Photoelectric fault tracer circuit

Photoelectric fault tracers can provide convenience for amateur radio enthusiasts to repair radios, tape recorders, speakers and other audio equipment. When
the DC current and DC voltage of audio equipment are basically normal, but the fault appears as a series of complex
situations , it is often difficult to judge by a multimeter alone, but charging faults The tracer can successfully find the cause of the fault.
    The picture shows the fault tracer circuit. The high-frequency or low-frequency signal picked up from the probe
is coupled to the base of VT1 via Ci. VT1
itself not only amplifies the signal, but also
detects the high-frequency signal. The residual high-frequency signal after detection
is bypassed . Since the impedance presented by the high-frequency choke LC to low-frequency signals
is very small, both the detected signal and
the low-frequency signal directly input from the probe can be smoothly added to the next
stage.
    In order to prevent overload distortion when the input signal is too large, a volume potentiometer RP is provided in the circuit to control the size of the input signal. G is
the DC blocking capacitor. VT2 and VT3 are composite amplifier tubes, and the amplified audio signals are output by the headphones.