3.2.4 GENERAL PRINCIPLES OF VM REPAIR

The main purpose of the repair of any device is to return it to the user in the working state, if possible without degrading its characteristics, preferably with a guarantee of its sufficiently long-term further performance. To achieve this goal is possible only by answering the following questions:

• Has the cause of the failure been unequivocally determined?
• This reason is eliminated by a qualified replacement of the elements (preferably on the appropriate scheme)?
• Was the analysis carried out according to the concept on the subject: could this malfunction entail others?

In order for the repair work to have a positive result, the following procedure should be followed:

1. Make sure the VM is faulty.
Before starting work, it is necessary, first of all, to make sure that it is the VM that has the defect, and not the video card in the computer. This is easy to do by connecting the VM to a known-working computer.

2. Opening the VM and assessing its condition helps to determine the approximate service life of the VM, the correctness of operating conditions. In case of severe internal contamination, it is necessary to clean all the boards and parts of the structure from dust, since the dust creates a heat insulating layer and disrupts the normal thermal operation mode of the parts. In addition, in polluted areas where high voltage is present, conditions for electrical breakdown are created. During the inspection, special attention should be paid to power and high-voltage elements, which include: TDKS, power supply transformer, diodes, high-power transistors, electrolytic capacitors and capacitors in the horizontal scanning unit. Inspection of the back side of the printed circuit board allows you to assess the quality of the soldering, while it is also possible to quickly detect the defect. First of all, you should pay attention to the soldering at the points of connection of massive parts, such as transformers, transistors on radiators, diodes. A characteristic sign of a defect in soldering is the appearance of cracks or a gray rim around the lead, clearly visible against the background of a brilliant solder. Such points are subject to compulsory soldering, during which a defect from poor tinning of the part may be revealed, which is manifested in the swelling of solder from the terminal.

3. Ensuring the inclusion of BP VM
Bring the VM in such a state that it can be turned on, and if necessary, repair the internal power supply. At the same time, it is necessary to check if there is a short circuit at the source outputs and to eliminate interference in its operation. At this stage, it is useful to make a control measurement of the output voltages of the power supply unit, first of all, the filament voltage of a CRT, in order not to damage it.

4. Definition of a faulty node.
When the VM is turned on, but there are violations in its work, it becomes possible to conduct a primary diagnosis. The purpose of this stage is to identify VM nodes in which malfunctions are possible, provided that the power supply is checked and generally works. Then the following nodes remain unchecked:

• Line scan node
• Frame scan.
• Video processing nodes.
• Mode control circuits.
• Protection schemes

At this stage, you should try to get a raster on the screen of the VM. Perhaps, at the moment of switching on, there will be no screen glow due to the lack of a signal from the computer or changes in the settings. If it is not possible to get a glow of the screen, then the voltages on the CRT pins and the presence of high voltage are checked. Further, according to external signs, and if necessary, based on the results of control measurements, a conclusion is drawn about the faulty node.

5. Diagnostics of faulty units.
At this stage, there is a need for schematic diagrams and information on individual components. Their presence makes it possible to quickly trace the passage of signals and present their approximate levels at the terminals of microcircuits and transistors. Simultaneously with the described actions, it is useful to once again carefully inspect the printed wiring in the area of ​​the suspicious node to identify possible defects that were missed during the inspection earlier.

6. Replacing defective parts.
It is desirable to replace parts with the appropriate scheme, however, it is not always possible. In this case, it is necessary, using the reference literature, to correctly select analogues. After replacing the defective parts, repeat step 5 to ensure that the node that has been repaired and that there are no other faults is working.

7. Analysis of possible causes of faults.
Produced after completion of major repairs based on all information received during operation. The purpose of the analysis is to identify the main cause of the failure and to make a conclusion about possible VM failures upon its further use.

8. Final diagnostics, tuning and testing are performed in conjunction with a computer.

From the moment the VM is turned on, the heating of the transistor radiator of the horizontal output stage is controlled by heating - it should not be excessive for the first 15 minutes. In the same way they monitor the key transistor of the power supply and other heating parts. The established mode occurs only an hour after switching on. At this time, the output voltage of the power supply unit, the magnitude of the pulse voltage at the collector of the transistor of the horizontal output stage of the oscilloscope (it should not exceed 1500 V), the high voltage on the CRT are controlled by a high-voltage probe (24 - 25 kV). It should be noted that every deviation from the normal values ​​of the measured values ​​should be analyzed for possible malfunction. After 1 hour of VM operation, you can proceed to the settings. On the computer, choose a service program, which, as a rule, comes with a video card. This program allows you to switch VM operation modes. Select the basic mode and check the brightness of the screen and the quality of focus. Next, select the graphics mode and display the color table. Set the dimmer on the front panel of the VM to the middle position and once again control the screen luminescence - all colors of the table should be normally distinguishable, if not, then adjust the accelerating voltage G2 on the TDX to get the desired result. At the same time monitor the quality of the focus and, if necessary, correct it with another knob on the TDX. Focusing quality is assessed by the visibility of individual lines of lines. After the adjustment of focus and brightness is completed, the video amplifiers are adjusted, controlling the correctness of the color rendition according to the color table. Adjustment should provide a white balance in all gradations of brightness, which is achieved by installing trimmer resistors on the board of the final video amplifiers. Two resistors are responsible for setting up each beam (they are usually signed by BIAS and GAIN). At minimum brightness, you should adjust the resistor BIAS, at maximum - GAIN.

9. Checking the correct switching of VM modes, for which the computer selects sequentially modes and in each of them control the raster dimensions, its position on the screen, geometry and synchronization of the line frequency.

10. Correction of geometric distortions of the “pillow” type raster, for which the construction resistor with the designation “PIN” is used. This adjustment is made to set the vertical of the raster along its lateral edges; it is very subjective and depends on the curvature of the screen surface and the viewing angle. At the same time, you should not achieve fine tuning in all operating modes of the VM, since this is often not provided for by the design. It should be noted that in case of impossibility of any adjustment, there may be a malfunction of the control unit or actuators in other nodes. In this case, it is necessary to repair them and repeat the VM setup.

11. Heat run.
As a final check of the VM after repair, it is recommended to carry out the so-called "heat run", for which a fully prepared for work with a fixed back cover and mounted on the VM stand is turned on together with the computer for a sufficiently long time (at least 2 hours). During this time, the temperature of all components reaches a steady state, i.e. simulated real conditions of the VM.