This article is mainly about the relevant introduction of Samsung 788DF color display, and focuses on the working principle and maintenance method of Samsung 788DF color display.
Samsung 788DF color displaySamsung 788DF is a CRT monitor owned by the brand Samsung, with a 17-inch display. This CRT monitor has been discontinued on November 22, 2007.
Basic parameters
Screen size: 17 inches
Visual size: (standard) 312 x 234 mm;
(Maximum) 325 x 244 mm
Picture tube type: flat
Picture tube characteristics: Samsung Dana flat picture tube, with negative ion health function
Dot pitch: 0.2mm
Horizontal scan: 30~70 kHz
Vertical scan: 50~160 Hz
The highest resolution: 1280×1024
Recommended resolution: 1280×1024
Input signal: RGB analog
Input signal connection/interface: 15-pin D-Sub
Adjustment function: OSD language
Deflection angle: 90 degrees
Audio performance: no speakers [1]
Plug and play: DDC 2B/2Bi
Power performance: AC90~264VAC, 50/60Hz
Power consumption: ≤55W
Shape design: composite coating
Dimensions: net size 401 x 378 x 410;
Packing 464 x 459 x 518 (width x height x thickness) mm
Product weight: (net weight) 14.3kg
(Packing) 16.2kg
Safety certification: TUV GS, MPR II
Other properties: MagicGreen / Cable: VGA cable
The working principle of Samsung 788DF color display circuitThe main power supply of this machine is composed of the new thick film switching power supply control chip DP904 as the core, which mainly supplies power for the microprocessor, kinescope filament, line/field scanning, video amplifier and secondary power supply circuits.
1. Introduction to DP904 chip
DP904 is a thick film circuit dedicated to switching power supply produced by Samsung. It contains a reference power supply, an oscillator, a current mode PWM controller, a drive circuit and a soft-start circuit, and a current detection type power output field effect tube with a withstand voltage of up to 650V, and has protections such as overcurrent, overvoltage/undervoltage, and overheating. Features. The pin functions and measured data are shown in the table below.
. Mains input and conversion
After the AC power supply is connected, the 220V mains voltage is sent to the common mode filter (ie anti-interference circuit) composed of R601, C601, L601, C602 and other components through the fuse FHI to filter out the high frequency interference in the mains power grid , Divided into two outputs: one is sent to the controlled degaussing circuit composed of the thermistor TH601 and the degaussing coil CN602 through the relay RL601; the other is rectified by the rectifier bridge stack D601, and the current is limited by the negative temperature coefficient thermistor TH602, and the capacitor C605 Filter to obtain a DC voltage of about 300V, which supplies power to the switching power supply circuit.
3. Switching transformer circuit
The about 300V DC voltage generated by the rectifier filter circuit is applied to IC601 (1) pin (ie the drain of the field effect switch tube inside DP904) through the (1)-(2) windings of the switching transformer T601. At the same time, after the start-up resistors R606 and R605 are stepped down, they are added to the IC601 (3) pin (power supply terminal), and the capacitors C606 and C607 are charged. When the chip (3) pin voltage rises to 15V, the internal oscillation circuit of the chip starts Enter the oscillation state, the switching power supply starts to work. At this time, the windings (1)-(2) of T601 will have high-frequency pulse currents, and due to electromagnetic induction, high-frequency pulse voltages are induced in windings (3)-(4) of T601. This voltage is rectified by D606, current limited by IL607, amplitude limited by D607, and filtered by C606 and C607 to obtain a DC voltage of about 32V, which replaces the turn-on voltage provided by R606 and R605 during startup to maintain the normal operation of the power chip IC601.
4. Voltage stabilization control circuit
The voltage stabilization control loop is mainly composed of circuits such as ZD602, Q601, IC601 (4) and other internal circuits. When the mains voltage rises or the load becomes lighter, causing the output voltage to rise, the sampling voltage generated by the (4)-(5) winding of T601 also rises. After the voltage is limited by the voltage regulator tube ZD602, it is sampled by R610 to increase the base potential of Q601 and deepen the conduction of Q601, which causes the potential of IC601 (4) to drop. The potential of IC601 (4) is changed by the voltage in IC601. After the voltage stabilization control circuit is processed, the time to control the conduction of its internal FET is shortened. The energy storage of the switching transformer T601 is reduced due to the shortening of the on-time of the switching tube, and the output voltage of the switching power supply drops to the specified voltage value to achieve voltage stabilization control . On the contrary, the voltage stabilization control process is reversed.
5. Protection circuit
(1) Power supply over-current protection TH602 is a negative temperature coefficient thermistor, which mainly acts as a current limiter at the moment of start-up, so as to prevent the rectifier bridge stack D601 and other components from being damaged by excessive inrush current.
FHI is a 3.l5A fuse which is used to prevent over-current of the mains rectifier, filter circuit, and switching power supply.
(2) The switch tube protection circuit consists of R603, C608, D603 and C609, R604, D604 to form two spike absorption circuits. Its function is mainly to eliminate the spike voltage generated by the switching transformer when the internal switch tube of IC601 is turned off to prevent integration The circuit is not damaged by breakdown.
(3) Over-voltage protection circuit. When the AC input voltage is too high or the voltage stabilizing circuit is out of control, causing the voltage across C606 to be higher than 34V, the zener diode 2D60l breaks down and short-circuits to ground, causing IC601 to stop working without operating voltage. Protect the role of the whole machine.
In addition, IC601 (3) foot also has an overvoltage protection function.
(4) Under-voltage protection circuit If the switching power supply causes IC60 (3) pin voltage to be lower than 9V due to abnormal operation, IC601 will stop working and enter the locked state.
(5) Overcurrent protection circuit IC601 (2) has an internal overcurrent detection resistor connected to the source of the switch tube. When the switch tube outputs overcurrent, the voltage across the overcurrent detection resistor rises and exceeds the set threshold (1V), its internal overcurrent protection circuit will act, the oscillator will stop oscillating, and the power output will be cut off.
(6) Overheating protection circuit When the junction temperature of IC601's internal die exceeds 160℃, the overheating protection circuit will act and the power supply will stop working.
In addition, C630 and R631 form an anti-interference network, which is used to filter out the peak high-frequency interference generated by the high-frequency rectifier diode from the turn-on to the cut-off moment, and protect the rectifier components from damage. D621 is set to prevent Q621 from being damaged by overvoltage at the moment of cut-off.
6. Synchronization circuit
The switching power supply is in a self-excited oscillation state when it first starts to work, and the oscillation frequency is determined by the internal circuit of the chip. After the line circuit starts to work, the output clamp pulse (CLAMP) of the microprocessor IC201 (27) pin is isolated by T602, and C613 and C611 are coupled to IC601 (5) pin. Therefore, the operating frequency of the switching power supply is locked by the line frequency.
7. Normal display and energy saving control
Under the control of the host power management signal (VESADPMS), the display has two working states: one is the normal working state. The other is the energy-saving state. Different states are displayed by the light-emitting conditions of the panel indicators.
(1) Normal display status: When the microprocessor IC201 (30) and (31) pins simultaneously detect the line and field synchronization signals sent by the host graphics card, after identification and processing by the internal synchronization signal processing circuit, from its (6) pin Output a high-level start-up signal and add it to the base of Q624 via R626, so that Q625 and Q623 are turned on successively. In this way, the controlled power supply of the whole machine is all turned on. At the same time, the (40) pin of the microprocessor outputs a high level, which is added to the control terminal of the power indicator OP1 via R215. OP1 is green, indicating that the machine is in a normal working state.
(2) Energy-saving state; when the mouse and keyboard are not working for a long time, the computer only outputs the horizontal synchronization signal or the vertical synchronization signal. After IC201 detects this information, it outputs a low-level control signal from pin (6) to enable Q624 and Q625 , Q623 cut off one after another, cut off the line/field scanning circuit and the power supply of the kinescope filament, the display goes black. At this time, the microprocessor's (40) pin outputs a pulse control signal to make the power indicator OP1 flash green, indicating that the machine has entered an energy-saving state.
Samsung 788DF maintenance method1. There is no grating, the indicator light is off, and there is no starting sound
For "nothing" faults, first check whether the fuse FH1 is blown and turned black or yellow to further determine the fault location.
(1) The fuse FH1 is blown and turned black or yellow.
If the fuse FH1 turns black or yellow. Even the glass tube burst, indicating that the AC input circuit, the rectifier filter circuit, and the degaussing circuit are seriously short-circuited, or the switch inside IC601 has broken down. The main method of repairing this kind of fault is the disconnection method, that is, first disconnect the thermistor TH602, and then use the digital multimeter to place the "diode" on the road test IC601 (1) pin. If the multimeter's buzzer sounds, it means filtering. Capacitor C605 breakdown or IC601 internal switch tube breakdown; if the buzzer of the multimeter does not sound, it means that the fault is in the AC input circuit, rectifier circuit or degaussing circuit. Disconnect the degaussing wire and plug CN602 and replace the fuse FH1 before powering on. If the fuse is not burned, it means that there is a problem with the degaussing circuit; if the fuse is burned again, it means that the fault lies between the AC input and the rectifier circuit, and it can be further tested to find out the faulty components and replace them. Tip: If the internal switch tube of the power supply thick film block IC601 is broken down, before replacing the IC601, be sure to check whether the components of the spike absorption circuit are normal, so as not to damage the power supply thick film block again after replacement: (2) The fuse is intact, but The output voltage is 0V.
The output terminal voltage is OV, indicating that the switching power supply is not working, and the fault location should be determined by measuring whether the IC601 (1) pin has a DC voltage of about +300V.
1) If there is no DC power of about +300V, it means that it falls between the AC input to the rectifier filter circuit. It is easy to find the faulty part by using the voltage inspection method.
2) If there is a DC voltage of about +300V, and it takes a long time for the voltage to disappear after shutting down, it means that the power circuit has not started to vibrate, and you should check whether IC601 (3) has a starting voltage. If there is no starting voltage, it means that the starting circuit is malfunctioning; if there is starting voltage, but it disappears instantly, it means that the power feedback circuit is abnormal, such as the breakdown of the rectifier diode D606 and the open circuit of the current limiting resistor R607.
3) If the above inspections are normal, it may be that the voltage stabilization system is out of control and the overvoltage protection circuit is activated. Check whether Q601, ZD602, 0605 are normal. If IC601 is damaged, it can be replaced and tested.
In addition, the +5V power supply, the abnormality of the microprocessor circuit or the failure of the power switch key can also cause "nothing" failures. If the +5V power is abnormal, most of the tor-side regulator integrated block lC621 (KIA78 (l5A) is damaged; if the power switch (POWER) fails, replace it to eliminate the fault; if the microprocessor (CPU) circuit is abnormal, check first The "three elements" of the CPU work, namely, whether the power supply, reset and oscillation signals are normal. If the CPU working conditions are met but the CPU still cannot work, it means that the CPU itself has malfunctioned.
Reminder: Check and repair the power supply without output voltage. If the power is turned on and then the power is turned off, the two ends of the 30OV filter capacitor should be discharged first to ensure the safety of maintenance personnel and measuring instruments
2. There is no grating, the indicator light is off, and there is a faint "click" sound in the machine
After the machine is turned on, there is a slight "click" inside the machine, indicating that the main power supply is basically working normally, and the main cause of the enemy formation is the load circuit. The method of disconnecting the secondary load circuit of the switching transformer one by one can be used to judge the fault range. When a certain load circuit is disconnected, the switching power supply resumes normal operation, and the protection circuit start is caused by a short-circuit fault in the load circuit of Fh. The most common damaged yuan stacks mainly include the following two types:
One is the breakdown of the secondary power switch tube and the line output tube of the secondary power supply rectifier diode of the switching transformer; the second is a local short circuit inside the line output transformer.
Reminder: The sampling of the switching power supply voltage regulator circuit adopts indirect sampling method. During maintenance, a dummy load (60W/220V electric bulb) should be connected to the output terminal of the main power supply (+51V) to prevent no-load protection.
3. Without grating, the power indicator light flashes green
There is no display after power-on, and the indicator light flashes, indicating that the synchronization signal input circuit is abnormal or the 15-core signal line is bad.
First, check whether the 15-pin signal line of the display is broken again or has a bad connection. After confirming that the cable is normal, check whether there is a synchronization signal input at the CPU (30) and (31) pins, if so. Then check whether the peripheral components of the CPU are normal; if not, it means that the fault lies in the synchronization signal input circuit.
Tip: If the IIC bus SCL and SDA terminals on the CPU circuit are faulty (for example, the IIC bus interface circuit has component soldering, loose joints, etc.), it will cause the CPU to fail to issue a normal startup command. Therefore, the line/field scanning integrated circuit cannot be started, which can also cause the above-mentioned failure phenomenon. Be careful when overhauling.
4. There are stains around the phosphor screen
Color spots appeared around the phosphor screen, saying that the image tube was obviously intelligentized or the image tube was damaged. The pick-up sound can be made by the relay to further judge the fault location. If the relay can make a pick-up sound after power-on, it means that the degaussing circuit is bad or the picture tube is damaged. The most common fault is that the degaussing resistor fails, and the fault can be eliminated after replacement; if you can’t hear the pick-up sound of the relay after power-on, it means that the fault is due to the degaussing. The control circuit should be tested to find out the fault point and eliminate it.
ConclusionThis is the end of the related introduction about Samsung 788DF color display. Please correct me if there are any deficiencies.
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