get started monitor repair

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Acrisoft- Monitor Reapair Course Lesson 01 General Introduction Lesson 02 Power Supply Introduction Lesson 03 Start up Lesson 04 Series Lamp Lesson 05 Optical Coupler Lesson 06 Regulator Lesson 07 Power Supply won’t work Lesson 08 CRT Lesson 09 Horizontal Circuit Lesson 10 The Horizontal output Lesson 11 Vertical Lesson 12 Vertical 2 Lesson 13 Brightness Control Lesson 14 Blanking Lesson 15 Degaussing Lesson 16 Removing spots Lesson 17 Horizontal Oscillator Lesson 18 Horizontal Sync Lesson 19 PWM based power supply Lesson 20 Power Supply Regulation Lesson 21 Secondary Power Supply Paths Lesson 22 The Suspend Circuit Lesson 23 Micro-controller Lesson 24 The RGB with power IC Lesson 25 H-size and ABL Lesson 26 LG / Goldstar vertical output Lesson 27 LM 1203 – Video Lesson 28 Solving Pincushion Problem Lesson 29 Preventing the HOT from going Shorted Lesson 30 The secondary FET getting hot and shorted MONITOR REPAIR COURSE

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Acrisoft- Monitor Reapair Course

Lesson 01 General IntroductionLesson 02 Power Supply IntroductionLesson 03 Start upLesson 04 Series LampLesson 05 Optical CouplerLesson 06 RegulatorLesson07 Power Supply won’t workLesson 08 CRTLesson 09 Horizontal CircuitLesson 10 The Horizontal outputLesson 11 VerticalLesson 12 Vertical 2Lesson 13 Brightness ControlLesson 14 BlankingLesson 15 DegaussingLesson 16 Removing spotsLesson 17 Horizontal OscillatorLesson 18 Horizontal SyncLesson 19 PWM based power supplyLesson 20 Power Supply RegulationLesson 21 Secondary Power Supply PathsLesson 22 The Suspend CircuitLesson 23 Micro-controllerLesson 24 The RGB with power ICLesson 25 H-size and ABLLesson 26 LG / Goldstar vertical outputLesson 27 LM 1203 – VideoLesson 28 Solving Pincushion ProblemLesson 29 Preventing the HOT from going ShortedLesson 30 The secondary FET getting hot and shorted

MONITOR REPAIR COURSELesson 01

The FBT of the monitor has two potentiometers; the upper potentiometer adjust the FOCUS and the lower potentiometer adjust the SCREEN control. The screen control is a DC voltage which feeds the grid 2 of the CRT on the monitors and also the TV sets. The higher the screen voltage, the higher the brightness on the screen.

Different from TV set, it is very common to happen internal failures in screen potentiometers in monitor FBT, what causes too bright screen to a very dark screen.There are usually three wires on the flyback. The thicker one is HV (high voltage) which is linked to CRT anode, the medium is the focus, and the thinner one is the screen voltage control. Screen voltage is about 400 to 600VDC.

Many multimeter will not read the real screen voltage, but something about 250 to 300 V.FOCUS

The medium wire is the focus control voltage. Its voltage varies between 4500 to 7500V. The correct focus will be adjusted by the upper potentiometer of the flyback.

In order to check focus voltage and HV it is necessary a special probe, because most of the multimeter measure only up to 1200VDC. You can use an appropriate probe for that, with meter coupled to it, or if you prefer a cheaper solution you can buy a HV adaptor multimeter.

Before buying the probe you must consult the salesperson to know the appropriate tip for your multimeter. The probe for digital multimeter are different from probes for analog multimeter. Differences also exist among the analog multimeters because the appropriate probe will depend on the impedance of the multimeter. So, write down all the data of your multimeter before acquiring the probe.

About HV (high voltage)

The picture below shows how to check HV voltage which is around 24KV to 25KV.

Horizontal output transistor (HOT)

In many monitors the horizontal output transistor has same format of the switching transistor of power supply. So, technician not used to monitor repairing may confuse those transistors.

If you look carefully, you will easily identify them even without the monitor schematic. The Horizontal Output Transistor H.O.T. is close to the flyback,while the switching transistor is close transformer (chopper). To make sure, you should look at the lower side of the printed circuit board.

The HOT has its collector (middle terminal) linked to a pin of the flyback, while the collector of the switching transistor is linked to the chopper. Modern monitors use transistor FET in their power supplies, the oldest use bipolar transistors. At the upper

picture you can see the transistor FET, that is usuallysmaller than the common transistor and usually begins with the letter K.Bipolar transistors have 2SC and 2SD prefix. Some FET have the manufacturer’s unique codes. In this cases they don’t begin with letter K.

The picture below will show you some format of vertical ICs.

Most of the Monitor use these two format, so beginners will easily identify them.Later you’ll identify other format of vertical ICs.In case of failure in the area of vertical you will see dark screen, just a horizontal line in the screen. We will also be able to have the screen closed just partially showing a black strip in the upper side of the screen or in the lower part.Monitor Repair CourseLesson 2Introduction to Monitor Power Supplies

In the beginning, when the first TV sets were made, large transformers were used in the power supply in order to lower or to increase voltage. There was not any regulator transistors, and the circuits were fed directly. There was not any regulation.Then the regulated power supply appeared where there always was a regulator transistor and a resistor in parallel, through where passed part of the current.The power supply was already stabilized, the voltage in the output of the source didn’t vary when there were certain variations in the input voltage.But even so it was still necessary a transformer when it was necessary to plug the device in a 220V outlet.Then TV sets appeared with switching power supplies in series. In that case there was a circuit composed by two large capacitors, with 300VDC over their extreme terminals and after passing through the transistor regulator there would appear 110VDC to supply the horizontal circuit of TV set.Notice in that the case it was not necessary a transformer, because there was always 300V entering the switching transistor’s collector. Then another type of switching power supply appeared: the parallel type. In this power supply type, the current doesn’t enter in the collector of the transistor and left by the emitter as in the secondary terminals.In the case of the parallel power supply type, a current circulates through the primary of the transformer that induces a current in the secondary and that makes the voltage appear in the second terminals.The transistor of the source acts as a switch that opens and close quickly in high frequency, and that determines the voltages that appear in the secondary of the transformer. Also notice in the illustration below, that the primary of the source is totally isolated of the secondary.This is the kind of power supplies used in modern monitors.

As you can see, there are several output at the chopper (transformer).As it is difficult to get the monitor schematic diagram we need, it’s this good to have an idea of the value of the sources of the monitors.

When we come across with the monitors with a inoperative power supply we need to know its values. All the monitors have a main source, that feeds the circuits of the horizontal output.This power supplies vary from monitor to monitor. In some monitors the horizontal is fed with 150V while in other with 50V.So, in order to know whether a source is high or not, we should observe the electrolytic capacitor that filters that source. For instance, if we have a cap of 100uf x 160V. the value of the source will be from 100 to 110V. But if we have an electrolytic cap with the value of 100uf x 100, then the value of the source will be from 60 to 75V.We also have other source which is the source that feeds the video output. That source is around 150V and it is easy to identify it because it uses a cap with superior voltage at 180V.There’s also the source that feeds the vertical that comes from CHOPPER, this source is usually around 20V and electrolytic cap is around 35V. We have the source of the heater of the tube that in the monitors, unlike TVs, are continuous current, that the source is of 6.3V and its capacitor is of 10V or 16V.It is important to say that for the fact of the heater to be DC (continuous current) it increases the chances of failure in this block, because we have the presence of the diode and of the electrolytic capacitor and it is very common this cap to become dry and to cause reduction in the heater voltage and consequently, loss of brightness.

In TVs, mot of the heaters of the tubes is fed directly with non-rectified voltage which comes from flyback.

BOOSTER CAOPACITORSSome monitor sources work with booster capacitors. Specially on 220V countries like UK. Those monitor, have always two large capacitors of equal value in the primary of the source. There is a manual 110-220 switch that we should turn off manually when the monitor is plugged to 220V outlet. Those monitor will always have 300VDC on the primary side of the power supply

SINGLE CAPACITORMost of monitors use only one large capacitor on the primary side of the power supply. The nominal voltage of the capacitor is 400V.

In the countries with 120VAC outlet as USA, there will only 150V to 170V on the terminals of the capacitor.However if the plugged to 220VAC, there will be 300V to 340V on the terminals of the capacitor.

That’s the reason why its nominal voltage is 400V.

Note: regardless of the voltage on the primary side, there will always be a fixed voltage on the secondary side.

MONITOR REPAIR COURSE

Lesson 3Let’s make a generic analysis of the operation and maintenance of monitors power supply and in the next lessons we will study other types of power suppliesIn this class we will study a power supply that uses only transistor in that.

It doesn’t use PWM based IC.We already spoke in the previous lesson, that we always needed a primary power supply to feed the switching power supply. This power supply comes directly from AC outlet without transformer, and then it is rectified by a diode bridge and filtered by unique capacitor.Then we will have 150VDC when plugged to 110VAC outlet or 300V plugged to 220VACoutlet. Most of the devices which have only one electrolytic capacitor in the power supply, works automatically in 120 or 220V.

If the capacitor nominal voltage is 200V, it can be only plugged to a 120VAC outlet. However if the capacitor is 400V, you can plug the monitor to 110VAC or 220VAC outlet. For the study of this lesson we will suppose that the monitor is plugged to a 120VAC outlet, then we have 150V over the terminals of the main electrolytic capacitor of the primary power supply.

All of the switching power supplies have a start-up resistor. These are high value resistor from 330k to 470k or two resistor of 180k, and varies from monitor to monitor. In the illustration above we can see R1 and R2 (180k).The 150V source decreases through the resistors and will polarize the base of the Q1 with 0.6V.For us to recognize start-up resistors in monitors without schematics, we must observe that they always have the terminal linked to main power supply (150V or 300V) and the other terminal to the base of the switching transistor or feeding some IC on the power supply. They are always of high value ( third color orange or red).The Q1 acts as a switch, that at first is open and it doesn’t drive. As its base is polarized with 0.6V, the transistor becomes a close switch. Then the current is induced in the coil of the illustration between the pins 9 and 11.As the current flows, a voltage appears on pin 14 of the chopper and goes by D1 and R3 and it end up reinforcing the polarization in the base of Q1. Of course the power

supply is not only that, there is also stabilization circuit that we will study later. But the start-up resistor, which many technicians don’t know, it is the reason of many failures in the power supplies.We will analyze some failures in the start up circuit.IMPORTANT WHENEVER YOU CHECK VOLTAGE ON THE PRIMARY SIDE OF THE POWER YOU MUST PUT THE BLACK MULTIMETER PROBE ON THE NEGATIVE PIN OF C1. NEVER PUT THE BLACK PROBE ON THE SECONDARY OF THE POWER SUPPLY BECAUSE IT IS ISOLATED, AND IT WILL RESULT IN VOLTAGE MESUREMENT ERROR.See solution of a problem below: there’s 150V on the collector of Q1, however the power supply won’t start always in these cases we should check voltage on the base of the transistor Q1 which must be at least 0.3V.If the is no voltage, the failure is in the start up circuit. It is not very correct procedure to remove parts from PCB to check them we can analyze them on the PCB by the voltages found. The failures are not always damaged components, as ICs, transistor, etc. It could be broken traces of fail in soldering usually cause by excess of the heat.

Therefore it is important to find failures by measuring voltages over the PCB.

Whenever there are two start-up resistor, measure voltage on the junction of the two resistors. There must be half of the voltage of the main capacitor. If there is more than half of main capacitor voltage, the lower resistor is open or altered. If there is less than half, the upper resistor is altered.I’ve check the voltage of Q1 and found 3V, as the junction base-emitter of the transistor Q1 is similar to a diode, it could not have more than 0.6V Could I say that base-emitter junction of the transistor is open?Yes, but before to replace it, I checked the voltage on its emitter. Measuring the voltage on the emitter I found 2.5V but how can it be, if the emitter is linked the GND (negative of power supply)? In that case the failure was bad soldering between the emitter of the Q1 and the GND.

MONITOR REPAIR COURSELESSON 4

We already talked about some analysis of failures on initial start-up of power supply. Opportunely, we will talk its stabilization. Now we will analyze some failures that happen in the primary of a monitor power supply.Primary it is the side of the “chopper” (transformer) that is linked to the switching transistor.

Secondary it is the side of the outputs of the power lines that feeds the flyback and all the circuits of the monitor, as vertical, horizontal, RGB output etc.

It is very common to find monitors with the primary fuse blown, and the first thing that we do is to replace it. Most of the time, the fuse will blow again.That is because there is a short on the primary of the power supply

Yes on the primary- short on the secondary of the power supply rarely will cause the fuse to blow.

When there is a short on the secondary, the power supply simply stops working.Whenever we find blown fuse on the primary side, we must test it by plugging the monitor to the AC out-let through a series bulb lamp device.If already have a series lamp bulb device, then you already know their advantages. If you still don’t have a series lamp, you can build a simple assembly as below.

Note: the lamp combination must be 2.5 times the power of the monitor. The series lamp will prevent the fuse to blow again in case of short.

If there is a short, the lamp will light at the maximum intensity. You will then be able to seek the short without problems.

The series lamps will also facilitates the location of the short because many components when measured out of the PCB with the multimeter, sometimes seem to be OK, but when put in the PCB and submitted to the voltage source, present the short.Failure: Blown fusePlug the monitor through a 160W series lamp

If lamps light at the maximum, a short exist.See how to plug a lamps

Then suck up the solder of the collector of the switching transistor (terminal of the middle). If the lamp fades, then the short can be on the transistor or still in the polarization circuit that is linked to its base.

To make sure depolarize the transistor. (See in the end of this lesson the topic on how to depolarize the transistor). If when turning off the collector of the transistor, the lamp does not fade, then short can be on the bridge diode. There are hard to find short as a ceramic capacitor in parallel with the diodes bridge or linked from the main power source to the ground.But using the series lamp it is easy to locate it because as soon as the capacitor is lifted from the PCB, the short disappears and the lamps fades or lights with less brightness.

Depolarizing the transistorTo simplify what means to depolarize, in this case it means to turn off.The transistor is polarized by the current that circulates base-emitter.

In this case the lamp is lighting strong and indicating a short or excessive consumption. Apply a jumper according to the illustration below. Do not remove any terminal on the transistor from the PCB

The jumper example also works for FETs. If the lamps continue the same, then the failure is in the transistor. If the lamp turns off then the failure is on the stabilization circuit (components linked to the base of Q1)