A JBLSUB140 has a source of audio (bass gun). After turning on, the indicator is on. After about 2S, there is a relay suction sound, but silent.
The amplifier part of this machine uses a separate component to form an OCL output circuit (the output end does not have DC electrolytic capacitors). It is characterized by no coupling capacitance in the signal transmission channel. powered by. Due to the DC coupling method, when the circuit fails, the power amplifier output is prone to DC voltage, and the danger of burning the horn is burned out. Therefore, the speaker protection circuit needs to be set. When a high DC voltage occurs at both ends of the speaker, protect the circuit action and disconnect output to protect the speaker.
When the machine is turned on, there is a succession sound of relay, indicating that the output end of the power is normal, and the positive and negative power supply is normal. The faulty part should be in the front stage. The actual and negative power supply is 19.6V, which is really normal. In the front signal processing circuit of the machine, there is a op amp TL4558, which should be the input -end small signal amplifier circuit. The op amp’s feet and ④ feet are positive and negative supply ends, respectively. The voltage of the foot is 1.5V, but the ④ foot voltage is 0V, which is obviously abnormal. Follow the line to check forward, and find that the ④ and ⑧ feet are connected in the middle of the power triode Q119 and Q117, respectively, as shown in the figure below.
Q117, Q119, and its peripheral components are composed of positive and negative voltage with active filtering and voltage voltage voltage circuits. Q118 is a positive and negative output voltage detection tube. R152 is sampled on the positive output voltage, and the R154 is sampled to the negative output voltage. Since the negative sample circuit has more resistance R153 than the positive sample circuit, the foundation of Q118 has a smaller positive bias (the measured value is 0.58V), that is, the Q118 is in a micro -guidance state, and there is one of the foundation of Q117. Small diversion effect. When the load changes cause the positive voltage of the Q117 output to increase, the base voltage of the Q118 is increased through the R152, the degree of incentives of Q118 increase, the resistance value of the inter-C-E junction becomes smaller, the base voltage of Q117 decreases, and the output voltage decreases. So as to play a stable role. When the output voltage drops, its voltage process is opposite to the above.
The negative output voltage is mainly regulated by D109. In addition, when the absolute value of the negative voltage of the output decreases, due to the sampling effect of R154 and R153, the B pole potential of Q118 rises, and the resistance value of the inter-C-E knot becomes smaller, which reduces the Q117 B polar voltage. That is, the positive voltage also decreases simultaneously, so that the absolute value of the positive and negative voltage is always consistent. Because in the OCL circuit, if the positive and negative power supply voltage is unbalanced, it will directly affect the zero level of the output end, so the sampling voltage voltage circuit centered on Q118 is set up.
The actual measurement found that the C pole voltage of Q117 and Q119 was+19.7V and -19.7V, respectively. However, the B polar voltage of Q117 is only 2V, which causes the positive voltage of the output to only 1.5V. The B pole voltage of Q118 has risen to 0.63V. Q118 is already in a state of strong guidance, causing the Q117 B polar voltage to decrease significantly. Carefully check the negative voltage voltage voltage voltage circuit and find the capacitor C119 leakage. The resistance value is about 1kΩ, causing the Q119 to be in a cut -off state. After replacing the C119 test machine, the fault was eliminated.
Before the failure was ruled out, the positive and negative voltage of the TL4558 was abnormal. This is because the TL4558 power supply voltage is too low and the op amp is not working. The output end is zero potential, and the output end of the amplifier is also zero potential, so the relay can still be sucked normally. During the maintenance, there was a problem with Q118 or peripheral capacitors. After removing Q118, the power output was basically normal, but 18V DC voltage appeared at the power amplifier output terminal, and the relay did not sucked.