Printer power board maintenance knowledge

Printers use low-voltage power supply, which mainly includes two categories: logic power supply and drive power supply. The logic power supply is generally DC +5V, which provides working power for various control chips. If it is a serial printer, the logic power supply also includes DC +12V, which provides the operating voltage for the serial chip. The driving power supply is generally DC +35V (some models are 36V or 24V), which mainly provides working power for the carriage motor, paper feeding motor, and printing needle driver.

There are many ways to repair power circuits, and they cannot be generalized. The following introduces the commonly used methods in actual repairs. Typical faults of the printer power system include: the panel indicator light does not light up after powering on, the whole machine does not respond; a certain group of voltage output is abnormal and the power transformer is damaged, etc. Based on the analysis of the causes of these failures, the specific maintenance methods will be introduced below.

1. The panel indicator light does not light up after powering on, and the machine does not respond. Under normal circumstances, after the printer is powered on and turned on, the power indicator light on the operation panel should light up, and the character carriage returns to the initial position at the left end. Then normal printing operations begin. The power indicator light is illuminated by the +5V DC power supply inside the machine. The +5V power supply is also used for all logic circuits and the word car motor that adopts "high-voltage drive, low-voltage locking". If there is no +5V power supply or the +5V power supply is abnormal, the indicator light on the operation panel will not light up, the logic circuit will not work, and the character carriage will not return to the left end and the printer will not respond. Common causes of this type of failure include the following.

(1) No AC power supply

First check whether the AC 220V input voltage is normal. Mainly check whether there is voltage output from the AC power supply network, whether the printer's power plug and wires are intact, and whether they are connected to the socket. Whether the contact is good, etc.

(2) The power fuse is blown

Usually, the fuse is divided into natural fuse and destructive fuse. If it is a natural fuse, only the middle of the fuse in the fuse tube is broken, but the tube shell is intact, and the fuse is clearly visible at both ends of the fuse tube shell. This is often caused by the fuse being broken after being used for too long; or the external power grid occasionally experiencing transient high voltage (i.e., the voltage fluctuation of the power supply network), causing the fuse to blow; or the printer being left unused for a long time and then being powered on to start the printer (the printer power supply has no fluctuations). When the surge suppression circuit is installed), the electrolytic capacitor in the DC filter circuit instantly flows through an excessive charging current, causing the fuse to fuse. For this, just replace the fuse with a new one. If it is a destructive fuse, you can see that the inside of the fuse has been burnt black, and sometimes the glass tube has even cracked. This destructive fuse is caused by a short circuit in the circuit. It is necessary to find out the cause of the blown fuse before replacing it with a new one. The short circuit point can be checked step by step using secant testing. Short circuits in power circuits are often caused by capacitor leakage, such as AC filter capacitors, post-rectifier filter capacitors, etc. In addition, if one arm of the rectifier bridge stack is short-circuited, it will also cause a short-circuit at the AC input terminal and blow out the fuse.

(3) The thermal protector in the power transformer is blown

As mentioned above, the power transformer step-down power circuit has a thermal protector in series with the primary winding of the power transformer. If it fuses, the primary winding of the transformer will be open-circuited and power supply will not be able to be supplied. After eliminating the short circuit fault at the input and output ends of the power transformer, the following methods can be used to repair it.

Remove the printer casing and frame, unscrew the mounting screws of the power transformer, remove all external connections from the power transformer, and take the transformer out of the casing. Remove the transformer's fastening mounting bracket. Peel off the green paper wrapping of the transformer wire package until both wire packages are exposed. Peel off the insulation layers of the two wire packages on the untapped side until you can see the insulating yellow wax tubes connected in series. This is the thermal protector. Solder down the fused thermal protector and short-circuit the wires at both ends. Check the DC resistance of each winding of the transformer and whether the insulation between the windings is good. After everything is normal, wrap the green shell paper of the insulation layer as it is and install the fastening frame. Finally, install it on the printer as it is and restore the corresponding connections. Install a 2A fuse after the power switch lead and before the AC filter circuit. At this point you can resume normal use.

(4) The DC fuse of the +5V circuit is blown

Some switch-type voltage stabilizing circuits without power transformers are installed at the input end or output end of the +5V voltage stabilizing circuit. There is also a DC fuse. If the fuse blows, this type of failure will also occur. The usual cause is a short circuit in the logic circuit.

2. The output voltage is abnormal or a certain set of voltages has no output

First check whether the AC and DC fuses in the machine are intact. If they are intact, it means that the circuit components of the whole machine and the power supply itself are faulty. Severe short circuit. Focus on checking the secondary rectifier filter circuit or overvoltage protection circuit of the high-frequency transformer. When the overvoltage protection circuit works abnormally, it will cause the photocoupler to be turned on incorrectly, causing the switching power supply to enter a shutdown protection state with no voltage output. For the rectification

filter circuit, you should focus on checking the +5V switch adjustment circuit, because most components in the printer are powered by +5V power supply, which has the heaviest load, so the failure rate is high.

If there is no output in a certain group of power supplies, it means that the main circuit of the power supply is normal. You should focus on checking the adjustment circuit or output loop of the power supply group. Generally, the fault point can be found by tracking and testing with a multimeter.

1.3.16 Quick maintenance method of power transformer

When there is an abnormality in the power circuit, you should first disassemble the machine for visual inspection and observe whether the power transformer and other components are overheated or burned. phenomenon, whether there is desoldering or disconnection in the circuit, whether there is any breakage in the printed circuit board, etc. Then perform a power-on test. You can start from the AC power supply end and check step by step backward, or you can check from the power output end forward to gradually narrow the scope of the fault and find out the damaged components. During specific maintenance, you can perform maintenance according to the logical flow shown in Figure 1-20. In the power circuit, there are no more than three types of power transformer faults: one is an internal open circuit of the power transformer, the other is an internal short circuit of the power transformer, and the other is poor insulation of the power transformer. Each is explained below.

1. Quick maintenance method of internal circuit breakage

There is no voltage output when starting up, and the whole machine does not respond. You can judge by measuring the resistance or voltage at the terminal of the transformer coil with a multimeter. When measuring resistance, disconnect the components connected in parallel at both ends of the coil. When measuring voltage, if there is no voltage in each secondary coil, but there is voltage at both ends of the primary coil, it means that the primary coil is open circuit. If only one coil of the secondary has no voltage, it means that this group of coils has been disconnected. Open circuit faults are mostly caused by moldy wires at the welding points, which often occur at the roots of the lead wires or at the welding points of the enameled wire ends during the winding process. Carefully check whether the soldering point of each terminal and pin is open. If there is no open circuit, it means that the fault exists inside the coil, and the "probing method" can be used to find the broken position. That is, use the low resistance of the multimeter, connect one test lead to one of the lead wires of the winding, tie the other test lead to a fine sewing needle, and pierce the edge-most turn of each layer of coil. When a certain layer is pierced, When there is no deflection on the surface, it means that the coil in this layer has been disconnected.

When the transformer disconnection fault occurs at the root of the winding lead-out, the transformer should be heated first, and after the insulating paint softens, use a sewing needle to carefully pick out the coil, and then use enameled wire or enameled wire with a similar diameter. Solder the multiple strands of soft wires to the picked out wire ends, and then solder the connected wires to the soldering lug. If there are baffles on both sides of the frame, the baffles must be bent or broken, and be careful not to damage the coils and lead wires.

When the circuit breakage occurs in the innermost layer of the winding, the iron core should be removed, the frame should be carefully pried open, the broken wire ends should be picked out, and then lead out after welding. Use thick cable paper to stick to the damaged part of the frame, apply insulating paint, and finally insert the iron core.

When the circuit break occurs on the voltage, current and temperature overload fuse packaged in the power transformer (whether there is a fuse inside the transformer will be indicated in the circuit diagram), the transformer should be disassembled. , after finding the fuse, cut off the fuse, connect and weld the leads at both ends and wrap them with insulating cloth, and reinstall the transformer. A separate fuse is installed outside the transformer, and the size of its fuse current should be appropriately selected according to the power of the complete machine.

2. Quick maintenance method for internal short circuit

Internal short circuit of power transformer includes short circuit between turns within the coil or between the coil and the core. When such a fault occurs, the transformer will heat up. At the same time, the output voltage of the secondary coil of the power transformer will also be abnormal, accompanied by burning of the AC 0.5A fuse. In this case, the method of measuring the secondary no-load voltage is usually used. The location where the short circuit occurs can be determined. When testing, first use a multimeter to measure the no-load voltage of each secondary winding. If the voltages are higher than the normal value, it means there is a short circuit in the primary winding; if the no-load voltage of a certain secondary winding is significantly lower than the rated value, it means there is a short circuit in the primary winding. There is a short circuit fault in this winding;

If the no-load voltage of each secondary winding is basically normal, it means that there is a short circuit in the electrostatic shielding layer. You can also use a multimeter to electrically block the DC resistance of each winding to roughly determine the location of the short circuit. The resistance value of the winding with a short circuit fault is lower than the normal value, but it is not easy to determine a slight short circuit. During the actual maintenance, it was found that the voltage of the secondary coil of the printer's power transformer was low and the wires were thick. Short circuits were rare and faults generally occurred in the primary coil.

The method to repair the internal short-circuit fault is to remove the transformer, together with the transformer core and coil, magnetic shielding cover, short-circuit shielding copper ring and fasteners, etc. The silicon steel sheets of some transformer cores are inserted very tightly, and the transformers must be insulated by dipping paint. When disassembling the silicon steel sheets, you can first place the transformer next to the stove and heat it to 75°C - 85°C (it will feel very hot to the touch and the insulating paint will smoke slightly) to soften the insulating paint between the silicon steel sheets, or put the transformer into gasoline. Soak for 1 to 2 hours to soften the insulating paint. Then use a screwdriver to remove the horizontal strips of the silicon steel sheet, and use a knife to separate the adhesion between the E-shaped sheets. Then use a scrap steel saw blade and grind it on the grinding wheel into an iron bar whose thickness and width are basically the same as the thickness and width of the middle tongue of the silicon steel sheet. Pay attention to grinding the end face straight. Insert the iron bar into the gap between the horizontal bars, align it with the middle tongue of the silicon steel sheet, hit the iron bar with a hammer, and push out the E-shaped silicon steel sheet. The operation diagram is shown in Figure 1-2l. After knocking out three or five pieces, use a screwdriver to gradually pry apart the adhered silicon steel pieces, and then you can remove all the silicon steel pieces.

If the transformer is re-wound, the enameled wires and skeleton of the original wire package will no longer be used. You can use a hacksaw to cut open the wire package and take out the core laminations. After the transformer coil is removed, you should draw a picture and record the position of each lead wire pin, then peel off the outer layer of insulating paint, find the enameled wire end, pull off the enameled wire by hand, let the winding rotate with it, record the number of turns of the coil, and repeat. Just go around.

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