TV repair course sample lesson

Starting TV repair career

Warning: This course if intended for people with electronics knowledge
Electronic devices may have potential shock danger

Preparing for the course

Supposing you are starting the TV repair career
You may want to fix TV sets of your relatives or friends without commitment or your own TV set

Necessary tools
In the future you will need other specific tools but as a beginner you will need only some cheap basic tools as:

Digital multimeter
Analog multimeter
Soldering iron
Desoldering pump
Cut pliers
Chain Nose Pliers
Screw drivers

Necessary spares
Dot not order many parts now because many of them will never be used
Most TV problems require cheap replacement parts as resistors and capacitors
But the hardest task is to spot them and this is the purpose of this course

We recommend the spares below:

Resistors: several types of 1/4w
Capacitors: several types
Solder roll


Other tools

An oscilloscope is a very useful tool for TV and other devices repair but you can acquire it later when you already have many customers
I myself worked several years without an oscilloscope in my career beginning

Advice to the beginners

Avoid making commitments with customers
In the beginning of my career because of my lack of experience I made commitments with my customers and had problems

Please see my tips below

1- Avoid very old equipments.
Even if you fix them, another failure will appear very soon

2- Avoid equipments with very insignificant failures
Suppose the only failure is a small spot or some snow over the screen
After you open the TV set, another failure may appear
Suppose the TV becomes dead. This can be a real problem

3- Never promise to fix it.
 Say you will analyze it and will fix it if the part is available for sale

4- Avoid repairing the TV set in your costumer house
You need a quiet environment to find out the failure and your bench is the right place for it

Dangers of the electronics repair

TV sets have internal high voltage as high as 24KV or more
Though this voltage is too high, it's has a very low current capacity

So there is no danger of electrocution
The danger is the shock itself
There are devices more dangerous than a TV set
A microwave oven is more dangerous because it has high voltage plus high current inside

Summary of shock danger
Stay away from large and heavy transformers and avoid being barefoot or wet when servicing any electronic device

In case of doubt, keep the device unplugged form the AC outlet

DEAD TV SET - Part 1

A dead TV set may have at least three causes

1- Failure on the primary side (easy to solve)
2- There is voltage on primary side, but not in the secondary side

Let's now analyze a dead TV set

The first thing we must do is measuring voltage between both pins of the main power supply capacitor which is shown below




In USA where the outlet voltage is about 120V AC, there must be about 140 to 170VDC over the capacitor
In countries like England where the voltage is 220V, there must be about 280 to 340V over this capacitor

If there is no voltage over this capacitor or the voltage is too low, there's no need to check the switching power supply

The problem may be:

1- Blown fuse
2- Blown low value resistor somewhere between the AC plug and the capacitor
3- The main switch may be bad
4- There must be some broken trace on the PCB

Notice that some TV sets have a small power supply apart from the main power supply
It is called stand-by power supply
In these cases there is a relay which turns on the main power supply


DEAD TV SET - Part 2

Dead TV set, no voltage on secondary side

This is the most common kind of failure found on TV sets
There is voltage on the main capacitor showed above, but no voltage on secondary side

This maybe caused by power supply failure or some short on the secondary side
You are more likely to find the failure quickly if you start by the secondary side

See below the most common failures:

In first place, the fly-back, also called FBT

If the problem is the fly-back, the power supply will increase if you remove the +B pin of the fly-back from the PCB

In second place, the horizontal transistor output

This transistor is also called HOT

To check the HOT proceed the tests as below

Remove the TV set from TV AC outlet
Set the analog multimeter to the 100 ohms scale, place the red probe on the collector and the black probe on the emitter as below

If you find low resistance, there is no problem in this case

Now place to probes the other way around as shown below
In this case the resistance must be higher than in the first case

If the resistance in both cases is the same, the HOT is probably shorted

Remove the HOT and check it again outside the PCB
If the resistance remains the same, the HOT must be replaced

If you replace the HOT you must replace also the snubber capacitor
It's the 1600V capacitor which is linked to the collector of the HOT
If you don't replace it, there is a risk of the HOT to become shorted again

Third cause, the secondary power supply diodes
They are fast recover diodes linked to the secondary side of the power supply
If one of these diodes become shorted, the power supply may be blocked

Fourth cause, shorted yoke
Shorted horizontal yoke may also act as a shorted fly-back



Suppose the power supply has zero volt
If you remove the yoke plug and the power supply increases to 100V, so the yoke is shorted
Note: Only the horizontal yoke can block the power supply

Another precaution

Fly-back and capacitors may explode
It's more common with capacitors then fly-backs

Don't be scared
Fly-backs only explore if the TV set is ON and if protection circuit has been removed,
and it's very rare to happen

As a matter of precaution keep the PCB always from your face when the device is plugged to the AC outlet

That's all for a while

Good luck and hope to see you in the next class