Monday, December 27, 2010

Weak current College】 【monitor repair case excerpt】.

<br> During the summer, I'm hard to kill the online sweat, friends he damaged monitor sent, and the life and let me show him repairs. This summer the weather is too hot, air conditioner, resulting in a voltage fluctuation and .subsides, and I'm here, also just use a conditioner for 220V/110V transformer adapted successfully a regulator, the transformer, nominal power: 2000W, all devices are working, not the slightest effect. There is no display on the monitor, listen .a little inside the "pop" and "pop" sound, according to a friend is in normal use, it does not light up, and then shut down restart, only see the LED flashes on the host computer, the monitor does not have any .reaction, suspected internal component damage cause protection circuits. Turn on the monitor, not powered multimeter measurement line output resistance stalls of the collecting electrode, the resistance is relatively large, the description is not a line pipe; measurement main power output, find the resistance .of 0 indicates the principal power load have component damage caused a short circuit, along the circuit to check and found that there is a model for the power tubes IRF634 breakdown short circuit, the following task is to identify the causes of damage to the pipe, .and then swap symbol is OK now. Checked display of related circuit, found IRF634 power tubes is the second power boost. Now the monitor to a different resolution can be full of effect, using a microcontroller circuit to control the secondary power boost pipe work status .to get different output voltage, the circuit of the relevant draw, received the following circuit diagram, the following on this circuit to do some analysis: 1. Figure 1 inductors L1, field effect tube Q1, diodes D2 and so a simple bootstrap booster circuit .. Through the field effect transistor Q1 of G pole to join certain switching pulse signal, the Q1 FET is off. When the inductor on L1 will produce a certain amount of EMF, and then the overlay to the main power supply, through isolation diode D2 .isolation by the capacitor C2 power filter, gives the output circuit. <br> 2. Figure 1 resistor R2, R4, capacitor C3, triode Q3, Q4, Q7, diode D1, formed a switch pulse excitation circuit. By IC TDA 4858, .ë • feet output switching pulse signal, the Q7 triode amplifier, the collector output to Q3, <br> Q4 two transistors formed by amplifying circuit, then will switch pulse signal coupling capacitor C3 to diode D1, resistor R2 the composition of the integral circuits ., truncated switch pulse of negative half-week and a half weeks, retention is the current limiting resistor R1 evacuation to FET Q1 of G, driven bootstrapping booster circuit. <br> 3. Figure 1 capacitance C6, potentiometer W1, resistor R11, R12 ., R16, transistor Q8, D6, D7, diode to form a second power supply voltage sampling amplifier circuit. The line output transformer L2,, ë °-end output pulse voltage via harmonic filter capacitance C6, then by a potentiometer Wl and resistance of .sampling circuit R16, voltage sampling, D6, D7 by diodes and resistors R12, R13, consisting of the integral circuits for integral treatment, send to the base transistor Q8 for current zoom in, zoom in by emitter output current limiting resistors R15, evacuation to .ICmA4858,, ë § feet. The sampled signal voltage directly to control IC internal switching oscillation width of the pulse signal, then the zoom IC first, followed by, ë • feet output to drive circuit. By the sampled voltage to switch pulse-width .modulation, naturally regulating the field effect transistor Q1 breakover time, changed the inductance of EMF on L1, which reached the regulated, the purpose of stabilizing the voltage. <br> 4. Figure 1 capacitor C5, diode D3, resistance R10 a anti- .misoperation over voltage regulator circuit. Its role is to prevent the power-on, as part of the normal working line requires a certain amount of time, during which time, in order to prevent a second power supply from line part is not working, and .cause the second power supply voltage control, so that the output voltage is too high, the burning of output section of the component. <br> When powered on, the second power is in its initial state, and failed to work properly now. At .this point in order to prevent a second power supply voltage is out of the ad hoc anti-misoperation action control circuitry. No Flash, and a pulse voltage via C5 coupling after the diode D3 for integral, truncated negative half of week, then the signal .voltage current limiting by R10, sent to the input of sampling circuit, enabling secondary power supply output voltage reduce, prevent second power supply output voltage to rise. <br> 5. Figure 1 resistor R5, only 3, R6, R7, R8, .R9, diode, triode D5 Q5, Q6 and capacitor C4, form a pulse switch signal drive amplifier circuit. The circuit through the rows of the output voltage of sampling inspection, automatic change pulse excitation boost circuit of supply voltage, thus further switch pulse- .width so that the secondary power supply output voltage more stable. Through the analysis on the face of the circuit, fast diode D2, are boosting tube IRF634 load, now the pipe damage on its load circuit has caused its power through corruption, D2, C2 .and line output tubes are suspected, inductors L1 if any turn to turn short-circuit, also in the work process and breakdown IRF634 spikes. Remove the D2, check the diode forward and backward resistance to normal; <br> Remove the D2, the .online measurement line output tube resistance values, no unusual; <br> Remove the inductors L1, because no valid way to check that it is good or bad, just put the original coil dismantle the rewinding, the coil is the number of shares of yarn .package line wrap up all the original coil down, carefully check the yarn package cable is damaged or worn short, as is the coil winding, layers and tiers, coupled with a thin layer of the paper insulation, good after drying after dipping back after welding .; <br> Check the filter capacitor C2, no leakage phenomenon, but its forward and backward multimeter measurement of resistance, he found one of the pins loose phenomenon, force a dial, has put the PIN pulled out - very clear, thisA capacitor .is bad internal pin so that boost pipe the output of high-frequency pulses, cannot be good filter to volatility of voltage to the line output tube, it will cause a short circuit, and eventually burned AC-boost power tubes, so that the power .output short-circuit protection circuit. A capacitor 22UF/250V and one had to of the monitor IRF634 troubleshooting! below is from the online search to two cases of maintenance processes: cases of 1, 3. <br> Maintenance procedure: power on, .the power led is not lit, switching power supply transformer with lighter "Calculator", according to maintenance experience, this must be a serious short circuit switching power supply load. Powered, detection line pipe has breakdown. Line pipe damage several reasons so: line .power supply is high, line backward capacitor failure, irregular pulse excitation, line load short-circuit, etc. First disconnect the line load, on the main power supply output filter capacitor C1 ends a 25W bulbs as false load. Measuring voltage capacitor C1 as .65V, normal. And detection line backward capacitance and no exception, and then a new line pipe, restore power supply line. Power on, hear the CRT have high pressure to produce sound, then immediately disappear, and the power led is off, switching .power supply also issued "Calculator", line pipe and breakdown. Because the line pipe in the short time breakdown damaged, obviously belong to the pressure, but the main power take off load and normal, second power supply has failed. <br> At .this point, in order to prevent the power line pipe and breakdown, in line with tube Q2 collector and received between a 472pF/2.5kV of backward capacitance. Power-on test second power output to 250V, serious high. This description is second out .-of-power. Careful analysis, second power supply can produce output voltage, this shows the second power supply switch pulse oscillation, magnification, and promote the part are OK, the problem is in the steady voltage sampling feedback loop. In order to narrow .the scope, will be sampling potentiometer W1 end and circuit board welding, <br> Power-on test second power output filter capacitor C2 voltage remains 250V (normally around for 65V), serious malfunction. And detection sampling amplifier Q8, 12V, bc is .extremely 0V, stating that the pipe is closing, the normal status should be enlarged. This allows the detection of sampling filter capacitor C6 at both ends to 14V, resistance to Rll, transistor Q8 base, the base voltage 0V. Remove the resistance test resistance .to infinity, color-coding to 10kfl troubleshooting, replacement. <br> Maintenance summary: as sample current limiting resistor divider, which led to the failure of Rll over-air signals does not send back for sampling of pulse-width circuit, so that .the second power cannot change switch pulse width for regulators, led to the breakdown voltage rise, the line pipe. Example 2, no 3. <br> Maintenance procedure: power on, the power led is not lit, built-in "Calculator" .sound detection line pipe has breakdown, and then carefully used various methods to detect the main power supply, power supply, the second line circuits are OK. Replace the line pipe, commissioning eight hours, there is no exception. But the next morning, a .power-on, see power lights for a brighter then off machines also issued "Calculator". Disassembled machine, find rows of tubes and breakdown. Then replace the line pipe, and look to the power supply and secondary power supply and substitution all backward capacitors ., power-on hours are normal. At this time and continuous switch several times the power switch, try this machine voltage performance are normal. And the next morning, a power on, fault again, this time you can afford, but to replace new .line pipe on Q2 waiting for the arrival of the second day. <br> The next day, when powered on, the multimeter closely monitoring second power supply output voltage of the capacitor field ends, I saw a post, the voltage from 0V to 140V ., and line pipe breakdown, which can be learned, turned out to be a power regulator. Careful analysis, this failure is only occurs when the morning post, could be the voltage circuit a symbol of bad influence of the temperature. But the power is .divided into primary power and the power of two parts, the Vice-President, the first check which part? by principle can know, even if the mains voltage is not good, in this short time line pipe breakdown, because the main voltage regulator output ., even after the power supply voltage, the Vice-President will select from the Deputy power voltage regulator loop to check. Analysis of circuits, found to affect just post secondary power supply and stabilizing effect is the most important part is the resistance of the R10 ., diode D3, capacitor C5 formed by the "anti-misoperation over voltage regulator circuit". <br> Carefully check the resistance of the R10, diode D3 is no exception, but the multimeter capacitance Chang Rx10k stalls check, has no capacity to appearance .also changed shape, normal capacity as 0.033uF/250V. It is located at line pipe heatsink side, estimates may be affected by the capacitance in high temperature baking and long-term damage. Replace the capacitance, the failure to completely ruled out. < .br> Maintenance summary: as the capacitor failure, just post line circuit is not stable, it can not be normal and stable secondary power supply output voltage of power supply of line circuit, causing a second power supply output voltage rise, thus making row tube .is damaged. <br> <br> Note: the color display of line pipe is commonly used without damping of line pipe, we substitute color display line tube must not substitute color TV in common with impedance tube line pipe, this will lead to insufficient .motivation, making row row tube heat intensifies, resulting in damage. Besides color line pipe of frequency characteristics nor monitor's line pipe..

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