MONITOR REPAIR COURSE

Lesson 1



Introduction


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






Different from the TV sets, it is very common to happen internal failures in screen potentiometers in monitors FBT, what causes brightness to vary from a too bright screen to a very dark screen. There are usually three wires on the fly-back. 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 multimeters will not read the real screen voltage, but something around 250 to 300V Focus The medium wire is the focus control voltage. Its voltage varies between 4500V to 7500V. The correct focus will be adjusted by the upper potentiometer of fly-back.





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





Before buying the probe you must consult the salesperson to know the appropriate tip for your multimeter. The probes for digital multimeters are different from probes for analog multimeters. Differences also exist among the analog multimeters because the appropriate probe will depend on the impedance of the multimeter. So, writes down all the data of you 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 the same format of the switching transistor of power supply. So, technicians 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 transistor output (H.O.T.) is close to the fly-back, while the switching transistor is close of the transformer (chopper). To make sure, you should look at the lower side of the printed circuit board.


The H.O.T. has it collector (middle terminal) linked to a pin of the fly-back, 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 usually smaller than a common transistor and usually begins with the letter K. Bipolar transistors have 2SC and 2SD prefix . Some FETs have the manufacturer's unique codes. In these cases they don't begin with letter K.

Vertical output IC

Almost all the monitors use integrated circuits in their vertical outputs. Some of them still use transistors.

The picures below show some formats of vertical ICs





Most monitors use those two formats, so beginners will easily identify them. Later you will learn about other formats of vertical output ICs In case of failures in the area of the vertical, you will se the dark screen, just with a white 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.


Lesson 2

Introduction to monitor power supplies

In the beginning, when the 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 that in that 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 series type source 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 voltages to appear in the secondary terminals.
The transistor of the source acts as a switch that opens and closes quickly in high frequency, and that determines the voltages that appears 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 outputs at the chopper (transformer). As it is dificcult to get the monitor schematics we need, it's this good to have an idea of the value of the sources of the monitors. When we come across with monitors with a inoperative power supply,we need to know it's values. All the monitors have a main source, that feeds the circuit of horizontal output. This power supply varies 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 100mf X 160v. The value of the source will be from 100 to 110v. But if we have an electrolytic cap of the 100mf X 100v, 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 also 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 source is of 6,3V and it's 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 failures in this block, because we have the presence of the diode and of the electrolytic cap and it is very common this cap to become dry and the to cause reduction in the heater voltage and consequently, loss of brightness. In TVs, most of the heaters of the tubes is fed directly with non-rectified voltage which comes from flyback.

Booster capacitors

Some 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 capacitor

Most of monitors use only one large capacitor on the primary side of the power supply. The nominal voltage of this capacitor is 400V In countries with 120V AC outlet as USA, there will be only 150V to 170V on the terminals of the capacitor. However if 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.



Lesson 3


Let'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 supplies. In this class we will study a power supply that uses only transistors in the. 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 the AC outlet without transformer, then it is rectified by a diode bridge and filtered by a unique capacitor. Then we will have 150VDC when plugged to 110VAC outlet or 300v plugged to a 220AC outlet. 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, and 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 resistors and could be just a resistor from 330K to 470k or two resistors of 180k, and varies from monitor to monitor. In the illustration above we can see R1 and R2 (180k resistors) . The 150v source decreases trough 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 a d terminal linked to the 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 transistor 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 closed switch.. Then a 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 ends up reinforcing the polarization in the base of Q1. Of course the power supply is not only that, there is also the stabilization circuit that we will study later. But the start-up resistor, which many technicians don't know, it is reason of many failures in power supplies. We will analyze some failures in that start-up circuit. 

IMPORTANT: WHENEVER YOU CHECK VOLTAGE ON THE PRIMARY OF THE POWER
YOU MUST PUT THE BLACK MULTIMETER PROBE ON 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 MEASUREMENT 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 0,3 V. If there is not voltage, the failure is in the start-up circuit. It is not a 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, transistors, etc. It could be broken traces or fail in soldering usually caused by excess of heat. Therefore it is important to find failures by measuring voltages over the PCB. Whenever there are two start-up resistors, measure voltage on the junction of the two resistors. There must be half of the voltage of the main capacitor . If there's more then half of main capacitor voltage, the lower resistor is open or altered. If there's less than half, the upper resistor is altered. I´ve checked voltage on the base 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 0.6 V. Could I say that the base-emitter junction of the transistor is open?  Yes, but before to replace it, I checked voltage on its emitter. Measuring the voltage on the emitter I found 2.5 V. But how can it be, if the emitter is linked to GND (negative of the power supply)?  In that case the failure was bad soldering between the emitter of the Q1 and the GND.



Lesson-4

We already talked about some analysis of failures on the initial start-up of power supply. Opportunely, we will talk about it's 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 fly-back 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's a short on the primary of the power supply Yes, on the primary - Short on the secondary side of power supply rarely will cause the fuse to blow. When a there is a short on the secondary, the power supply simply stops working. Whenever we find a 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 you already have a series bulb lamp 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 prevents 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 lamp also facilitates the location of the short because many components when measured out of PCB with the multimeter, sometimes seem to be OK, but when put in the PCB and submitted to a voltage source, present the short.
Failure: blown fuse Plug the monitor through a 160w series lamp. If lamp lights at maximum, a short exists.

See how to plug the lamps




Then suck up the solder of the collector of the switching transistor (terminal of the middle). If the lamp bright 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 the short can be on the diode bridge. There are hard-to-find shorts as small ceramic capacitors in parallel with the diodes bridge or linked from the main power source to the ground. But using the series lamp is easy to locate it because as soon as the capacitor is lifted from PCB, the short disappears and the lamp fades or lights with less brightness .

Depolarizing the transistor

To simplify what means to depolarize, in this case it means to turn off.The transistor is polarized by the current that circulates base-emitter. Then if we want to depolarize the transistor, we must apply a jumper between base and 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 o the transistor from the PCB





The jumper example is also worth for FETs.
If the lamp continues 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).



Lesson 5


We already talked about the start-up of the switching power supply. Now we will make an introduction in the stabilization of the switching power supply.We will begin slowly and in blocks because this is the most complicated part of the power supply. We will make one generic analysis for later in the other lessons, to do an operation analysis and location of failures in specific models of monitors. The main part in the stabilization of a switching power supply is the optical coupler. It is easy to recognize the optical coupler because they have 4 or 6 terminals.
See the two models in the illustration below:





On a side there's a light emitter diode and on the other there's a receiving phototransistor Both are sealed inside of the same case and completely isolated electrically, only communicating by light emission.






If you want, you can do this experience to see the optical coupler behavior. Perform this test on a test PCB circuit and not in the monitor, because this test type without care could cause damages to the monitor. On pin 1 we will apply 12V through a 100K trimpot so that we can control the current that will be applied on the emitter diode of light. The more current on the diode, the more will be light intensity emitted for the internal transistor.The more light in the internal transistor, more it drives and decreases collector-emiter resistance. If we connect a multimeter on pin 4 of the IC and increase the current by rotating the trimpot, we will notice that the voltage on pin 4 will fall. However if we rotate the trimpot the other way, reducing the current on the light emitter diode, the voltage on pin 4 will arise. If you measure the voltage on pin 1 of the IC you will notice that there won't be great variation on it because it's impossible to exist more than 0.6 or 0,7v between two terminals of a diode . However when you rotate the trimpot in order to reduce its resistance, the internal current on the diode increases a lot.
On pin 4 we could have variations from 0 to 12V. If I apply a jumper using a screwdriver between the pins 1 and 2, I will eliminate the action of the diode and then the voltage in the pin 4 will arise to 12V. Because the internal light beam stops. You can also put a LED between the pin 4 and R1 instead of measuring with it the multimeter, you can visualize the current variations in the leds . In this case you should change R1 for a resistor of 1K In the next lessons we will study how the optical coupler works in a monitor power supply.



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