The tester has no operating controls and draws current only when the loudspeaker sounds. It comprises three oscillators that drive a common loudspeaker via buffering OR gates, each oscillator consists of a permanently fitted transistor and the transistor on a test (TOT). The oscillator frequency depends on the time constant of the appropriate RC network (R5C2, R11C4 or R15C5) and the current amplification of the TOT. The n-p-n sections are virtually identical. but. of course. the supply lines have been reversed. The buffer stages differ slightly to ensure that the loudspeaker remains connected correctly. The stage for testing n-channel FETs is basically derived from the design of the n-p-n stage. in operation, the gate must be negative with respect to the source. r Transistors T1, T3, and Tc must be high-gain types to prevent TOTs with a low f3 being assessed as non-functional.
The tester has been designed to enable unknown transistors to be tested without any danger of damage. The pinout of bipolar transistors may he EBC or BCE. Test leads with terminals EBCE or even CEBCE (both n-p-n and p-n-p) are. therefore. recommended. The transistor to be tested is connected with its pins to different terminals until the loudspeaker sounds. This does not only give the pinout but also makes it clear whether the TOT is are n-p-n or p-n-p-type. The frequency of the emitted tone indicates whether the emitter and collector have been connected correctly. When they have not. the current gain is only about 1. so that the oscillator frequency is high. If the two connections are interchanged. the current gain higher arid We oscillator frequency lower.
Owing to the symmetry of n-channel FETs it is not possible to distinguish between source and drain. When the loudspeaker sounds. it means only that the gate has been connected correctly.
The test leads may be terminated into crocodile clips or into quick-release, spring-loaded terminals.