We repair AON on our own. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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What should I do if the phone with automatic number detection stops working? Of course, you can ask for help in the workshop. But very often repairing the device is quite within the power of a radio amateur of average qualification. The author of the proposed article talks about the method of repairing ANOA with mains power.

Let's consider the most difficult and unpleasant case that happened to your device: the digital part of the caller ID stopped working. i.e. microprocessor controller. There is no indication on the display or a sign in one of the categories is brightly lit (as experts say, the system "hangs"). At the same time, the phone does not respond to pressing the keys or picking up the handset. There is a concern that a short circuit may occur when connected to a telephone line.

You need to start the repair by checking the power supply unit (PSU). If it is working, we connect the phone board to the PSU and check the supply voltage directly on it. If, with a working power supply unit, it dropped or noticeable ripples appeared, this indicates a breakdown inside one of the microcircuits of the device. In this case, a defective element can be determined by strong heating or by enumeration, turning off the power circuits of the microcircuits one by one.

If after these steps the device still does not work, you will have to study its device in more depth. All AONs powered by the mains are divided into two groups: those based on the 280 processor (possible analogues are T34VM1, MME, KR1858VM1) and those based on a single-chip microcomputer. The construction of the digital part of these groups of phones is fundamentally different, so we will consider the process of repairing devices of each type separately.

To repair a device built on the basis of Z80, you can use the scheme presented in [1]. Repair should begin with checking the operation of a quartz oscillator assembled on two elements of the K555LN1 chip and a frequency divider (K555IE7 or K155IE5). If these nodes are working properly, there should be a 6 MHz signal at pin 80 of the Z4 processor, and 9 MHz at pins 18 and 5808 of the KR53I1 timer.

At the next stage, it is necessary to check the correctness of the formation of the reset signal (pin 26 Z80) when the lithium is turned on, as well as the constant presence of a high level at pins 17, 24, 25 of the processor. If the reset node does not work clearly, it can be improved as recommended in [2].

After that, turn on the device, after removing the ROM chip from the panel. At the same time, there should be pulses at pins 19, 21 and 27 of the processor, indicating attempts to read from the ROM, and a complex dynamic signal on the data bus. This signal must be considered on the oscilloscope screen, paying attention to its amplitude. If the pulse span is small (less than 3 V), this indicates a breakdown of the input or output of one of the microcircuits connected to this bus conductor. If there are no pulse signals at all on these pins, then the processor is faulty.

When the signals on the data bus are restored, it is worth trying to run the program with another ROM chip containing a similar firmware. If this does not work, you should check the operation of the device's peripheral microcircuits. At this stage, the use of a test program (the author uses "Phon LS") will greatly speed up and facilitate the setup process. At the same time, the KR573RF4 (2764) chip containing the test is inserted into the socket instead of the standard ROM.

Once launched, the program does the following::

a) checks the functioning of the ports of the I / O device (I/O) KP580BB55A. If any problems with the air-blast in the left digit of the indicator are detected, the message "Err 8255" will be displayed in single characters;
   b) checks the indicator by digits, and then by segments. Defects of the indicator are visually determined, as well as microcircuits K555ID4, IK555IR23;
   c) tests the RAM. When faulty memory cells are found, as well as in the absence of RAM sampling pulses, the message "Err RU10" appears. In this case, it is necessary to check the presence of power and the correct formation of the signal and CS of the memory chip. If these signals are normal, the RAM needs to be replaced. By the way, versions of the "Rus" program with an index higher than 17 already contain a built-in RAM test;
   d) tests the channels of the programmable timer KR580VI53. The inscription "Err 8253" will appear in the absence of the RD signal at pin 22 of the timer, but do not worry: this signal is not necessary for the program to work. The message "CLC Err" will appear in case of malfunctions in channel 0 of the timer (formation of a processor interrupt request). The same result will be in the absence of the clock frequency of the timer. Finally, to check the 2nd channel, the program plays a musical fragment, which allows you to evaluate the correctness of the formation of sounds by ear. If the melody is distorted, the timer should be replaced.

At the same time, for KR580VI53 it may be necessary to select a microcircuit, while analogues of this timer (8253 or KR1810VI54) can be used without selection;
   e) testing the keyboard;
   f) checks the analog components of the apparatus. Here, of interest is the test of channel B of a port configured for information input. In this case, the indicator displays the status of all eight bits of the port in the form of zeros and ones. This allows you to quickly detect air-blast malfunctions related to telephone line maintenance (analysis of the position of the handset and the state of the line, receiving a call and a signal message from the PBX).

I will add that the incorrect operation of peripheral microcircuits may be due to the lack of control signals, in particular CS (crystal selection), therefore, before starting the test, it is useful to check the functioning of the K555LL1 microcircuit involved in the generation of these signals.

From the same stage, you should continue to work on the establishment in the case when there is no test program. Some difficulty here is created by the presence of software-dependent feedback (channel 0 of the timer), since if this channel fails, there will be no dynamic indication on the display. To eliminate this situation, you can artificially generate a processor interrupt signal. To do this, for the time of adjustment, it is necessary to disconnect output 16 of the Z80 processor from the board and connect it to the node shown in the figure (after the repair is completed, the previous connection must be restored).

Then it remains to check the functioning of two nodes - RAM and the I / O port. First, the presence of control signals of microcircuits is checked (for KR537RU10 - pins 18, 20, 21, for KP580BB55A - 5, 6, 36). The absence or low amplitude of the WR, RD signals of both microcircuits indicates a processor malfunction. If there are no short pulses at pin 18 of KR537RU10, the RAM sampling unit is faulty (see above). If the RAM chip has a CS signal, but the I / O port does not have it, most likely the RAM is damaged (a defect in the system memory area). In the presence of a sampling signal of both named nodes, it can be assumed that the air-blast unit is malfunctioning.

After the indication on the display is restored, you should check the operation of the keyboard, for which you need to check the signal at terminals 24 and 25 of the air-blast. When a button is pressed, pulses with an amplitude of at least 3 V should appear on one of these outputs. If their amplitude is small and pressing requires a lot of effort, then the keyboard pads or conductive rubber are dirty, which often happens with older devices. If, with a normal range of pulses at pins 24 and 25 of the port, keystrokes are not processed, this microcircuit is faulty.

To repair a device assembled on the basis of a single-chip microcomputer (most often used 80c31, 80c32, 80c52, etc.), we can recommend the scheme given in [3].

Repair of the digital part of such a device is easier than that based on the Z80. After checking the supply voltage, you need to make sure that the crystal oscillator is working, for which you need to check for the presence of pulses with a frequency of 10 MHz at pin 18 of the micro-computer. Here and below, the pin numbering is given for the DIP package (two-row pin arrangement).

At the next stage, pin 9 of the microcomputer must be disconnected from the board and connected to the power plus through a 10 μF capacitor, after which they begin to control the signal levels on the address and data buses (see above). If broken elements are found, the corresponding microcircuit is replaced.

Next, they check the operation of the buffer register (its type is K555IR22, 74LS373 or 74NS373), which forms the eight least significant bits of the address for RAM and ROM. At pin 11 of the register, there should be short load pulses (ALE signal), and at all outputs - signals with TTL levels. If the register is correct, proceed to the ROM test after making sure that the PSEN signal is present (pin 29 of the microcomputer).

The easiest way to check a ROM chip is to replace it with a known good one containing a similar program. It may not be successful. Then you have to conclude that the micro-computer is malfunctioning.

In addition to these microcircuits, it is necessary to mention the LSI of RAM. Diagnosis of this node is simple, since all versions of the software have a built-in RAM test and, if the latter fails, they display the message "Err RAM". The table shows the main types of memory chips used in AONs, as well as their symbols, which are issued by the Rus-25c + version during the test.

I will list the remaining microcircuits of the digital part: registers that control indicator segments and analog circuits (K555IR22, K555IR23), elements that write to these registers (K561LE5); indicator familiarity decoder (K555ID10). Checking these knots is simple and does not need any explanation.

When the digital part of the caller ID is adjusted, they proceed to repair the analog one - the nodes responsible for interacting with the telephone line.

Among the active elements of the analog part, he touched three key transistors (KT940A are usually used):

a) a pulse key (IK), which issues dialing pulses and service signals, including voice ones, to the line;
b) an "auto-raise" transistor that connects a load resistor to the line during auto-dialing and number identification;
c) conversational key.

Such a construction is typical, but there are exceptions, in particular, a p-n-p transistor (KT3157A) or a combination of two transistors of different structures can be used as an IR.

To check the analog part, you need to turn on the device in the telephone network and test it in all basic modes (call, number identification, conversation, dialing, auto redial).

Below are typical breakdowns of the analog part and methods for finding them.

1. When the device is connected to the telephone line, the voltage in it decreases, it is impossible to get through to this number (constantly "busy"). Having disconnected the phone from the power supply, it is necessary to measure its input resistance from the side of the telephone line terminals, the normal value is from 100 to 300 kOhm. If the resistance is from several tens of ohms to tens of kilo-ohms in both directions, the reason is damage to one or more key transistors. If the resistance is low in only one direction, one of the diodes of the bridge connected at the input of the device is faulty. Another possible cause of the malfunction is a defect in the printed circuit board, visually this manifests itself in the form of a blackening of the plastic area.
2. There is no indication on the display that the line is free, and after the end of the conversation, the counting of the conversation time continues (for versions of "Rus"). You need to check the line status sensor circuit. In Caller ID on the Z80, at pin 20 of the KR580BB55A port, when the telephone line is free, it should be high, and when it is busy, it should be low. If so, then the port is bad. In the AON on the micro-computer, at its output 15, with a free line, there must be a low level (an inverter is used). You should find a break or replace the defective element.
3. When a call is received, the device rings once or twice, and then disconnects from the line (the caller hears short beeps). This common failure is associated with a break (burnout) of the resistor connected to the line as a load. It should be rated at 0,25 watts.
4. When a call comes in, the phone does not ring. We need to check the call detector circuit. In caller ID on the Z80 - the presence of pulses during the call at pin 19 of the KR580VV55A port. If these pulses are present, then the port is faulty. In AON on a micro-computer, the pulses should be at pin 14 of the last one. In both cases, a capacitor failure in the voltage divider circuit is likely. It happens that the caller ID, which worked normally on one telephone line, stops ringing on another. The reason for this is the non-standard PBX ringing signal, because in later versions of the program, not only the presence, but also the frequency of the ringing voltage is analyzed. For example, with a bipolar ringing signal, the pulse frequency at the output of the diode bridge becomes twice the standard and may go beyond the frequency band allowed for the call. In this case, the phone can be restored by connecting it to the line through a diode (for example, KD209A). The polarity of the diode is selected experimentally by the presence of a beep in the tube. It should be added that in the Rus-25s+ program, as well as in some versions of the Rus-23 version, the frequency band selection is implemented in software.
5. The phone rings when there is no call on the line. It is necessary to check the detector circuit (see above), a defect in the capacitor of the voltage divider circuit is possible. If the phone starts ringing when dialing a number on a parallel device, it is necessary to reduce the resistance of the lower arm of the divider (for caller ID on Z80, this is a resistor connected between terminal 19 of the UBB and the common wire). Another possible reason is that the frequency capture bandwidth is set too wide (for the Rus-25s+ version). In this case, you need to set the standard bandwidth.
6. "Auto redial" does not work, numbers are not determined. Malfunction in the signal amplification channel from the line. For the device on the Z80, it is necessary to check the formation of the signal by the K554SDZ comparator. When you start the "auto-dial" on the output 18 UBB should be a signal with a frequency of 425 Hz. If this signal is present, then air-blast is faulty. In the apparatus on a microcomputer, the signal at its output 13 is checked. In the early AOH models, the amplifier was assembled on two or three CMOS logic elements connected in series and operating in a linear mode. Later, they began to use a two-stage inclusion of an op-amp. You should check the passage of the signal through this circuit, paying attention to the input isolation capacitor - it must be high-voltage (for example, K73-17).
7. Auto redial works, but numbers are not detected. This situation does not necessarily indicate a malfunction of the device. In general, you need to check the system settings, if there is any doubt about their correctness, you should restart the program. Numbers may not be determined for other reasons. In particular, an outdated or departmental PBX may not be equipped with caller ID equipment at all. Modern stations, on the contrary, often have electronic blocking, the removal of which is a paid service. Some PBXs require individual selection of parameters (type and number of requests, a pause between them, see the operating instructions for the device).
8. When the handset is off-hook, it is impossible to dial a number: after dialing any number, the line is reset (long beep). "Auto redial" when the handset is down works fine. The defect is associated with a break in one of the transistors of the microphone amplifier, while the equivalent resistance of the speaking node becomes several times higher than the norm. Because of this, when the handset is off-hook, the voltage in the line is 25 ... 40 V instead of the prescribed 5 ... 15 V. As a result, after dialing a digit, the PBX devices do not record the "off-hook" state and disconnect the subscriber.
9. There is no beep in the handset with a working telephone line. The breakdown is caused by a malfunction of the conversational key, one of the transistors of the telephone amplifier, or the telephone capsule itself. You should also check the operation of the microswitch located under the handset hook.
10. There is a beep on the handset, but the microphone does not work. Open the tube and having connected the device to the line, touch the positive output of the microphone several times with tweezers. If clicks are heard at the same time, then the microphone is faulty, otherwise it is a defect in the microphone amplifier. If the handset is connected via a cord with J11 connectors (for example, in the body of the "Technica"), you must additionally check the presence of contact in the connectors. If the connection is poor, replace the cord.
11. Noise in the handset during a conversation. It is possible that the electret microphone has broken contact between the body and the negative electrode. Lightly crimp the rim of the microphone with pliers or replace the microphone. Another possible reason is the oxide capacitor in the microphone power filter circuit has failed.

Literature

1. Korshun I, Timakov S. Phone of a business person. - Radio, 1993, No. 9, p. 33-36.
2. Nikishin D. Once again about the improvements of GAs on the Z80. - Radio, 2001, No. 2, p. 38.
3. Dolgov O. Multifunctional telephone set "Telink". - Radio, 1996, No. 6, p. 43-46.

Author: D. Nikishin, Kaluga; Publication: radioradar.net

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