MONITOR REPAIR LESSON

Troubleshooting a dead monitor

Checking the primary side

If the monitor is dead the first thing to do  is to verify if there is voltage on the drain of the switching FET of the main power supply
In case there is not voltage, you have to seeking for some open thing like blown fuse, resistors or broken traces.
The normal voltage on its drain is about 150V when the monitor is plugged to a 120V AC outlet (USA users) or 300V when the monitor is hooked to a 220V AC outlet (UK users)




But if you check it and find voltage on its drain, it means that the rectification is good and the power supply unit is blocked for some reason

In this case, you must verify if there is start-up voltage on the main power supply
If the unit uses the IC 3842 or 3882 as shown below, the start-up voltage must be about 12V



For other ICs check its datasheet to find out which is its start-up pin

If the start-up voltage is not present, you must seek for blown start-up resistors
The start-up resistors are high resistance resistors such as 220K or 470K resistors

In many cases you find the start-up voltage, but the power supply does not work

In this case the problem can be:

1 - Short on the secondary side

2 - Short on the optical coupler or on some part hooked to it

3 - Short on some part in the primary side itself

Step for analysis of short on the secondary side

Remove or suck up the collector of the HOT.

If the power supply works, then it means that the power supply has no problem
In this case the problem is on the horizontal output: Fly-back, HOT or yoke.

In some monitors if you remove the HOT and do not apply a load resistor on its place, some capacitors will explode because the power supply may increase beyond its normal value. In other lessons you will see how to apply load resistors

If you donīt want apply a load resistor you must turn the monitor on only for a brief moment
If the main power supply won't work yet, maybe there's a short in any of the secondary lines

You can check that with the multimeter in the scale of ohms (100R)
For checking shorts, unplug the monitor from the AC outlet and check the resistance of each one of the sources of the secondary.

First put the red probe of the multimeter on the source and the black tip on the ground, then  invert and put the black tip in the source

Check all of the sources of the secondary.
When you put the red tip in the cathode of a diode, it is normal to find low resistance.
However when inverting the tip you should find a higher resistance.

If you find the same resistance on both sides, there's a short on that source.

In the illustration above when the black tip is on the cathode and red tip is on the ground, the resistance should be higher.

Important: To measure the resistance of the source of the heater you must remove CRT PCB, because the CRT heater resistance is very low and it will show  a false short. The heater is the lowest secondary power supply line

Never measure on the anode of the diode , measure on the cathode because the anode is hooked the a coil of the chopper which has low resistance and this coil has a terminal hooked to the GND. Therefore there will always be low resistance on the anodes that are hooked to the chopper.

If the short is in a medium power supply (between 20 and 35v), probably the cause can be a short in the vertical output.
In this case, if you don't know which pin is +B of the vertical IC, to remove the IC from the PCB and the short will disappear.
Replace the IC and the problem will be solved

If the short is on a higher source as 53 to 150v, probably this is the source of the horizontal output.
Then you must remove the HOT or to suck up the pin of the fly-back +B.

If the short is on the highest source (usually 150v) then it is probable that the short is in the RGB output.
There are monitors which use three RGB output transistors, others uses  a power RGB IC.

A shorted  RGB IC can also cause the main power supply to be dead because it's hooked directly to the power supply

If you found no problem on the secondary side, then you must check the primary side

In monitors that use 3842 IC on its power supply, check capacitors hooked to its pins # 3 and #4
Also replace small electrolytic capacitors as shown below:

It's recommended to replace the capacitors because many of them can pass the multimeter test and still be bad
At last, if the problem is not found, the optical coupler must be replaced

Below you can see two types of optical couplers





In lesson 5 we show how you can  test an optical coupler and how does it work
You can choose between testing or replacing it without testing

That's all for a while

Good luck and hope to see you in the next class

---

This lesson is part of monitor repair course