 via VD2. Voltmeter V can measure that the voltage at both ends of C2 will continue to rise as time goes by. When the potential at E reaches the peak voltage of VBT, E~B1 of VBT will automatically conduct, and the voltage on capacitor C1 will pass through E~B1. Discharge causes the emitter junction of transistor VT1 to be reverse-biased and cut off, and H1 goes out. If the single-junction transistor VBT under test is good, the capacitor C1 will charge and discharge repeatedly, and H1 will flash alternately between light and dark. If H1 is always on, you can swap the B1 and B2 pins of the VBT for testing (this will also make it easier to correctly distinguish the Bl and B2 pins), otherwise the single-section transistor is a bad tube.