Troubleshooting of the power supply system of the

Troubleshooting of video recorder power supply system

early video recorders delivered from the factory generally adopt series regulated power supply, which is equipped with bulky power transformer. Various voltages required by the machine can be obtained from the secondary corresponding winding of the transformer. The basic types of such power supply are: voltage transformation, rectification and filtering, plus a small amount of clock oil for regulated and non regulated output. Because this kind of power supply has obvious shortcomings compared with the switching regulated power supply, most of the video recorders produced in the later stage have replaced the series regulated power supply with the switching regulated power supply, and the multi-channel voltage output has also evolved from the discrete component structure to the integrated circuit. The structural form of these two circuits is similar to that of other electrical equipment power supply, and the circuit principle and maintenance method are also the same, but the whole power supply system of the video recorder has the following characteristics compared with other types of power supply:

① the power output is divided into controlled and uncontrolled, and the controlled output is controlled by the microprocessor

② there are many auxiliary circuits of the power supply system, which are distributed in other circuits

③ in different working states, the output power of the power supply varies widely

this paper only describes the main characteristics and maintenance methods of the video recorder power supply system. Several factors affecting the normal output voltage

generally, the main power switch is not set in the power circuit of the video recorder (only a few machines are set at the rear). As long as the plug is connected to the power supply, the power circuit will start to work. We can preliminarily judge whether the power circuit works normally by observing the display condition of the display screen. Figure 1 shows some circuits of the power supply system of Panasonic L15 video recorder, which is a common model in China, and the switching power supply is omitted in the figure. The secondary of the switching transformer has five groups of outputs, among which S7 and S8 windings provide the display screen filament voltage, S6 provides - 30V voltage to the operating circuit board, S5 mainly provides 45V voltage to the TV demodulator, S4 provides about 6V voltage to the ⑥ pin of the multi-channel regulated output integrated block ic1102 (stk5392), and the main power supply of the machine circuit is provided by S2. In addition to providing the non regulated 14V power supply to the main circuit, S2 also outputs the uncontrolled regulated 12V voltage through q1102. In addition, S2 is also connected to the ② pin input of ic1102. There are three outputs of ic1102, of which ⑦ pin outputs uncontrolled 5.3v voltage, ④ pin and ⑧ pin output controlled 12V and controlled 5V voltage respectively, and the control command is sent by the pin of microprocessor ic2001 (mn6740vcpk) through pin ② of qr1001, d1001 or ic1102. When it is suspected that the power circuit is faulty, it can be judged by measuring the voltage value of each pin of p1101. If there is no voltage at each pin, the fault point is generally in the switching power supply part. If only part of the voltage value is abnormal, just check the relevant circuit. For example, when the voltage of pin ④ of p1101 is normal and pin ② has no voltage output, the data can be correctly tested by eliminating these faults. The inspection focus is on q1102, r1104, d1114 and c1132. If pin ④ also has no voltage output, the inspection focus is on d1109, c1119, d1113, l1103 and c1120. Pin ⑤, ⑦ and ⑧ outputs of p1101 are provided by ic1102. When the multi-channel stabilized voltage output integrated block receives the correct command and the output voltage is abnormal, it is usually the integrated block or its auxiliary circuit fault

example 1 fault phenomenon: a Panasonic L15 video recorder is powered on, and the display screen shows it, but when you press the power switch, the power indicator light is not on, and the tape cannot be put into the tape bin

maintenance: for the above fault phenomena, the power circuit and system control part should be mainly checked. First, detect the voltage of each pin of p1101. It is found that when pressing the power switch, the ③ pin command changes normally, while the ⑤ pin and ⑧ pin always have no voltage. Therefore, it is judged that it is a power circuit fault. Unplug the p1101 plug, remove the power component and check it separately. There is still no voltage at pin ⑤ and pin ⑧ of p1101. After checking the peripheral circuit of ic1102 and finding no abnormality, it is determined that ic1102 is damaged. Replace ic1102 and repair the fault

example 2 fault phenomenon: a Hitachi 427 video recorder displays normally, the power indication is controlled, and the tape cannot be placed in the tape bin

maintenance: manually put the tape into the tape bin, make the mode switch at the stop mark, power on, and the display is normal. Press the playback button to load the tape in place, but the drum motor and the main shaft motor do not work. It is suspected that the fault is in the servo circuit. Detect the servo circuit, find out that the power supply voltage in the circuit is very low, and finally determine that the fault is in the power circuit after inspection. Measure the voltage of each pin of the multi-channel stabilized voltage output integrated block ic851 (stk5372), and the controlled output 5V and 12V of the ① pin and ④ pin are lower than the normal value, but the voltage of the ② pin at the command input end is much higher than the standard value by 0.7V, reaching 8V. Measure that the command output of the microprocessor at the ④ pin of pg851 is normal, so it is suspected that the ic851 is damaged. Because there is no replacement of the integrated block of this model on hand, try to reduce the resistance of the grounding resistance r857 (22K) of the ② pin of ic851 in order to reduce the voltage of the ② pin, Finally, replace r857 with a resistance of 3.6k, reduce the voltage of ② pin to 0.6V, and the controlled output voltage rises to the normal value

in the above two cases, the instruction task cannot be completed due to the damage of the multi-channel regulated output integrated block. If the instructions sent by the microprocessor cannot reach the controlled terminal correctly, the fault phenomenon generated by the machine is also similar, such as the damage of d1001, C1001 and qr1001 in Figure 1. The fault phenomenon is roughly the same as that in example 1. When testing the faulty machine, if it is difficult to determine whether the fault is caused by abnormal instructions, you can disconnect the command end, add analog command input to see whether the controlled output is normal, and then you can immediately determine the cause of the fault. In the power supply circuit of some models of video recorders, the controlled output is composed of discrete components. When the controlled point is increased, the circuit is slightly complicated. Figure 2 shows the power controlled output unit of Toshiba dv98 video recorder. The structure of these fixtures has its own advantages and disadvantages. The power switch command output by the pin of microprocessor icx01 (tmp47c870n4845z) is divided into two channels through electronic switch ql60. One channel controls the + 9V output of q817 through q818, and the other channel simultaneously controls the + 9V output of q811 and + 5V output of q814 through q813 and q812. It can be seen that if the command is normal, there is no output if ql60 is damaged, no output if q818 is damaged, no output if q817 is damaged, and no output if q811 or q814 is damaged

example 3 fault phenomenon: a Toshiba V94 video recorder displays normally, and the tape cannot be put into the tape bin

maintenance: the tape cannot be put into the tape bin, which is often caused by the non rotation of the loading motor. The reasons are: ① no loading detection signal, ② no control signal of the loading drive integrated block, ③ no power supply of the loading drive integrated block, ④ the loading drive integrated block is damaged. Which of the above four reasons is the fault machine? Only for the drive integrated block ic60 Bayer materials technology is 12 of the world's largest polymer production companies (TA