CRT TV screen shrinking and expanding

CRT TV Screen Shrinking and Expanding – How to Fix

IntrIntroduction

Is your CRT TV screen shrinking and expanding? This issue is common in older televisions and usually points to voltage instability, faulty components, or circuit failures. In this guide, we’ll cover the possible causes of CRT TV screen size fluctuations and provide step-by-step solutions to fix them.

Common Causes of CRT TV Screen Shrinking and Expanding

Before attempting a repair, it’s important to understand what might be causing the issue:

1. Power Supply Voltage Fluctuations

The power supply unit (PSU) of a CRT TV needs stable voltage. If the voltage varies, the CRT TV screen size may change unpredictably.
Solution: Check the power supply circuit for faulty capacitors, voltage regulators, or dry solder joints.

2. Defective Horizontal Output Transistor (HOT)

The HOT is responsible for maintaining the horizontal deflection in the TV.
A weak or damaged HOT can cause CRT TV screen instability.
Solution: Test the HOT using a multimeter. Replace if defective.

3. Bad Flyback Transformer

The flyback transformer provides high voltage to the CRT TV. If it’s malfunctioning, it can cause screen distortion.
Solution: Look for visible cracks, burns, or sounds indicating a failing flyback transformer and replace it if necessary.

4. Faulty Horizontal Deflection Circuit

The horizontal deflection circuit controls the width of the CRT TV screen image. Any problem here can cause shrinking and expanding.
Solution: Inspect the circuit for defective diodes, resistors, or capacitors and replace them.

5. Electrolytic Capacitors Failure

Aged or damaged capacitors in the power supply or deflection circuit can lead to instability in screen size.
Solution: Identify bulging or leaking capacitors and replace them.

Voltage Check Points

To diagnose CRT TV screen shrinking and expanding issues, use a multimeter to measure voltage at the following points:

Power Supply Output: Should be stable at around 110V–130V.
Horizontal Output Transistor (HOT) Collector: Must be within 110V–130V.
Flyback Transformer Secondary Output: Should be between 20kV–30kV.
Horizontal Driver Transistor: Voltage should match the circuit diagram specifications.
Voltage Regulator ICs: Ensure they provide the correct regulated voltage output.
Electrolytic Capacitors: Check for fluctuating voltage indicating possible failure.

Components Check Points

When troubleshooting CRT TV screen shrinking issues, check the following components:

Capacitors: Look for bulging, leaking, or high ESR values.
Diodes & Resistors: Test for continuity and correct resistance values.
Flyback Transformer: Check for visible damage, overheating, or buzzing sounds.
Horizontal Output Transistor (HOT): Test with a multimeter for shorts or open circuits.
Voltage Regulators: Verify they provide a stable output voltage.
Solder Joints: Inspect for dry or cracked joints and re-solder if necessary.

Equivalent Replacement Components

If a component is faulty and needs replacement, here are some common CRT TV equivalent components:

Horizontal Output Transistor (HOT): If using BU508A, it can be replaced with BU2520DF or 2SC5149.
Flyback Transformer: Replace with the same model number; generic equivalents depend on TV model.
Voltage Regulator IC: Replace STR50103 with STR5412 or an equivalent with matching specs.
Capacitors: Replace with the same capacitance and voltage rating (e.g., 2200µF 35V should be replaced with an equivalent).
Rectifier Diodes: 1N4007 can replace many standard rectifier diodes in the circuit.

Step-by-Step Repair Guide

Step 1: Safety Precautions

Before starting, always follow these safety measures:

Unplug the CRT TV and let it discharge for at least 30 minutes.
Use an insulated screwdriver to discharge the anode cap.
Wear safety gloves to prevent electric shocks.

Step 2: Inspect the Power Supply Section

Open the CRT TV back panel and locate the power supply board.
Look for swollen or leaking capacitors.
Test the voltage regulator ICs and replace them if faulty.
Voltage Check: Measure the B+ voltage from the power supply; it should be stable at around 110V–130V.

Step 3: Check the Horizontal Output Transistor (HOT)

Locate the HOT, usually near the heat sink.
Use a multimeter to check for continuity.
If defective, replace it with the same or compatible model.
Voltage Check: Ensure the HOT collector voltage remains steady at around 110V–130V.

Step 4: Examine the Flyback Transformer

Inspect for visible damage, burning smells, or unusual sounds.
If the flyback transformer is defective, desolder it and install a new one.
Voltage Check: Use a high-voltage probe to test the anode voltage (should be between 20kV–30kV).

Step 5: Test the Horizontal Deflection Circuit

Look for burnt resistors, shorted diodes, or faulty capacitors.
Replace any damaged components accordingly.
Voltage Check: Measure the horizontal driver transistor voltage to ensure correct operation.

Step 6: Reassemble and Test the TV

After replacing the necessary components, carefully reassemble the TV.
Power it on and check if the screen size remains stable.

Preventive Maintenance Tips

To avoid future CRT TV screen fluctuations, follow these maintenance tips:

Use a voltage stabilizer to prevent power fluctuations.
Keep the CRT TV dust-free to prevent overheating.
Regularly inspect capacitors and circuit components.

Conclusion

CRT TV screen shrinking and expanding is often due to power supply issues, faulty capacitors, or problems in the horizontal deflection circuit. By following this guide, you can diagnose and fix the CRT TV screen size problem effectively. If you’re unsure about handling electronic components, consult a professional technician.

For more CRT TV repair guides, check out our CRT TV Vertical Collapse Fix and Flyback Transformer Troubleshooting posts!

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