3.2.7 FEATURES OF THE DEVICE OF THE NODE OF TREATMENT OF THE VIDEO SIGNAL VM. REPAIR PROCEDURE OF VIDEO SIGNAL VIDEO TREATMENT

Input devices provide a connection between the VM and the computer and the passage of video signals to the final video amplifiers.

The main requirements that must be met by the input circuits and video signal processing nodes are: transfer of video signals and synchronization signals from a computer to VM nodes without distortion, as well as their stability over time, so that the image on the screen has maximum clarity, raster stability and retains its brightness parameters . These requirements must be consistent with the VM class, its modes of operation, and the limit parameters of a CRT.

Figure 59 - Typical input device and video amplifiers

The type and characteristics of the signals input to the monitor are presented in the table.

Inspection and repair of the video signal processing unit

Troubleshooting in the video signal processing node is performed after the power supply and scan nodes have been restored, so that the screen can be lit, i.e. so that all voltages on a CRT are close to the workers. The first inclusion for checks can be made without connecting a signal from the computer. Turn the knobs for setting the brightness and contrast on the front panel to the maximum position and turn on the power of the VM. In the absence of a luminous raster on the screen, check for the presence of all necessary voltages on the CRT, including the high voltage at the anode, and the glow of red from the filament in the area of ​​the base. If it is absent, remove the CRT socket and measure the resistance of the filament directly on the leads with an ohmmeter - it should be less than 3 ohms. A break in this circuit or a large resistance indicates a defect and the need to replace a CRT. If there is heat and all voltages are normal, you should try changing the position of the G1 setting (usually the lower knob, SCREEN) on the TDX to achieve a moderate raster glow and then check the effect of the focus setting (upper FOKUS knob), evaluating the result from the raster edges sharpness or observing individual lines . In the course of these checks, possible malfunctions of a CRT are identified, they may be: internal breakages of the leads from the electrodes and short circuits between them.

At the next stage, the VM is connected to a computer and the operation of the video signal processing unit is checked using a text image or graphic tests. In this case, additional malfunctions can be revealed both in the CRT and in other nodes, however, defects are more often manifested in electronic circuits than in the CRT itself.

Typical symptoms of a video signal processing node are:

• Complete absence of the image on the raster - you should check the connecting cable, the contacts in the connectors, the power supply of the IC, and the reverse circuit.
• Increased raster brightness, low uncontrolled image contrast indicate damage to the transistors of the final video amplifiers, malfunctions of the ABL system or protection circuits for excess high voltage.
• Brightness and contrast adjustments do not work - this may be due to a defect in variable resistors or a horizontal scanning unit.

The above faults can be called global, that is, until they are eliminated, it is impossible to evaluate the operation of the node as a whole. After overcoming global faults, you can fully use all the adjustments to obtain an image sufficient to assess its quality. Image quality control is performed by the pictures obtained when running test programs. In the case of testing video nodes, the program in the computer must provide test images for the following checks and adjustments:

• Focusing and assessing the size of the spot from the beam, clarity.
• Set the brightness and contrast.
• Evaluate and tweak white balance and color.
• Check the purity of color on the screen field.
• Evaluation of the transient characteristics of video amplifiers in the low-frequency region.
• Estimates of the work of the information system rays.

When checking for test images, the following malfunctions can be detected:

• • The impossibility of obtaining sufficient brightness of a single beam - this may be caused by the aging of the cathode of a CRT, a defect in the IC or transistors; for an EGA-type VM, malfunctions may occur in the video signal processing unit (ROM, etc.).
  • Poor color purity - manifested as a divorce or uneven glow across the screen field, this is due to magnetic interference, the source of which may be a demagnetization loop (if it does not work or works, but does not turn off), and CRT defects (its deflecting coils) are possible.
  • The distortion of the boundaries of the transitions from the bright edge of the image to black, which appear in the form of "tynuchek" or repetitions, as a rule, this is due to faulty electrolytic capacitors, matching resistors, cable.
  • Instability of focus, brightness, chromaticity - it is usually observed due to unstable voltages obtained from sources at other nodes, or soldering defects and poor contact in trimmers.
  • Malfunctions in the horizontal scanning and control units that lead to changes in the supply voltage or the inclusion of protection circuits (ABL, high voltage excess).

After obtaining a stable image in one of the main operating modes of the VM, repeat the verification of the characteristics of the tests, both of this mode, and of all possible others for this VM.