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

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

Most problems with equipment using switch mode power supply (s.m.p.s) like television and monitor are power related, this is because power supplies runs under considerable stress due to high voltages and current 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 CRT monitor too.

Case One: CRT Television 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 power 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 okay 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 measurements of the voltages on the primary side of the power supply.

This is 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 the 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 learn 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 repairing the set.

Therefore I encourage you to narrow down to the faulty circuit (area) by using voltage testing only. This one is very fast and valuable method I have come to learn.

Now set your meter to DC range of 1000VDC and use the main capacitor as your ground.

First check the voltage across this main capacitor and if you get around 300 vdc on 220vac 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 dry joint on the power path.

Be sure this main DC(300vdc or 150vdc) is reaching the power regulator ic collector/drain pin, Now still using the same ground move the positive probe and measure the vcc-IN pin of the power regulator ic.

Depending on the regulator used you can check the correct vcc-IN Supply (start up Voltage) 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 a open circuit or start up resistor is open.

Follow backward and find what is preventing the start up voltage from reaching the vcc-IN pin.

Another hint: if you see this vcc is less than normal and pulsating there is a possibility that there is a short on that pin, to confirm just lift up that pin and if the voltage increases then it means there is a short on that pin internally and the ic must be replaced.

There is also a filter capacitor for this source 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 cause the power supply not to start. Usually it is a very low ohm resistor, less than 2 ohms and around 2 watts so it is a big one.

If up to here you have checked and everything is okay, then you should suspect a short on the secondary side of the s.m.p.s.

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 lift up one leg and test again. If you still get two reading then you can conclude that diode is shorted.

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, ics, diodes, capacitors.

Remember when doing voltage testing on the secondary circuit you should be using the COLD GROUND of the set.

The best cold ground to use is the turner ground (out part) or the CRT ground return.

If you find still there is no voltage at the secondary output or the voltage is low, it is the time to substitute for the main B+ Load with a dummy load.

You can make a dummy load by using a 40 watts bulb/250 vac, a bulb holder, crocodile clip and a wire(read and black) and connect them as shown on the picture below.

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 s.m.p.s is okay and the problem is on the secondary side.

CTV common Complaints

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TV Basic Repair Guide: Common Problems

This post is for those who want to know the common problems that we will encounter in repairing Television set. I will be posting some common problems and their possible solution and tricks to repair this problem.

DEFECT: Single Horizontal Line In The Center

POSSIBLE HINTS : 

1.) Check first the vertical IC. Some Vertical IC’s may have loose contacts on solder joints on its legs because of the heat and cold condition on its operation.

2.) Check the supply voltage if present in the main vertical section, sometimes 24 volts from the flyback and other supplied by 24v and -24v plus ground.

3.) If the supply voltage is missing, check the line from flyback having 18-24 volts, possible culprit is an open fusible resistor.

4.) Defective yoke, maybe there is a broken or burn part of the coil inside it.

4.) If the supply voltage is present, possible problem is that the Vertical IC is defective or the Vertical Sync coming from the Jungle IC, better to check also the jungle IC in this situation.

DEFECT: VERTICAL  LINE ONLY

POSSIBLE HINTS :

1.) Defective yoke, maybe there is a broken or burn part of the coil inside it.

2.) Loose contact connection on one of the legs of the yoke on the main board.

3.) Defective non electrolytic capacitor.

DEFECT: NO SOUND

POSSIBLE HINTS :

1.) check first your speakers, it is usually the most common culprit after repairing a television with power supply problem based on my repair experience.

2.) Check if the supply voltage for audio ampifier is present, if not check the line.

3.) If the supply voltage is present, try doing a hum test by touching the input of the audio IC, refer to the datasheet to find the input pin of the IC. Replace the audio IC if everything is done.

DEFECT : NO POWER

POSSIBLE HINTS :

1.) Check first the power cord.

2.) If the power cord is ok, check the fuse

( note the fuse is burnt, don’t replace it right away. Check first the circuit if their is a short on the circuit specially on the primary power section, check all the diodes surrounding the main power IC)

3.) If their is no shorted diode and no short on main IC, now check the Horizontal Output if it is busted or shorted.

Color Telivision

Introduction

Television at the crossroads

Television in substantially its present form has been with us for nearly 50 years. It is a tribute to the National Television Standards Committee (NTSC) that the color television standards agreed upon in the early 1950s have performed remarkably well making quite efficient use of valuable radio spectrum space and the psychovisual characteristics of the human eye-brain system. However, HDTV (High Definition TV) will supplant and ultimately replace the current standards. We will all come to expect its superior resolution, freedom from noise and ghosting, and pure CD sound. Yet, the perceived quality of TV broadcasts and cable will never likely be the major issue with most consumers. Content will continue to be the biggest problem.
It is likely that in roughly 15 years, HDTV - digitally processed and transmitted as 1s and 0s - will completely replace the current system. Acceptance in the marketplace is by no means assured but with the merging of TV and computers - with the Internet as a driving force - it would seem that the days of the stand-alone analog TV set are numbered.

Basics in TV Repair Guide

Here are some of the useful information that we must know if we want to learn about TV Repair, but before this, you must also have the knowledge about the electronics components and the symbols that we see when dealing a schematic diagrams and printed on the circuit boards. You must learn the basic functions of the Resistors, Diodes and other semiconductor components that are available on electronics circuits. For more information about the Semiconductors, here is a website that can help you understand more About Semiconductors.

In this diagram, it shows the different section in a Television Set. This can be serves as a reference when referring a television set. You must have the knowledge on how to diagnose problem so can just choose on what section you will troubleshoot to decrease the time of your troubleshooting.

Here are the parts list from the above diagram:

1. Speaker

2. Yoke

3. Video Amplifier

4. CRT Socket

5. High Voltage Section

6. Bridge Diode

7. Regulator

8. Audio Amp (green square) Audio Section

9. Tuner

10. Flyback

11. Chopper

12. EEPROM (Memory IC)

13. Vertical

14 Horizontal