Repair of power supplies for printers. The world of pc peripherals Fig. 3. HP LJ1320 Power Supply Voltage Converter Configuration

Repair of printer power supplies (educational program).

The printers use switching power supplies that convert AC mains voltage into multiple output DC power rails for various printer components. one.

The power supplies are located inside the printer on a separate board or on the power supply board along with the high voltage power supplies for the imaging system (primary charge, development, transfer, etc.). The power part of the power supply is most often represented by a pulsed flyback voltage converter with or without a control microcircuit. Regulation and stabilization of the output voltages of the source is carried out by the method of pulse-width modulation (PWM) and is carried out, as a rule, by a specialized microcircuit PWM controller based on the feedback signal. Since the PWM microcircuit - the controller is included in the primary circuit of the power supply, the feedback taken from one or more output power buses is fed to the microcircuit through a galvanic isolation - an optocoupler.

The power supply protection circuits against overvoltage at the output and increased current consumption or short circuit are also implemented through blocking the operation of the PWM control microcircuit - the controller. The blocking signal from the secondary circuits of the power supply is also fed to the control microcircuit through an optocoupler.

At the input of any switching power supply, there is an input filter circuit designed to provide protection against various problems of the primary network. The most important elements of this part of the power supply, which must be checked at the stage of troubleshooting, are the input fuse and varistor. These two elements provide protection against short circuits in the primary circuit of the power supply and in the heating element of the stove, as well as protection against overvoltage of the input voltage of the power supply. Almost all input circuits of the printer's power supply are protected by a diode bridge from a current pulse when the printer is turned on; it is provided by a thermistor.

Figure: 1. Functional diagram of the laser printer power supply.

The number of output power buses varies from one to three, and they are all formed in the classical way - by rectifying the EMF from the secondary windings of the power transformer. A typical variant is the formation of buses + 3.3V, + 5V and + 24V at the output. The voltage assignment is as follows:

1. Bus +5V - used asstandby voltage, as well as for power supply of digital, analog circuits, etc.

2. Bus +3.3V - supply voltage of digital microcircuits, controllers, microcircuits on the interface board, line start sensor in the laser-scanner unit.

3. Bus +24V (+ 12V)- supply voltage for the power components of the printer: motors, electromagnetic clutches, solenoids, scanner lamp power supplies, etc.

When repairing switching power supplies for printers, you should strictly follow the general rules of electrical safety, the main provisions of which are as follows.

One of the most dangerous paths of current flow through the human body is the hand-to-foot direction; therefore, it is prohibited to repair pulse power supplies in damp rooms or in rooms with cement and other conductive floors. The use of a dielectric mat reduces the likelihood of current flowing in the direction in question.

No less dangerous is the path of the current through the arm-arm section. Therefore, it is forbidden to repair switching power supplies near grounded structures (central heating batteries, etc.). All manipulations with the switched on pulsed power supply unit should be carried out with only one hand in clothes with long sleeves, oversleeves, a tool with insulated handles. All of these points reduce the likelihood of electric shock. It is strictly forbidden to solder on a switched on pulsed power supply unit.

Repair of power supplies for printers in the switched on state should be carried out in stationary workshops at special workplaces where there is an isolation transformer.

A particular danger to human life is that part of the switching power supply circuit that is energized by the input network (on the printed circuit board of the power supply unit, it is usually marked with shading).

It should be remembered that the elements of the toner fixing unit - "fuser" are also under mains voltage.

After turning off the switching power supply (during its repair), it is necessary to discharge the electrolytic capacitors of its circuit, or maintain a certain pause after turning off, so that the capacitors are discharged through the circuit elements.

A full and high-quality repair of switching power supplies will be performed only if if the master has a clear knowledge of the operation of the power supply unit, its circuit, and knows the practical methods of finding and eliminating defects.

The repair will be carried out with less time and using the minimum, really necessary number of radio components only if the radio mechanic is fully familiar with the basic methods of repairing radio equipment. These include the following methods:

Method of external manifestations is based on the information content of the printer during operation, by the nature of the manifestation of malfunctions during the printing process, it is possible with a high degree of probability to judge the operability of the switching power supply, and also to roughly determine the group of radio elements, among which there may be a faulty one.

The installation analysis method allows using the human senses (sight, hearing, touch, smell), find the location of the defect by the following signs: burnt radioelement, poor-quality soldering, a crack in the printed conductor, smoke, sparking, etc.;

Measurement method based on the use of measuring instruments when searching for a defect; voltmeter, ohmmeter, LC meter, oscilloscope.

Replacement method is based on replacing a questionable radio element or module with a known good one. If after such a replacement the external manifestation of the defect has disappeared, then it is obvious that the defect has been eliminated.

Exclusion method based on temporary disconnection (in case of possible leakage or breakdown) or jumpering of terminals (in case of possible breakage) of questionable elements.

In a switching power supply, group stabilization is used to stabilize the output voltages. It is characterized by the fact that with an increase in the load current of one of the secondary rectifiers, the load of the pulse transformer increases and this affects the values \u200b\u200bof the output voltages of all rectifiers connected to it. Therefore, when searching for a defect, both the continuity of the load circuits and the disconnection of suspicious circuits should be widely used.

Method of exposure based on the analysis of the circuit's response to various manipulations performed by a radio mechanic: changing the positions of the setting variable resistor slides, jumpering the terminals of transistors in DC circuits (emitter with base, emitter with collector), changing the supply voltage (with control of the PWM circuit operation by the oscilloscope), bringing the tip of a hot soldering iron to the body of a dubious radio element, forced cooling with compressed air, etc. manipulations.

Electric run method allows you to find periodically manifested defects and check the quality of the repair performed; on average, the run time should be about 4 hours).

Knocking method allows you to identify installation defects (with the power supply turned on) by tapping on the chassis with a rubber mallet, etc.

Equivalents method based on the temporary disconnection of a part of the circuit and its replacement with a set of elements that have the same effect on it, these include: auxiliary sources of constant voltage, load equivalents, etc.

In practice, a radio mechanic must use the listed methods not only in their “pure form”, but also their combinations, and the richer the arsenal of methods for finding defects that a radio mechanic possesses, the more flexible he will be to use and apply them according to circumstances. The result of such manipulations by methods will be higher productivity of his labor, cheaper and better quality of his repairs.

Repairs to the printer's power supply should always be done after preliminary diagnostics of both individual elements and the entire power supply as a whole. Such diagnostics are necessary in order to assess possible damage, identify defective elements, exclude repeated failures and interference when the power source is turned on after repair work.

As a rule, any specialist has his own method of checking and diagnosing a faulty source, which has been developed over the years on his own work experience. However, any specialist should when carrying out repair work adhere to certain rules that will reduce the likelihood of errors and repeated failures when repairing a printer power supply:

1. Before performing the main work on the repair of the source, you must make sure that there is a supply voltage in the network, the serviceability of the power cord. This check is performed using a conventional tester.

2. Diagnostics of the power supply unit must begin with a visual inspection of the parts and the state of its printed circuit board. At this stage of diagnostics, all existing visible external defects of radioelements are usually detected. Usually, in this way, faults of a fuse, varistor, thermistor, many resistors, transistors, capacitors, the state of chokes and transformers are determined.

A defective fuse with a glass body is determined visually by the absence of a conductive sting, by a metal deposit on the glass, by the destruction of the glass body, sometimes it is covered with a heat-shrinkable cambric, in this case its serviceability is checked by resistance with an ohmmeter. A failed fuse may indirectly indicate a malfunction of the input varistors, input rectifier diodes, key transistors or the control circuit for the image fusing unit.

Varistors, thermistors, as well as capacitors in the input circuits of power supplies, in case of failure, often have mechanical damage to the case. They turn out to be split, cracks are visible, the coating flies around, soot can be observed on the hull.

In case of failure, electrical capacitors are swollen or also have damage to the case, in which electrolyte can be sprayed onto adjacent radio components.

When the resistors burn out, the color of the case changes, traces of soot may appear. In some cases, cracks and chips of protective paint may appear on the resistor case.

When a power transistor breaks down, the destruction of its body is most often observed, cracks and chips are observed, in some cases there is soot on the neighboring radio elements.

It will not be superfluous at this stage to make a visual inspection of the power supply board, assess the integrity and quality of printed wiring, serviceability of conductive tracks and soldering points of radioelements, determine the deformation of the board as a result of its improper installation or incorrect temperature mode of operation.

In short, at the level of visual inspection, it is necessary to carefully inspect all parts of the printer's power supply, paying attention to violations of the integrity of the case, discoloration of radioelements, traces of soot, the presence of foreign objects, the slightest damage to the printed conductors and places with suspicious soldering quality.

3. The next stage of diagnostics is to determine the type of power supply unit, the scheme for constructing a power converter, the determination of circuit solutions and the appointment of any other power supply circuits. At this stage, it is also necessary to determine the element base and the type of microcircuits and transistors used, prepare a schematic diagram of the power supply, identify radioelements, check the revision of the source board and compare with the existing circuit.

4. After all the previous steps, you can start searching for faulty items. They start by checking the fuse at the power supply input. In the event of a burnout, the rectifier bridge diodes, thermistor, varistor, output filter capacitor, key transistor, current resistor, primary winding of the power transformer, heating element of the fixing unit, triac in the voltage control circuits of the heating element are subject to mandatory check. With this test, we identify a short circuit at the input of the power supply, if present.

A mandatory item at this stage is to check the health of the control microcircuit (PWM controller) of the printer power supply. To do this, you must have technical documentation for the microcircuit, the purpose of the legs, a resistance map at the terminals. It is imperative to ring the control output of the microcircuit (DRV) for the power switch, if it is made on an external case, and the resistance of the microcircuit for power supply, the Vcc pin. In both cases, the resistance should be very high. Since the control microcircuit of the printer power supply is included in the primary power circuit, at the initial stage of operation of the power supply, it is powered from the +310 Volt power bus through a resistive voltage divider, and in operating mode the microcircuit is powered from the additional winding of the power transformer of the transformer. For this reason, it will not be superfluous to ring the power circuit of the microcircuit with an ohmmeter: measure the resistance of the resistive divider; ring the additional winding, check the serviceability of the rectifier diode from the additional winding and the smoothing capacitor for the power supply for the microcircuit.

Bipolar or field-effect transistors can be used as a power switch in the power supply. They should also be checked for breakdown, as this is one of the most common power supply faults.

The bipolar transistor can be tested with a multimeter for voltage drops of the base-collector and base-emitter junctions in both directions. In a working bipolar transistor, the transitions should behave like diodes, but it must be remembered that some bipolar transistors may include a built-in diode between the collector and emitter and a resistor in the base-emitter circuits that will ring when ringing.

When checking a field-effect transistor, it must be evaporated for a reliable check. For example, for diagnostics of field effect transistors N-channel view, the multimeter must be switched to the diode test mode, the black probe is placed on the drain (D) of the transistor, and the red one on the source terminal (S), the multimeter should show a voltage drop across the internal diode - 502 mV, the transistor is closed. Further, without removing the black probe, touch the red probe of the gate output (G) and again return it to the source (S), the tester shows 0 mV field-effect transistor opened. If you touch the gate lead (G) again with the black probe, without releasing the red probe, and return it to the drain (D), the field-effect transistor will close and the multimeter will again show a voltage drop of about 500 mV.

If a faulty transistor is detected, it is also necessary to check all of its "strapping": diodes, low-resistance resistors, electrolytic capacitors in the base circuit and the primary winding of the power transformer.

Checking the power supply circuits of the fixing unit must be performed through the "dial-up" of the heating element, the protective thermal fuse and the measuring thermistor. The resistance of the heating element should be in the range from 60 to 180 Ohm, the thermistor at room temperature from 300 KΩ to 1000 KΩ.

Figure: 2. The pinout of the triac.

The main malfunction of the control circuit of the printer stove can be considered the failure of their triac, see Fig. 2, since a sufficiently large current flows through it. Checking this microcircuit can be quickly performed without unsoldering it from the board. To do this, you need to "ring" her contacts with a tester. In a soldered state, with a working triac, the tester should show the following resistance values:

Between the terminals T1 and T2, the resistance should be very large (infinite) when "dialing" in any direction;

The resistance between the terminals T2 and G should be infinitely large for a "continuity" in any direction;

The resistance between terminals T1 and G should be very small within the range from 50 to 150 Ohm with a "continuity" in any direction - this is the resistance of the resistor that is connected in parallel with the terminals.

This diagnostics allows you to determine the breakdown of the triac, however, the most accurate information about the state of the triac can be obtained only by testing it after it is evaporated from the circuit or replaced with a known faulty one. If at the initial stage of diagnostics it is necessary to repair only the power supply itself, then the toner fixing unit in the printer can be disconnected from the power supply, and the power supply can be diagnosed without it, even at subsequent stages of diagnostics with the supply of mains voltage.

Testing secondary diode rectifiers can also be performed using a multimeter for open and short circuit without unsoldering from the circuit. If the check revealed faulty diodes, then it is imperative to check all the output electrolytic capacitors of this output bus - there is a high probability of their failure.

5. Based on the results of inspections, it is necessary to draw a conclusion about defective elements, the possibility of replacing them with the same ones, or analogs with the same characteristics. The selection of parameters must be carried out using the appropriate reference books and technical information for these radioelements. When selecting analogs and searching for characteristics of radioelements, it will not be superfluous to use information sources on the Internet. When selecting analogs, it is most responsible to replace powerful key transistors and elements of secondary output stages (diodes, capacitors, chokes).

6. Next, all defective elements are replaced. Particular attention should be paid to the installation of a powerful key transistor (or a powerful hybrid microcircuit) on the radiator. The case of a power transistor is usually connected together with its collector (drain), so it must be isolated from the heatsink. The insulation is installed between the radiator and the transistor body, mica gaskets, special heat-conducting rubber are used, and if the body is completely plastic, then only heat-conducting paste can be used. After installing and sealing the transistor, you must once again make sure that there is no contact between its collector (drain) and the radiator using a conventional tester.

When replacing a fuse, do not forget that the operating current is approximately 4A to 10A. Replacing with a high-current fuse may damage other elements of the power supply or the printer itself.

7. After replacing all faulty elements, you can make a test run of the power supply, but first it must be provided with a load, since all switching power supplies without load are not stable or fail. Therefore, before switching on, make sure that all output circuits of the source are connected to the load. If it is necessary to turn on the power supply of the printer when its loads are disconnected (motors, main control board, interface board, etc.), then instead of them, you can load it with equivalent external circuits. Most specialists, as a load for the power source, connect resistors of the appropriate rating and wattage to the output power supply buses or ordinary electric lamps for + 12V and + 24V with a power of 10-60 W, you can use car lamps. To control the level of the required output voltage bus, it is advisable to connect a voltmeter to the output of the power supply before turning it on, with which it will be possible to control it.

At the stage of preliminary preparation before turning on, you can also put a 220V electric lamp with a power of 100-150W instead of a mains fuse, which will give a visual representation of the current consumed by the source as a whole. If, when the power supply is turned on, the lamp shines brightly, then this will indicate excessive power consumption and a possible short circuit in the primary circuit of the power supply, and with normal current consumption, a slight glow of the lamp will be observed. When applying this method, it must be remembered that it is a violation of safety regulations, for this reason it must be used with extreme caution.

At the moment of switching on, it is necessary to comply with all safety measures; visually observe the operation of the power supply unit in protective glasses, because when turned on, electrolytic capacitors, power switches, diode bridge diodes, varistors and a thermistor may fail, all these radio elements, if they fail, can explode with the destruction of their case. During the period of initial switching on and operation of the power supply, you need to pay attention to the appearance of possible sounds (whistling, clicks). The appearance of smoke, a burning smell will indicate an unresolved problem and a malfunction. Sparks and flashes are usually observed when fuses, power switches and diodes fail.

For all emergency situations there must be a possibility to quickly turn off the standwith a tested power supply from the mains.

As we promised last year, we continue the series of publications dedicated to one of the most massive and popular modern laser printers. In today's issue we take a look at the HP LaserJet1320 printer power supply. It is the power supply unit in laser printers that is a module that many people are trying to repair, and even those who do not know very well what laser printing is. But this, in fact, is not so important when repairing a power supply. And, what is most interesting, the repair of power supplies very often gives a positive result. As for the HP LaserJet 1320 printer, its power supply maintainability is very high. And therefore, the failure of the power supply in this printer is not a serious problem, especially since now you have its schematic diagram.

The Hewlett Packard LaserJet 1320 printer uses a switching power supply. The peculiarities of this class of power supplies are higher efficiency, smaller weight and dimensions. The printer's power supply is physically located on the mechanism controller board (Fig. 1), which in the documentation is called the DC-Controller board, and many experts call the main (mother) board of the printer.

Figure 1 HP LaserJet 1320 Engine Controller (DC-Controller)

The HP LaserJet1320 printer power supply is a single-cycle switching power supply built on a flyback self-oscillating circuit. A distinctive feature of the power supply unit of the LaserJet 1320 printer from the power supply units of other previous models is the simpler circuitry of the primary part. This printer does not use the high-power PWM controller IC, which was a custom IC in all previous models. The failure of this microcircuit was tantamount to the failure of the entire board. the acquisition of the microcircuit was very problematic. Now, as a key element of the pulse converter, a field-effect transistor is used, which has many analogues. This is what makes the power supply circuit of the Laser Jet1320 printer not very difficult to repair.

Input circuits

The input circuits of the source provide protection against noise, current surges and voltage surges of the primary supply network. The connector for the printer's network cable is J101.

As part of the input circuits of this printer model, the following protection elements can be noted: current fuse FU101, varistor VZ101, varistor VZ102, thermistor TH101.

Varistor VZ101 protects the primary part of the power supply from overvoltage. In the event that the surge in the mains voltage exceeds the response threshold of the varistor VZ101 (620V), the resistance of the latter is significantly reduced, and a significant current begins to flow through it (and therefore through the input fuse FU101). As a result, the fuse "burns out", but the rest of the electronics is often left intact.

The TH101 thermistor, which is an NTC (negative TCS) thermistor, protects the diode bridge against inrush current.

Capacitors C111, C112, C101 - C106, varistor VZ102 and choke L102 provide filtering of both symmetrical and asymmetrical impulse noise of the supply network.

The rectification of the alternating current of the network is carried out by a diode bridge consisting of four diodes D111 - D114, and the smoothing of the rectified voltage is provided by the capacitor C107, on which, as a result, a constant voltage of about 300V is created.

The input circuits also provide some protection for the fixing unit (stove) from network noise (but the stove control module itself is not shown in the diagram - we will consider it in the next issue of the magazine).

Inverter

The inverter converts the rectified mains current into a pulse current of the T501 transformer. The key element of such a converter is the Q501 N-channel MOSFET. Since a sufficiently large power is dissipated on the transistor, it is placed on the radiator. The transistor is made in a TO-220 package. The internal block diagram of the transistor and its pinout of the case are shown in Fig. 2, and table 1 shows its main characteristics. Failure of this transistor is not a major problem. Even if there is no possibility of purchasing the 2SK3565 transistor, then instead of it you can use almost any powerful field-effect transistor, i.e. He has a lot of analogs, you just need to open the manual on field-effect transistors.

Fig. 2 Transistor 2SK3565

Table 1. Characteristics of the 2SK3565 transistor

Parameter	Value
Drain-source voltage [V DS ]	900 V
Drain-gate voltage [V DGR ]	900 V
Source-gate voltage [V GSS ]	± 30V
Constant drain current[I D ]
Pulse drain current[I DP ]
Power dissipated in juice [P D ]
Resistance of the drain-source transition in the on state [R DS ( ON ) ] (atID=3 A andVGSS=10 V)	2.0 W
Drain leakage current [I DSS ]	100 uA
Rise time[tr ]	30 ns
On time [ t ON ]	70 ns
Decay time [tf ]	60 ns
Shutdown time[t OFF ]	170 ns

A very unusual solution used in this power supply is to install a safety resistor R560 in the primary current flow circuit. Its presence is designed to protect the primary winding of the pulse transformer from "burnout" when the Q501 transistor breaks down. In addition, the resistor is able to protect the Q501 itself from thermal breakdown when the primary current increases. Resistor R560 is a Safety Type resistor that burns out when a large current flows through it, and in fact, these resistors are slow-blow fuses.

The inverter is a single-cycle flyback converter of an autogenerating type. The starting circuit of the converter is a resistive divider R570 / R501 / R502 / R503. Self-generation of the Q501 transistor is provided by the tertiary winding of the T501 transformer (pin 5 - pin 6) and a frequency setting circuit consisting of a capacitor C502, resistors R504 / R505 and diode D502.

The duration of the pulses at the gate of Q501 can be limited by the transistor Q502. This allows the on-time of Q501 to be adjusted, i.e. allows you to change the amount of energy stored in the T501 transformer. This is how the output voltages of the power supply are regulated and stabilized. In turn, the Q502 transistor is controlled by a feedback optocoupler - PC501, which generates a signal proportional to the + 24V voltage.

Transistors Q503 and Q504 form a protection trigger that allows you to block the operation of the inverter (and therefore the entire power supply) in the event of emergency operating modes at the output of the power supply. These emergency modes are:

Significant overvoltage + 24V;

Overload (short circuit) in the + 24V channel.

Information about the occurrence of these emergency modes of operation is sent to the trigger through the protection optocoupler - PC502. If the trigger is triggered, the operation of the power supply is blocked, and it will be possible to restart it only after turning off the printer using the power switch.

Secondary rectifiers

The difference between the power supply of the LaserJet 1320 printer and the power supply of all other printers is that the pulse transformer has only one secondary winding. This means that only one output voltage (+ 24V) is generated directly with the help of the transformer. The rest of the secondary voltages (+ 3.3V and + 5V) are obtained by pulse conversion of + 24V voltage (Fig. 3). In this case, voltage converters + 3.3V and + 5V are step-down.

Fig. 3. HP LJ1320 Power Supply Voltage Converter Configuration

Channel current protection + 24V

Overload and short circuit in the + 24V channel are determined using a current sensor consisting of two parallel resistors R524 and R525. The voltage drop across this current transducer is directly proportional to the load current, and this voltage drop is monitored by the IC502 operational amplifier. If the channel current becomes too large, a high level signal is generated at the output of the amplifier (on pin 14), which leads to the opening of the ZD503 zener diode and the "turn on" of the PC502 optocoupler. As a result, it triggers the primary circuit trigger (transistors Q503 and Q504).

Overvoltage protection in + 24V channel

Overvoltage protection in the + 24V channel is provided by a zener diode ZD502, an optocoupler PC502 and a trigger on transistors Q503 and Q504. When the voltage in the + 24V channel is exceeded, the ZD502 zener diode opens and a current begins to flow through the LED of the PC502 optocoupler. This, as we have already considered, leads to the triggering and disconnection of the power supply.

+ 24V channel feedback circuit

The stabilization of the + 24V output voltage is carried out by the method of pulse-width modulation. The duration of the pulses at the Q501 gate is determined by the potential based on Q502, which, in turn, is directly proportional to the current through the LED of the PC501 optocoupler. The PC501 optocoupler is controlled by three elements at once:

Operational amplifier IC502 (output pin 8);

Comparator IC501 (output pin 13);

Transistor Q210.

The current value of the PC501 optocoupler is determined by the IC502 operational amplifier, depending on the voltage difference at its inputs (pin 9 and pin 10). A reference voltage obtained using a zener diode ZD510 is supplied to pin 10, and to pin 9, an output voltage of + 24V is supplied through a variable divider. A variable divider consisting of resistors R519, R520, R521, R522 makes it easy to modify the board at the installation stage to fit the parameters of the installed elements. In particular, in the presented diagram, resistors R522 and R521 are excluded from the divider by a jumper installed between the contacts CP4 and CP2.

Transistor Q210, opening on command from the microcontroller (CPU), provides a maximum current flow through the LED, which allows the power supply to be blocked. This provides protection in the event of an error in the operation of the printer mechanisms.

Overvoltage protection in + 3.3V and + 5V channels

Comparator IC501 (output pin 13) provides protection against overvoltage in the + 5V channel. The + 5V channel voltage is compared with the reference voltage, which is obtained by dividing + 24V using a resistive divider R283 / R280. When the comparator output goes low, the maximum current flows through the PC501 optocoupler LED and the power supply is turned off. This protection is necessary in case of a breakdown of the Q505 transistor, as a result of which a much higher voltage, namely + 24V, can be applied to the + 5V low-voltage channel. Through the considered comparator IC501, protection is also provided against overvoltage in the + 3.3V channel. If the Q510 transistor in the + 3.3V channel breaks through, then the + 24V voltage through the D509 diode is also applied to the + 5V channel, as a result of which the IC501 comparator works in a similar way.

Voltage regulator + 5V

+ 5V voltage is obtained by pulse conversion of + 24V voltage. The key element of the buck converter is the Q505 transistor. The pulses at its gate are formed by comparator IC501 (output pin 1) and transistor Q507. The pulses received at the drain of the transistor Q505 are smoothed by the capacitor C517, and the choke L502 stores the energy necessary to maintain the current in the load. Diode D506 recharges capacitor C515 with the energy stored in inductor L502 during periods when Q505 is off.

Current protection of the powerful transistor Q505 is provided by the current sensor R532 and the transistor Q506.

Protection against exceeding the input voltage of the pulse converter is carried out by the ZD505 Zener diode.

Voltage regulator + 3.3V

Voltage + 3.3V, as well as + 5V, is obtained by pulse conversion of + 24V voltage. The key element of the buck converter is the Q510 transistor, which looks like a microcircuit. The pulses at the gate of Q510 are formed by comparator IC501 (output pin 14) and transistor Q509. Smoothing of the converter pulses is provided by the capacitor C517, and the storage inductor is L503. The load current when Q510 is turned off is supplied by diode D508.

Current protection of transistor Q510 is provided by current sensor R543 and transistor Q508.

Overvoltage protection in the + 24V channel is provided by the ZD506 zener diode.

I offer cooperation to IT companies for the repair and maintenance of copier printers MFP fax power supplies for UPS monitors.

We remove the side covers - they are latched. We unscrew the shown screws and release the rear panel stem from below and remove it.

Unscrew the 2 screws of the top cover on the back and the screw shown on the right, release the wires from the connector from the control panel bar by lifting the cover and remove it. You can immediately pull out the ribbon cable from the bottom of the interface board.

Rear view of the device, fuser (stove), main board. The bottom arrow shows from which in the first figure the rear panel stem is pulled out. We release the wires by sliding and removing the plastic holders. The pad on the far left in the rice is taken out. Disconnect the thick wires of the heating element.

Disconnect the solenoid connector and unscrew the 2 fan screws

Unscrew the 3 screws of the gear block, remove it, remove the rubber shaft gear

Remove the 3 black screws securing the stove to the metal frame on one side, disconnect the diagnostic connector and bypass the wires in order to make it easier to pull out the fuser

On the other hand, unscrew the 8 screws of the interface board, if you have not removed the lower cable, we do it.

We also remove 3 screws after moving the board to the side

Detach and release the wires of the paper exit sensor and the thermal sensor

Front view. All that can be cleaned is done

Rear view after removing the stove. Above is the laser unit. If necessary, remove the cover and clean it carefully. Using a bolon with compressed gas, we clean the contacts of the high-voltage part of the board

2 screws hold the top of the stove. delete it

Raise the middle plastic part of the stove slightly in the direction of the green arrow and release its rods from the metal base of the fuser

We squeeze the springs of pressing the thermoelement with the film to the rubber shaft, turn the latches 90 degrees, take out the springs and metal strips

The clamping mechanism is more clearly presented.

After passing the power wires of the thermoelement, take it out of the fasteners

We change the bushings (bushings) of the rubber shaft, remove it

Lubricate, put back and assemble the device in reverse order

In laser printers, electronics failures are rare. However, power supplies (PSUs) are not taken into account in these statistics, because the probability of their failures is determined, first of all, by the stability and quality of the supply network. Therefore, power supply failures in laser printers occur at exactly the same frequency as power supply failures in other peripheral and consumer devices. Despite the fact that in most cases printer repairs are carried out by replacing a faulty module or faulty board, at present it would be imprudent to "throw away" the power supply board without even trying to find out the real cause of its malfunction.

The schematic diagram of one of the most popular printers of the class of workgroups "HP LJ 2300" presented in this article will help our readers to approach the issue of diagnostics of the power supply more professionally.

The power supply unit of the "HP LJ 2300" printer is built according to the pulse converter circuit (see the schematic diagram in Fig. 1). The diagram of the power section of the block is practically no different from the diagrams of similar assemblies of other Hewlett Packard printers. The greatest differences between power supply circuits can be found only in their secondary circuits.

Figure: 1. Schematic diagram of the power supply

The network cable is connected to the printer through the J101 connector. The input circuits of the unit (fuse FU201, varistor VZ201, resistor TH201 and mains filter elements) provide protection against interference, current surges and voltage surges of the supply network.

Varistor VZ201 protects the primary circuits of the power supply unit from overvoltage. If the mains voltage exceeds the pickup threshold of the varistor (620 V), its resistance is significantly reduced and a significant current begins to flow through the FU201 back-up fuse. As a result, the fuse burns out, but the rest of the elements of the primary circuits most often remain intact.

PTC thermistor TH201 protects diode bridge D201 from inrush current. Capacitors C201, C209, C210, C204-C206, varistor VZ202 and chokes L201, L202 provide filtering of both symmetrical and asymmetrical impulse noise of the supply network.

The pulse converter is built on the basis of a PWM controller with a built-in power switch (MOS transistor) IC601 of the STR-Z2064 type. The STR-Z2064 microcircuit is custom-made (it is not supplied to service centers), so its failure leads to very great difficulties in repair. The power transistor as part of the microcircuit is connected in series with the primary winding of 1-2 pulse transformer T601 (pin 1 - drain, pin 14 - source). When the transistor is turned on, current begins to flow through the primary winding of the T601 pulse transformer (see Fig. 2).

Figure: 2. Fragment of the power switch switching circuit

The flow of a constant current component and, accordingly, the magnetization of the core of the transformer T601 is prevented by the capacitor C640.

The IC601 microcircuit is triggered by the voltage from the output of the mains rectifier, which is fed to the pin. 5 (VCC) through a divider (R651-R653 R671 R672 R657). This circuit creates a starting current, the value of which is in the units of milliamperes. The magnitude of this current is not enough to ensure the normal functioning of the microcircuit in the operating mode, therefore, in this case, the microcircuit is powered from the 3-5 T601 winding through the D616 C636 rectifier. The same voltage is used to power the PC601 optocoupler in the protection circuit.

The stabilization of the output voltages is carried out by the PWM method according to the feedback signal supplied to the pin. 8 (CONT) of IC601, which is formed by the PC602 optocoupler. The LED current of this optocoupler is generated by an adjustable Zener diode IC605 (TL431). The voltage of the +3.3 V channel is supplied to the control input R of the IC605 through the divider R695 R697 R698. In addition, the channel voltage +24 V is also fed here through the resistor R696. Thus, the feedback signal is proportional to the voltage fluctuations at the outputs of the +3 channels , 3 and +24 V. An increase in these voltages leads to an increase in current through the IC605 and through the LED of the PC601 optocoupler. As a result, the phototransistor of the optocoupler opens and the potential at the pin. 8 IC601 crashes. As a result, the duration of the current pulses through the primary winding of the T601 decreases, which leads to a decrease in the output voltages to nominal levels. In the event of a decrease in output voltages, the potential at the pin. 8 IC601 increases, the duration of the control pulses increases and the level of the output voltages stabilizes.

Blocking the PWM controller IC601 in the event of emergency modes of operation of the power supply is carried out by applying a high level signal to the pin. 4 IC601, which is formed by the PC601 optocoupler. This happens in the following cases:

Excessive voltage at the outputs of the channels +5 and +3.3 V;

Excess current in the +24 V channel;

Excess current in the channel +3.3 V.

The voltages at the channel outputs are controlled by the R666 ZD608 and R667 ZD609 circuits, and the currents - by the IC503 Q605 circuit. If any of these emergency modes occur on the pin. 1 PC601 optocoupler supplies a voltage causing current to flow through its LED. As a result of this, the phototransistor of the optocoupler opens, and on the pin. 4 IC601 goes high.

Let's consider each of the options for emergency operation of the power supply.

Overvoltage in channels +5 and +3.3 V monitored by zener diodes D609 and D608, respectively. If a voltage that exceeds their stabilization voltage is applied to any of these zener diodes, the current through them rises. Opening any of the zener diodes leads to the supply of voltage to the pin. 1 PC601 optocoupler and current flow through the optocoupler LED.

Excess current in the +24 V channel monitored by the Q605 transistor and the R693 R694 current sensor included in the emitter-base junction of the Q605 transistor. If too much current occurs in the +24 V channel, the voltage drop across the current sensor increases. As a result, the potential of the base of the transistor Q605 relative to its emitter decreases, which leads to the opening of the transistor, supplying voltage to the pin. 1 PC601 optocoupler and to the appearance of current through the optocoupler LED.

Excess current in the channel +3.3 B is tracked by one of the IC503 comparators. A current sensor is connected between the two inputs of this comparator, which is a resistor R688 (22 mOhm). The voltage drop across this resistor corresponds to the current in the channel. If the current in the channel rises, then the potential difference between the pin. 9 and 8 of the comparator IC503 increases, the comparator switches, and a low potential is formed at its output (pin 14). This signal opens the Q605 transistor, which leads to the flow of current through the LED of the PC601 optocoupler and blocks the PWM controller IC601.

As you can see from the presented diagram, the printer's power supply generates the following voltages:

24V, used to power motors, high voltage sources, solenoids, relays, fans, etc .;

3.3 V, used to power the entire digital part of the printer - controller and formatter chips, memory, etc .;

5V, used to power the sensor optocouplers LEDs, laser LEDs, parallel port and USB interface circuits.

All output voltages of the power supply unit are formed by rectifying pulses taken from the secondary windings of the T601 transformer. In channels +3.3 and +24 V, the rectifiers are implemented in a half-way-one circuit on diode assemblies DA601 and DA602. In the +5 V channel, the rectifier is implemented according to a one-cycle circuit on the D619 diode. The +5 V voltage is additionally stabilized by an IC602 type 78M05 integral stabilizer.

Since the process of starting a switching power supply is the most dangerous and the overwhelming number of faults occur at this very moment, the developers provide such a mode of switching on the source, in which the duration of the pulses of the primary winding of the transformer increases smoothly. This process is called "soft start". In this power supply, a "soft start" is provided by a capacitor C638 connected to the pin. 7 STR-Z2064 controller. At the moment of starting the microcircuit, this capacitor begins to charge, and as it is charged, the duration of the output pulses of the microcircuit smoothly increases.

A stove control circuit is also connected to the primary circuit of the power supply unit, which includes a Q301 triac, an RL301 relay and an SSR301 optocoupler (not shown in Fig. 1).

Consider typical power supply faults. Traditionally, the primary part of the power supply unit is most susceptible to failures, namely the input circuits and the PWM microcircuit of the IC601 controller.

Before considering the main faults of the power supply unit, we will discuss the issue of diagnosing the IC601 microcircuit. Due to the fact that the author does not have information about the functioning and internal architecture of the STR-Z2064 microcircuit, there is no need to talk about its full functional check, but in most cases this is not required. The failure of this microcircuit can be detected in a very simple and effective way - by checking the internal power transistor. In the event of a breakdown of its transitions, and this problem is the most typical, pin. 1 and 14 will be short-circuited. The presence of low resistance between these conclusions indicates the need to replace the microcircuit. In a working microcircuit, the resistance between the indicated pins is more than 20 megohms. However, it is necessary to remember about the presence of a protective diode in the drain-source junction of the internal transistor. Therefore, if the "+" ohmmeter is applied to pin 14, and "-" to pin. 1, then the forward resistance of the diode (hundreds of ohms) will be monitored. In addition, in the event of a breakdown of the internal transistor, a very large current begins to flow through the microcircuit, which very often leads to the destruction of the chip case. In this case, problems with the microcircuit can be identified visually.

The table lists the malfunctions typical of this power supply. If you encounter problems with the power supply in question, you can suggest the following procedure:

1.Using an ohmmeter, check the fuse FU201. A break in the fuse, except for a malfunction in the power supply circuits, is possible due to a malfunction of the stove control circuit. Therefore, to check the power supply unit, you can disconnect the stove power cable from the J303 connector and remove the Q301 triac. If you turn on the printer without the fuser, a fatal error condition will occur, but the PSU will function normally.

2. Visually assess the integrity of the VZ201 varistor, TH201 posistor, IC601 microcircuit. At the same stage, the quality of the capacitors is immediately visually assessed (swelling of the cases, although this problem is not typical for HP printers).

3. Proceed to diagnostics when the printer is turned on, and at this stage it is necessary to check the following voltages:

At the output of the diode bridge (about +300 V);

On pin. 5 microcircuits IC601 (about +16 V);

At the output of the power supply (voltage +3.3 V, +5 V, +24 V).

Table. Typical power supply faults

Malfunction manifestation	Items to be checked
The printer does not turn on. There is no voltage of +300 V at the output of the diode bridge D201	FU201 fuse; Thermistor TH201
Fuse blows when turned on	Varistor VZ201; Diode bridge D201; IC601 STR-Z2064 microcircuit
The printer does not turn on. At the output of the diode bridge D201, a voltage of +300 V is present	- The presence of a voltage of about +16 V at the pin. 5 microcircuits IC601; - starting circuit R651, R652, R652, R671, R672; - capacitor C636; - diode D616
When the printer is initialized when the motors are started, it turns off	Diode assemblies DA601, DA602, D619; Resistors R693, R694, R688; Capacitors C649, C650, C659, C647, C651, C657, C658
The printer does not turn on. Output voltages +5 V, +3.3 V, +24 V pulsate	- The presence of a short circuit in the load; - power circuit IC601: capacitor C636 and diode D616; - diodes of secondary rectifiers DA601, DA602, D619; - current sensors A resistors R693, R694, R688; - protection circuit ZD608 ZD609 Q605; - feedback circuit IC605 PС602

RM1-2316 Power Board from HP LaserJet 1022 Printer

Introduction. There is a category of repairs that will not take a narrow-profile specialist no more than 5-10 minutes, while a wide-profile specialist can take quite a long time. We want to talk about such a repair. There is such a power supply unit RM1-2316 from the HP LaserJet 1022 printer, which uses a rather unusual fuse, from the habit of seeing this protection element is rather difficult, it is even more difficult to find a replacement for it.
Malfunction according to the customer. The HP LaserJet 1022 printer does not turn on after spilled liquid on it.
Primary diagnostics. A burnt-out textolite is observed around the power switch, while the power switch Q501 (2SK2900) itself turned out to be intact. However, cleaning the board from coal gave no results. In addition, the absence of the classic PWM-Power key bundle plunged the repair engineer into a stupor, the absence of a current sensor, which means that the short-circuit and no-load mode is not provided, the absence of a PWM controller, as a result, another circuitry forced to look into the circuit board RM1-2316 from HP LaserJet 1022 printer. When searching for a circuit, I came across a competent description of the operation of the power supply for "dummies".

Inertia fuse R523 0.22 Ohm (safety type resistor). The photo shows an already replaced resistor - an ordinary resistor0.22 * 1W

Repairs. The circuit itself spoke for itself, it was enough to change the inertial fuse R523 (0.22 Ohm safety type resistor - Safety Type). Honestly, we are faced with such protective elements for the first time, since the current sensor is much more effective and, accordingly, is more widespread. We could not find such a resistor, so we installed a standard current sensor from the Benq Q7T4 monitor 0.22 Ohm * 1W. However, such a replacement did not help, the printer turned on through the lamp gave a short circuit at start-up and the protection in the printer's power supply was triggered. The engineering repair of the power supply began, taking into account the prevalence of elements and the simplicity of the power supply, then by replacing the elements with analogs, we came to the conclusion that it was necessary to replace the HF transformer T501, which was replaced with a less suitable HF transformer. However, the power supply stubbornly refused to start, going into protection when turned on. We were saved from further bullying over the power supply by a printer repair mechanic who accidentally passed by and became interested in how we got a printer to be repaired, as it turned out, the HP LaserJet 10xx printers, in principle, do not start through the lamp. As soon as I ran the printer directly into an outlet, it all worked. Formally, in this repair, we found ourselves hostage to our traditions of repairing switching power supplies, if we plugged the printer directly into the outlet, we would have repaired the power supply after replacing the R523 resistor (0.22 Ohm), however, the usual repair methods played a cruel joke on us and more prevented than helped in the repair.
Conclusion. As practice shows, in "experienced" hands, even replacing the fuse can take 2 hours. For this reason, we made a similar publication - from which two conclusions follow:

• Running the HP LaserJet 10xx printer through a lamp is STRONGLY not recommended.
• The use of inertial fuses, although not the best idea, has shown its viability, no matter how the resistor R523 (0.22 Ohm) coped with its task and saved all other elements from failure.