【 Weak current College 】 multimeter maintenance display fault method 】

First, the power supply circuit fault diagnosis

Display most of the switching power supply, the failure is mainly divided into two broad categories: the first is that the power supply is interrupted; the second is the power supply is not interrupted. A power failure, mainly has following several.

1. no raster, no image

This kind of failure may be a total power failure, or it may be a fault line scan circuitry.

To monitor the rear door is open, observation of insurance tube is damaged, there are several circumstances.

(1) insurance tube was severely damaged. When visible fuse has been broken, and burn the blackened, indicating that power some serious fault, the common is the power switch transformer primary group before circuit faulty or switch tube transmitter sets and collector breakdown short circuit.

(2) fuse is burnt out, but not dark. This indicates that the power part no serious fault, mostly output tube diode breakdown, breakdown or backward surge current impact will fuse blow (commonly display uses a period of time, because fuse long power, current, increasingly reduce overload on power surge current is large, easy-to-own damage).

(3) Insurance Authority intact. Indicates power no obvious faults that could be the output circuits work in unhealthy or switch the power supply is not working properly (e.g., no vibration).

2, image, raster, but raster spoke when large small

The main reasons for this failure are:

(1) power supply circuit pulse width adjustment circuit failure, power switching valve's breakover time is not working properly, so that the output voltage change.

(2) regulator to adjust the output voltage of the potentiometer to adjust the potentiometer and bad or damaged.

(3) voltage circuit sampling, comparison circuit faulty.

3, grating, images, accompanied by the sound of "Squeak", twist image

This failure:

(1) power filter is open or missing capacity capacitor, so that the output voltage ripple coefficient increased interference caused by the level of the image, the image is "S" shape distortion, interference with the underworld.

(2) switching power supply synchronization pulse signal coupling components open circuit, switch the power free oscillation and the oscillation frequency is below the row frequency switching power supply transformer, issue a "Squeaky" sound, to display the form as interference.

(3) switching power supply line oscillation frequency too low to display rows in output transformers and switching power supply transformer hair also "Squeak" sound.

2. output circuit fault diagnosis

Monitor output circuit has two main roles: first generation Imaging desired voltage, such as picture tube anode voltage, accelerating voltage, filament voltage, focus voltage; second, the supply line deflection coil frequency 15.75kHz of Sawtooth wave current, making row deflection coil produces vertical magnetic field, when the magnetic field generated by electron beam thanks to the left or right of deflection, resulting in line scan line, due to the line output circuits work in high-voltage and high current, output level consumption power covers whole power of 70 percent, the output circuit failure rate is high. Output circuit fault mainly has following several.

1. no raster, no image

This failure commonly line output, line output transformer, capacitor, backward diode backward.

Open the cover, the focus should check the output transformer and surrounding the appearance of the component view is partially burned, discoloration or scratches, capacitor is leakage, rose crack and drum burst and other phenomena, whether or not the line output tube shell color, rupture. If you do not find obvious fault, the available measurement line output tube multimeter-collector-to-ground resistance, normal forward resistance shall be in more than 3 k Ω (different modelscircuit detect values vary). If detection value is less than 1.5 k Ω, the main consideration should be given to the line output tube is breakdown, breakdown, damping diode is backward capacitor or S correction capacitor is breakdown or leakage.

It should also be further multimeter test output transformer secondary DC power supply circuit to ground resistance (just to test).

(1) 800-900V voltage output on resistance of about 3 M Ω.

(2) 180 ~ 190V voltage output on resistance value of about 1 M Ω.

(3) 12 ~ 17V voltage output on resistance of about 25 Ω.

The above test, you can power up detection, color display line output level is directly by the power supply and very high DC voltage, mostly 100 ~ 120V (according to the determination of the actual voltage monitor circuit diagram). If it detects a line output tube collector still voltage low, indicating that the circuit has a short circuit fault; if output tube collector voltage too high, you should check the power circuit; if output tube collector voltage is zero, you should check whether the normal power supply circuit, there is no power supply circuit breaker.

2, grating or Pincushion distortion

The failure of the main reasons are:

(1) deflection coil damage.

(2) correction of the transformer's primary and secondary winding.

(3) correction transformer quality is too low.

3, grating a bright vertical line

The failure of the main reasons are:

(1) line break inside the deflection yoke.

(2) lines of deflection circuit is not working or not working properly.

(3) lines of deflection circuit connectors or bad.

4, rows are not synchronized, or bad line synchronization

This kind of symptom and row frequency control circuit failure result rows are not synchronized phenomenon has similarities. Distinguishing fault is with the frequency of automatic control circuit or row frequency control circuit party not: found rows are not synchronized, adjust row frequency potentiometer, if regulationRow frequency can be instantaneous or short-time synchronization, synchronization coverage is very small and unstable, and indicates malfunction in row frequency automatic control circuit, the AFC circuit; if adjusting row frequency potentiometer rows cannot be transient or frequent short time synchronization, indicates that the fault lies in row frequency control circuit.

Rows are not synchronized, or fault of the main reasons are:

(1) AFC phase detector failed.

(2) lines of Sawtooth voltage forming circuit in damage.

5. central location offset

Central location offset mainly to show character or graphic to the left or right shift, but the image is stable. Such failures are relatively pulse shifter circuit failure, available multi-meter AFC input and integrator of DC voltage is normal. If the voltage is not working properly. If the voltage is not working properly, you can detect the supply circuit and filter capacitor is good.

3. line oscillation circuit fault diagnosis

Line fault of the oscillation circuit are the following:

1. no grating

When checking IC line oscillation level is faulty, you can use DC

Pressure detection method, i.e. the measurement of the voltage value leads to the foot is the same as with normal or near that IC internal and detection tester for electronic circuit work basically normal, otherwise the associated circuit faulty instructions. Common fault causes are:

(1) line oscillation circuit integrated circuits add this reason capacitance.

(2) integrated circuit is damaged.

2, display character or image out of sync

Adjustable row frequency potentiometer, if the character or image can be instantaneous or short time synchronization, indicates that the fault in the automatic control circuit line; if adjusting row frequency potentiometer cannot instantly or short time synchronization, indicates that the fault lies in row frequency circuit, you should check the control circuit of related components.

IV. field scan circuit fault diagnosis

1. field oscillating circuit failure

Field oscillating circuit failures are mostly due to the IC and external RC circuit caused by poor, common failures are:

(1) field sawtooth wave generation circuit is faulty, the display is a horizontal light line.

(2) variable capacitor in the RC circuit for a horizontal manufacturers or raster distortion.

(3) resistance RC circuit, open circuit or short circuit as a horizontal line.

2, field-level and field output motivation level fault diagnosis

Spot stimulation level failure, mostly as a horizontal light lines, and farm output stage failure symptoms. How to determine the field-level failure or incentive field output stage fails, the judgment is disconnected field excitation level output to field in the output stage pipeline people end line, multimeter R x 1 k Ω, red table pen grounding, black table amount not stop locations contact field in the output stage of the pipeline's side, artificially to farm output stage into a bunch of frequency low positive pulse signal. If gratings have upper and lower stands aside, the field output level is normal; if there is a horizontal-line fault present incentive level previous circuit. To determine the field output circuit failure, you should check:

(1) check the field output-level power supply voltage is normal.

(2) check the field in the output stage push-pull circuit point voltage is normal (midpoint voltage mostly field for voltage half).

(3) check the field output circuit working state.

(4) check whether the field deflection coil good performance.

Farm output stage common failures are: a horizontal light line or a horizontal light belt; field range is small; field line

3, farm output feedback circuit fault diagnosis

Farm output feedback circuit failure, grating the top or bottom edge will cause the volume field linear distortion. Common fault attributable to the feedback circuit breaker or feedback circuit breaker or feedback circuit elements.

4, field trouble scanning circuits

(1) grating with Flyback. The presence of fault out hidden circuits, focus should check whether reduced blanking-capacitor capacity, you can replace a well-performing capacitance.

(2) field is not synchronized, the main cause of the failure are:

① field integral circuit resistance.

② field oscillation Timer components resistance or capacitance deterioration or damage.

③ field integration capacitor performance bad or damaged.

(3) a horizontal light line, the main cause of the failure are:

① field oscillating circuit poor contact.

② field oscillating circuit component damage.

③ field output pipe damage (if integrated circuit, it is possible that the field output circuit is damaged).

④ field output circuit coupling or output capacitor is damaged.

⑤ field deflection coil damage.

⑥ field oscillating circuit timing element resistor or capacitor is damaged.

(4) field of the fault reason mainly linear distortion are:

① field output tube performance adversely.

② field scan sawtooth formation capacitor damage or poor performance.

③ yard linear compensation component scanning circuit damage or poor performance.

5. video circuit fault diagnosis

Video circuit main role is to move the video signal amplification, the CRT screen received the luminance level. For a color display, in order to meet the color needs to be red, green, blue three color signal to display adapter receives over, and brightness signal together to form a color image.

In inspection, first detection depending on output tube collector DC voltage is normal. Should distinguish between the picture tube, together with the wooden pull-out end, measurement check tube holder is damaged. If the socket is good, is the picture tube and its subsidiary circuit faulty. Then measured as thePut the output pipe collector voltage, if correctly, then the picture tube is bad, otherwise it depends on the driver or a faulty scan circuitry.

If the brightness control, grating appears too dark or too light, you can check the video output circuit bright signal input terminals DC voltage is normal. This can tell is the brightness control circuit failure, or video output circuit. Video circuit failures are:

1. image missing colors

Available multimeter measuring three depending on tube (red, green, blue) collector voltage. If a Visual drop tube integrated electrode voltage is high, mostly the tube is damaged, open-circuit condition.

2, grating is a single color, and very bright, retrace lines

Available only as a multimeter measuring three tubes (red, green, blue) collector voltage. If a Visual drop tube collector voltage low, mostly the pipe damage, is short circuit condition.

3. partial color

There is a common symptom; balance circuit adjustment trimmer potentiometer incorrectly or bad. You can adjust the white balance again by moving the color saturation of the potentiometer switch off, adjust the red, green and blue trim potentiometer, the image is in black and white State. If there is a fine tuning adjustment potentiometer and useless, it can be removed, use absolute alcohol after cleaning and drying. If using a multimeter measurement is still poor, you should replace the trimmer potentiometers.

6. brightness circuit fault diagnosis

The main fault brightness circuits include the following:

1, a brightness control

Cause image brightness out of the main reasons is the brightness potentiometer, Deputy brightness deterioration or damage, potentiometer and DC clamp circuit faulty and unable to work properly.

Picture tube inside the gate and a gun cathode short-circuit, will create the brightness out of control, and accompanied the Flyback line and grating grave partial color.

2, contrast difference

The main reason for the failure is contrast adjustment circuit component deterioration or damage.

3, brightness too bright and accompanied back to line sweeping

The failure of the main reasons are:

(1) pair the brightness potentiometer performance bad or damaged.

(2) depending on tube saturation, make the CRT-cathode voltage is too low, beam current increases.

(3) CRT with voltage is too high, beam current increases, adjustable output transformer on the accelerator potentiometer is resolved.

(4) direct coupling of luminance channel components with open circuit or short circuit, the right to use depending on tube collector potential decline.

Dim 4, grating

The failure of the main reasons are:

(1) automatic brightness limits ABL circuit is not working properly.

(2) of the picture tube.

(3) CPT operating voltage is not working properly.

5, image fuzzy

The failure of the main reasons are:

(1) color CRT performance adversely.

(2) color CRT anode voltage is too low or focus.