The device and repair of the power supply unit of the PANASONIC KX-F50 fax machine. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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A considerable share of the Panasonic KX-F50 series devices is a cheaper modification, designed for a mains voltage of 120 V (the frequency does not matter much). One of the reasons for the failure of such devices is their inclusion in the 220 V power grid without a step-down transformer. As a result, there is a need for repairs. Power supplies model KX-F50B (220 V) suffer the same fate due to sudden power surges in the network. In addition, even with a working power supply, a malfunction of some of its elements is often detected, which can lead to the disappearance of the machine's printing functions both in the document copy mode and when receiving fax messages.

The Panasonic KX-F50 fax machine is the base for subsequent models - KX-F60, KX-F90 and KX-FUO, all of which have similar power supplies. The block diagram of such a block is shown in fig. 1.

The mains voltage through the toggle switch and mains fuse goes to the mains filter, which prevents the ingress of impulse noise from the operation of the unit into the network, and then is rectified by a high-voltage rectifier. The ripple of the rectified voltage is smoothed by the filter.

The filtered DC voltage is supplied to the high-voltage key and through a quenching resistor to the power circuit of the PWM controller that controls the key. Voltage pulses from the key output are fed to a pulse step-down transformer. The voltage taken from the secondary windings is rectified, filtered and additionally stabilized by the corresponding circuits. The main consumers are listed in Table. 1.

The top channel of the power supply according to the diagram, which outputs a voltage of +24 V, is designed to power the thermal head. It is controlled by a special device on a signal from the central processor of the fax machine.

The schematic diagram of the power supply unit model KX-F50 is shown in fig. 2.

(click to enlarge)

The image of the scheme is as close as possible to how it is given in foreign literature. The KX-F50B model differs from it mainly in the ratings of some elements (for this version, they are indicated in brackets). Note that there may be differences depending on the year of manufacture of the device.

AC mains voltage is supplied through an interference suppression filter, consisting of elements C401, 1.401. L402, C402-C404. rectified by diode bridge D401 and smoothed by capacitor C451. Varistors C405 and C406 perform the function of protection against short-term power surges (in case the third wire of the power cord of the fax machine is grounded). The TN401 thermistor prevents current surges when the device is connected to the mains.

A PWM controller is assembled on the IC451 chip. The conversion frequency is approximately 120 kHz. The output voltage is regulated by the method of pulse-width modulation (PWM) by changing the duration of the open state of the Q405 pulse key with the accumulation of energy in the core of the T401 pulse transformer.

The signal about the output voltage at the output is taken from the +24 V channel. Feedback is provided by the RS401 optocoupler. The voltage sensor is a precision divider R412R413, the voltage from which is applied to the IC401 regulator. On fig. 2, the controller chip is shown in a simplified way. It will be discussed in more detail below. With an increase in the voltage at the output of the power supply, the voltage at the middle point of the divider will exceed the exemplary one, the transistor will open, the current through the LED of the optocoupler PC401 will increase, the phototransistor of the optocoupler PC401 will open, and the duration of the pulses at the output of the PWM controller will decrease.

With a noticeable excess of voltage at the source output of +24 V (for example, when entering the operating mode at the time of start), the D410 zener diode opens. the current through the LED of the optocoupler PC402 increases, the phototransistor PC402 opens, the frequency of the master oscillator of the PWM controller decreases.

The +5 V voltage is generated from the +24 V voltage by the converter on the IC402 chip, which also works on the principle of PWM regulation. The operation of the +12 V and -12 V channels is provided by linear stabilizers IC403 and IC404, respectively.

The TN402 thermistor is located on the heat sink of the power supply. When copying an all-black document for a long time, or if it is overloaded for any other reason, the heat sink will overheat. The resistance of the TN402 thermistor decreases from 25 kOhm (at 25°C) to 2 kOhm (85°C). The signal from the thermistor is digitized by an ADC on the IC201 chip located on the main board of the fax machine and read by the central processor of the device. In the event of an emergency, a software block occurs.

The power supply board contains the RLY401 relay, which, at the command of the processor, supplies +24 V to the thermal head when the document copying mode is turned on or when a fax message is received. In these modes, a high logic level occurs on pin 11 of the CN402 connector, the Q404 transistor opens, the RLY401 relay is activated, its contacts close and +24 V voltage is supplied to the thermal head.

Elements FB1, FB6, FB7 are chokes with a small inductance, formed by a ferrite tube, put on a conductor. Their purpose is to suppress high-frequency interference on voltage pulses that form at the moment of transfer of the accumulated voltage from the secondary windings of the transformer to the load when the power switch of the PWM controller is closed.

A thermal head is connected to the CN404 connector, and a fax machine board is connected to the CN402 connector.

Oscillograms at various points of the power supply are shown in fig. 3.

(click to enlarge)

The period of all pulses is approximately equal to 8 μs. All oscillograms, except for 8, 12. 13., were taken relative to point A, which is common for the primary circuits of the power supply and is galvanically connected to the mains. For this reason, the oscilloscope case should not be grounded, and the power supply itself should be better connected to the network (for the duration of measurements) through an isolation transformer with a transformation ratio of 1: 1.

As practice shows, the main elements that most often fail are the FUSE 401 fuse, the D401 diode bridge, the C451 capacitor, the Q405 power transistor, the IC451 PWM controller chip, the C405, C406 varistors.

The failed power supply is removed from the case after removing the top rear cover of the machine. Repairs begin with an external inspection of the elements. A malfunction of the varistors and capacitor C451 can be identified by swollen cases. Failed varistors are best removed from the board and not replaced, since their effectiveness as protection elements raises some doubts. In a 120 V power supply, the D401 diode bridge can be replaced with four KD212A diodes. assembled by surface mounting according to the scheme of the rectifier bridge.

After replacing the faulty elements, a voltage of 458 ... 18 V is applied in parallel to the capacitor C30 from an external source and the presence of control pulses at the gate of the field-effect transistor Q405 is checked with an oscilloscope. If they are not available, the IC451 chip will need to be replaced. After that, you can short-circuit the resistor R454 and check for pulses on all windings of the transformer, referring to the waveforms and making a correction for the fact that the input voltage of the power supply does not exceed 30 V. If the forms of all the waveforms are normal, you can safely turn on the power supply to the mains or to an external high voltage source, having previously connected a load with a resistance of 5 ... 10 Ohms to the + 5 V channel output. After that, the voltage at the power supply outputs is checked. If everything is in order, the power supply is installed in place.

The main microcircuits of the power supply, their manufacturers and analogues of microcircuits from other companies are given in Table. 2.

Microcircuits IC403 and IC404 are 7812 linear voltage regulators manufactured by various manufacturers (domestic analogue - KR142EN8B).

The IC401 microcircuit is an element called in foreign literature a three-electrode diode shunt regulator. This is a monolithic reference voltage source IC, the equivalent circuit of which is similar to a zener diode with a low temperature coefficient of stabilization voltage (Fig. 4). It can be used as an analogue of a zener diode with an adjustable (using two external resistors) stabilization voltage from about 2.5 V to 36 V and a stabilization current from 1 to 100 mA. The domestic analogue is KR142EN19 (see Radio. 1994. No. 4. p. 45. 46).

IC402 controls the DC/DC converter based on PWM. The microcircuit (Fig. 5) contains an internal thermally compensated reference voltage source, a comparator, a controlled generator with a device for limiting the pulse duration in the event of an overcurrent, a buffer amplifier, and a power output switch.

IC451 is an IC for PWM-based switching power supplies with built-in protection against overloads both in current and in voltage, with the possibility of external start / block control (Fig. 6). It can be used as a PWM controller with a conversion frequency up to 500 kHz.

(click to enlarge)

The transistors used as a high-voltage key of the O405 power supply (2SK1357 - for modification of 220 V and 2SK1488 - for 120 V) are field-operated. MOS structures with n-channel. running in enrichment mode.

The main parameters are given in table. 3.

Author: S. Ryaboshapchenko, Odessa, Ukraine

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