Diagnosing Common Television Malfunctions & Repair Tips

Common Television Malfunctions

Troubleshooting TV Reception Issues

• Black Screen with MAT

  If there's a breakdown in the demodulator circuit or amplifier, the applied signal at the reception stage will not produce a corresponding video signal, resulting in a black screen.

• White Screen with Retracements

  A breakdown in certain integrated circuits can cause permanent activation of video lines. This results in a screen where all three electron beams are permanently activated, displaying a white screen with visible retrace lines.

• Contrast Differences Between Broadcasters

  This symptom occurs when some stations are received with sufficient signal strength compared to others with a weaker antenna signal, leading to noticeable contrast variations.

• Snowy Image

  This symptom always originates before the intermediate frequency (IF) stage. It can result from a fault in the tuner, an issue with the supply voltage, or an insufficient antenna signal level.

• Limited Channel Reception

  If a band switch transistor fails, the TV may stop tuning into broadcast channels, potentially receiving only a limited selection or just radio signals.

Diagnosing TV Control System Problems

• TV Does Not Start Up

  If the control chip fails to generate the boot command via the on/off line, the device will remain in its standby state.

• TV Does Not Start, Power LED Flashes

  This symptom indicates that the microcontroller is working but cannot initiate equipment operation. It is often caused by an EEPROM issue within the control system.

• TV Does Not Accept Commands

  The fault could be in the keyboard controller (integrated within the chip) or in the key matrix itself.

• Stations Not Memorized

  As is often obvious, the fault lies in the control system's memory or the battery that retains information when power is absent.

• TV Fails to Execute Specific Commands

  If one of the output ports breaks, you will notice an inability to control analog functions (e.g., brightness, volume).

• Remote Control Unresponsive

  In this case, you need to discern if the problem is with the remote control or the television. A quick way to check the remote is to press a key while pointing it at a television camera (or phone camera) to see the infrared (IR) signal.

Essential TV Diagnostic Checks

• Activity Check

  If the control buses are active and data is flowing through their lines, the microprocessor is likely executing its program normally.

• Clock Signal Check

  If the microcontroller's crystal shows a sinusoidal signal at a few megahertz, it indicates that the integrated circuit is powered and the clock is functioning.

• Power Supply Check

  If there is no signal from the oscillator, check if the polarization (voltage) reaches the microprocessor.

• Reset Terminal Check

  If there is no activity, and the powered oscillator is functioning, check the status of the reset terminal.

Understanding Intermediate Frequency (IF)

The Local Oscillator (LO) operates at a frequency equal to the desired channel's frequency plus the Intermediate Frequency (IF). This LO signal is mixed with the Radio Frequency (RF) signal from the antenna. This mixing process results in a shift of the signal's spectrum:

• The luminance carrier frequency moves to 38.9 MHz.
• The sound carrier frequency, which is 5.5 MHz higher than the RF luminance carrier, becomes 5.5 MHz less than the IF luminance carrier.