Repeating malfunction of the GRUNDIG-GR1450 TV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Repeating malfunction of the GRUNDIG-GR1450 TV. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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The GRUNDIG-GR1450 TV stopped switching from operating mode to standby mode (STANDBY) upon command from the remote control when pressing the ON/OFF button.

Part of the circuit diagram of the node that executes the mentioned command is shown in the figure (positional designations are indicated in accordance with the TV circuit diagram).

Repeating malfunction of the GRUNDIG-GR1450 TV

Let's take a brief look at his work. The signal from the remote control, received by a working TV, is processed by a specialized microcontroller CTV222S-PRC1 (U101). A high level voltage appears at its output STDBY (pin 41), which opens transistor Q103. In turn, this causes transistor Q505 to turn off and therefore transistors Q506 and Q507 to turn off. As a result, the +115 V supply voltage stops flowing through transistor Q507 to the horizontal output stage and the TV switches to standby mode.

The check showed that transistor Q506 had failed. It turned out to have a “broken” collector junction. In this case, transistor Q507 remained open, regardless of the signal level at the STDBY output of the microcontroller.

After replacing the faulty device, the TV's functionality was restored, but not for long. The second time the TV was turned off by a command from the remote control, the same transistor was again damaged. Another copy suffered the same fate. It became clear that the transistor operates in a mode close to the limit (most likely, in terms of the permissible power dissipated on the collector), and during transient processes resulting from turning off the horizontal supply voltage, the power released in the transistor exceeds the permissible and it goes out of service. building.

Obviously, it was necessary to select a pnp structure transistor with parameters no worse than those of the 2N5401 and higher power dissipation. Such a device has been found. This is the domestic KT505A. After installing it, the TV's performance was completely restored.

Author: V. Chudnov

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