3.2.8 CRT CONNECTION CIRCUIT AND ITS REPAIR METHOD

Electron Beam (CRT) is used for visual display

Figure 60 - Typical CRT connection scheme

signal outputted to its electrodes. The connection scheme of a CRT ensures the creation of the correct distribution of potentials inside the tube for a qualitative display of the signal outputted to it. The magnitude of the voltage (approximately) applied to the electrodes of a CRT and their purpose are presented in the table:

Accelerating voltage is applied to a separate anode contact on a CRT cylinder with a special high-voltage wire. Its excessive magnitude leads to an increase in X-ray radiation when electrons strike the mask, and an underestimated magnitude worsens the focusing conditions of the beam, therefore, it must be fairly accurately established. For color CRTs with a screen size of 14 ", the voltage should not exceed 25 kV (usually set to about 24.5 kV), in large-sized color CRTs (19-20") it can reach 27-40 kV, its exact value is taken from the service instructions.

In a CRT with a flat screen (with a size larger than 15 "), the so-called dynamic focusing is used, since the time of flight of electrons from the gun to the edges of the screen and its middle is different and it is necessary to adjust the focusing conditions in order to maintain the minimum spot size on the horizontal line scan. dynamic focus controls typically refer to a line scan node.

Diagnosis (requires special attention to the implementation of the rules of TB !!)

Diagnostics of the connection scheme of a CRT is carried out by successively performing the following checks:

• the presence of filament voltage is checked (compliance with 6.3V)
• The presence of accelerating and accelerating voltages and their positive difference with respect to the cathode is checked (the CRT should not be locked by the modulator).
• The beam current is checked for each channel (there must be the same current value through the channels)
• The presence of high voltage is checked by indirect signs - the presence of screen electrification or using a special high-voltage voltmeter.
• Voltage change is verified
  • on the modulator with the rotation of the brightness adjustment;
  • on A1 while rotating the focus adjustment.

A sign of good performance of the power supply circuits of a CRT is the presence of a glow on the monitor screen and the ability to adjust the brightness and focus of the tube.