Free Report

By Humphrey Kimathi

Disclaimer The reader of this book is expressly warned to consider and adopt all safety

precaution that might be indicated by the activities herein and to avoid all potential

hazards.

The author particularly disclaims any liability, loss or risk taken by individuals

who directly or indirectly act on the information Contained herein. The author

believes that the information presented here is sound, but readers cannot hold him

responsible for either the actions they take or the result of those actions.

Email: humphrey@humphreykimathi.com

Website: http://www.humphreykimathi.com

Solving power supply problems (s.m.p.s)

using control i.c.

Most problems with equipment using switch mode power supply (S.M.P.S)

like television and monitors are power related.

This is because powers supplies run under considerable stress due to high

voltages and currents involved around these circuits.

In order to understand how to solve S.M.P.S problem I am going to show you

how to analyze common failure mode in CRT television, you can use this

principle in repairing other equipment using switch mode power supply like

LCD Television and Monitors, DVDs etc.

1. Television is dead

Here dead I mean the set does not show any sign of life, power ON LED is

not lighted and nothing happens even after power is switched on.

First confirm the television cable is not broken inside, you can check this by

measuring the voltage across the AC power input connector on the circuit

board.

If the voltage is reaching the circuit board, then look for the fuse and see if it

is blown, if the fuse is okay check also the surge limiter is open or not.

If these two components are ok then most likely there is no short on the

primary side of the power supply note: Short in the primary side of the power

supply usually cause the surge limiter or the fuse to blow.

From now on we shall be using the main capacitor negative as the Ground for

all the measurement of Voltages on the primary side of the power supply.

I have modified my meter ground probe with a crocodile clip so that I can

attach it to ground point of the equipment I am repairing and therefore I use

only one hand to probe the circuit.

This is a safety precaution because as you know for there to have a potential

difference there must be a connection between the live and the ground.

So if you are using your two hands and incidentally touch the live and the

ground then current will flow through your body and this can be fatal. But if

you are using one hand it is not possible.

One thing I have leant in electronics is that if you master doing your

troubleshooting using voltage tests you will have an added advantage because

in electronics it is not always the component that fail but it could be just a

crack on the circuit board that is preventing the voltage to flow or even dry

joints and therefore if you concentrate on looking for the bad component you

are going to waste a lot of time and end up not even repairing the set.

Therefore I encourage you to narrow down your faulty circuit (area) by use of

voltage testing only. Fast and valuable method I have come to learn.

Now set your meter to DC range of 1000 VDC and use the main Capacitor as

your ground:

First check the voltage across this main capacitor and if you get around 300

vdc on 220 vac outlet or 150 vdc on 110 vac outlet then it is assumed the

supply is okay up to the main capacitor.

If there is no Voltage at the main capacitor then check backward looking for

open component or circuit board cracked or a dry joint on the main power

path.

Be sure this main DC (300 vdc or 150 vdc) is the reaching the power

regulator ic Collector/Drain pin

Now still using the same ground move your positive probe and measure the

vcc in pin of the power regulator ic.

Depending on the regulator used you can check the correct vcc from the data

sheet for the ic used from the internet.

These two voltages are very important and if any is missing the supply will

appear dead, if the vcc in is missing this means there is an open circuit or

start up resistor is open.

Follow backward and find what is preventing the start up voltage reach the

vcc pin.

Another hint if you see this vcc is less than Normal and pulsating there is a

possibility that there is Short on that pin, to confirm just lift up that pin and if

the voltage increases then it means there is a short on the pin and the ic must

be replaced.

There is also a filter capacitor on that pin usually 100uF/35V please check the

esr of this capacitor or directly replace it if you don’t have an esr meter.

There is also a resistor linked to the source pin of this ic called the current

sense resistor which sometimes open or change value and this can also cause

the power supply not to start. Usually it very low ohm less than 2 ohm and

around 2 watt so it a big one.

If up to here you have checked and they are okay then we should suspect a

short on the secondary side of the smps.

First measure for short of any of the secondary diode using analogue meter

set to X1, when you measure across any diode on the secondary you should

not have two reading and if you get two reading just lift up one leg and

measure again. If you still get two reading then you can conclude that diode

is short.

Also when using this method and find that the diode has two reading in

circuit and when lifted up you get one reading, it could also mean there is a

short on that source (line)

You can follow that source looking for the shorted component; usually the

most common components to short are transistors, ic, capacitors, and diodes.

Remember when doing voltage testing on the secondary sources (circuit) you

should be using the COLD GROUND of the set.

The best ground to use is the turner ground (outer part) of the CRT ground

return.

Also note that when measuring the voltage at output of the chopper

transformer at the secondary diodes use the diode cathode to measure the dc

voltage and when measuring at the anode of the same diode use the ac range

in your meter.

If you find still there is no voltage at the secondary output or the voltage is

low then it the time to substitute for the Main B+ load with a dummy load.

You can make a dummy load by using a 60watts bulb/250v ac, a bulb holder,

crocodile clip and two pieces of wire (red and Black) and connect them as

shown on the picture below.

Note you can also this bulb to discharge the main Capacitor, just clip the bulb

across the positive and negative leg of the main capacitor and you will see the

bulb flash briefly if the capacitor was holding Charge.

Now identify the main supply to the fly back called the B+ and disconnect

the supply just before the fly back B+ vcc pin.

Don’t cut the circuit board to do this just lift up one side of a resistor( high

wattage) in series with this B+ voltage or sometime they use a jumper wire

just before the B+ vcc pin of the fly back and attach your dummy load

between this point and the cold ground.

If the bulb light steadily and you measure between 90 Volts dc to 140 dc

depending on the size of the television then you can assume that the smps is

okay and the problem is on the secondary side.

Hint: A short on the secondary side can cause the power not to start

irrespective how far the short is.

If you find that when you use the dummy load the power is starting but when

you remove the dummy load the power is blocked then suspect short on the

components after the take off point which include the following for this

television using IC for RGB amplifiers.

when you find the power supply is not starting and you use the dummy load

and find that the power is okay but if you connect the B+ supply again you

find the power supply is blocked then suspect component in parallel with the

B+ (parallel means one leg is on the B+ supply and the other leg is on the

ground)

Any component which is shorting the B+ to the ground will make the power

supply to appear dead From the circuit above the following components can

cause the B+ to go to the ground directly and hence can block the Power

supply from starting.

1. Snubber capacitor C403

2. Horizontal output transistor (HOT)

3. C444

4. Fly back Transformer

5. IC901 (TDA6107Q) on the CRT neck board

6. C901 and C902 RGB source filter Capacitor

If any of the components marked on the above picture is shorted to the

ground it will cause the power supply to appear dead.

By Humphrey Kimathi(Author)

Basic electronics course by Humphrey Kimathi

CRT Television Repair Course by Humphrey Kimathi

DVD Player repair guide by Humphrey Kimathi

CRT TV Real Life Repair Experience Guide Vol.1

Recommended books

LCD Monitor Repair by Jestine Yong

Troubleshooting and Repairing S.M.P.S. by Jestine Yong

LCD Monitor repair real case histories Vol. 2 by Jestine Yong

LCD Television Repair tips by Kent Liew

V3.0- Collection of LCD & LED Television Repair Tips by Kent Liew

LCD TV Repair by Kent Liew