Circuit Fault Tracker

When repairing amplifier circuits such as radios, tape recorders, power amplifiers, and stereos, the fault finder can quickly and accurately determine the location of the fault. It is small, light, and easy to carry, and can be used as a standing tool for electronics enthusiasts.

1. Circuit Schematic
The circuit is shown in the attached figure. Use the probe to touch the measured point, and the received electrical signal is coupled to the base of transistor VT1 through capacitor C1, and pre-amplified by VT1.

The amplified signal is transmitted to the direct-coupled two-stage amplifier circuit composed of triode VT2, compound tube VT3 and VT4 through high-frequency choke 1, and the emitter of the compound tube outputs the signal, which drives the speaker Y to make a sound. When the probe touches the measured point and Y is silent or distorted, it means that there is a fault in the circuit.

2. Select components
VT1 and VT2 are high-gain triode 9014, with small penetration current and β>80; VT3 and VT4 are 9012, and the β value can be selected between 50 and 100. Inductor 1 and capacitor C2 form a high-frequency filter circuit. 1 uses a 10mH color-coded inductor with model 1H2A. Other models of color-coded inductors are also acceptable. The light-emitting diode VD can be selected arbitrarily. The speaker plays a monitoring role.

You can choose 8Ω/0.1W dynamic paper cone speaker, RP is used to adjust the signal size, the resistance value is 5.1kΩ, and it is required to be linked with switch S. The probe can be modified with a large veterinary injection needle, or a small multimeter probe.

The parameters of other components are as marked in the figure, with no special requirements.

3. VT1 and VT2 working point adjustment
The part that needs to be adjusted in this circuit is the static working point of VT1 and VT2. Before adjustment, first turn the potentiometer RP to a moderate position, select a signal generator with a frequency of 800Hz~1kHz and a level in the range of -14~0dB. The probe touches the output end of the signal generator, and connects the alligator clip of this device to the ground wire of the signal generator. If there is no signal generator, the probe can be touched to the collector of the pre-stage triode of the radio amplifier. The alligator clip is connected to the negative pole of the radio power supply or "ground". When tuning the radio, the speaker Y can hear the broadcast sound.

Adjustment of VT1 static working point: When a weak signal is heard, it means that VT1 is in saturation state. Adjust the resistor R1, the signal gradually increases, and the collector potential Vc1 of VT1 gradually increases until the sound is clearer. At this time, IC1 is about 0.6mA, VT1 is in the amplification state, and the adjustment can be stopped. Be careful not to let Vc1 rise close to the power supply voltage, otherwise VT1 will tend to the cut-off state. If no signal is heard, it means that VT1 is in the cut-off state. Adjust the resistor R1 to gradually reduce Vc1 from a high potential, so that VT1 enters the amplification state. The signal sound reaches the best effect.

The adjustment of VT2 static working point is the same as VT1. Adjusting resistor R2 can make the signal continue to increase from clear, but never adjust the signal to harsh. After the adjustment is completed, adjust the potentiometer RP, and you will obviously feel that the signal can be large or small. At this time, the measured VT2 collector current Ic2 is 5~10mA.

It should be noted that when adjusting the resistors R1 and R2, it is best to first connect 1kΩ and 10kΩ resistors in series, so as to avoid accidentally adjusting the resistance to 0 Ω, in which case the power supply voltage will be fully added between the base and emitter of VT1 and VT2, thus burning out the transistors. After adjustment, use a multimeter to measure the actual resistance value, and then fix the resistances of R1 and R2.

IV. Precautions during use
1. Turn on the power (the switch has a volume control adjustment), the indicator light VD lights up, and then you can use it.

2. Connect the alligator clip of the device to the ground wire of the machine to be checked, turn on the power of the machine to be checked, and turn the volume switches of both to the maximum position.

3. Touch the probe to the output terminals of each level. When a certain level is touched, there is a signal in speaker Y, indicating that the level and above are normal (the signal in the speaker refers to the signal sent by the radio broadcast, tape sound source or signal generator, etc.).

4. The inspection method generally starts from the bottom stage and goes forward step by step. Since the signal of the last stage is strong, the signal volume can be appropriately reduced to prevent overload distortion.

5. When the voltage of the measured part is greater than 160V, a capacitor with corresponding withstand voltage should be connected in series.