Thursday, December 23, 2010

Thank you for your support and happy new year

Hum-tech electronics once again wish to thank all our visitors to electronics repair made easy for visiting us,

Five months down the road after the birth of hum-tech electronics we have registered over 10,000 Visitors from over 115 countries.

Also let me take this opportunity to inform all my followers that being end of the year i will be breaking for a short vacation and i will be back on 4th January 2011.

Last but not the least i wish you all happy new year 2011 and happy Christmas for my Christian followers.


Friday, December 17, 2010

Capacitors are the cause of many problems in electronics for many decades...please sit back and watch how they used to be tested those days!!


Friday, December 10, 2010

LG Television NO PICTURE- AUDIO OK blue background

This is a LG television made in Egypt (or is it made or assembled in Egypt) and the customer complain was No picture but audio is okay.

So I went down to his residence to check on it…remember most of my repairs are done on the site and therefore I normally do some preparation for the task ahead of the time.

In this assignment I had prepared myself with the basic equipment plus a circuit diagram of the LG television (any) because mostly TV circuit diagram for a particular model are very similar in terms of major circuit modules.

In this case from the symptoms given by the owner via the phone I was very sure the problem was likely to be on the CRT circuit board, so I went down to his place and found he was not around but the houseboy was around to assists me with what I wanted.

Asking him about the television he told me that there was another technician there and I wanted to know how far he went into the television even before I opened it and he confided with me the technician came and after a keen observation on the screen for about ten minutes without his eyes blinking he told them that the problem is new in the world and therefore he sees no hope on the television and therefore the best place for it is in the dustbin or make it a toy for his children.

My friend that is why it is very important always to get a second opinion even in the medical world. I mean if your doctor tell you that your leg need to be amputated (God forbid) please ask him/her for some times and try another doctor to hear his opinion.

For me I was very happy with this technician because he was honest with himself and never attempted to repair it because he could have done even more mess. So I opened the set and went straight to the small CRT circuit (RGB)


I did a voltage testing on the points highlighted and filled the figures (voltages) on the sheet of circuit diagram I had carried with me as shown Below.

 Now let us analyze the fact at hand…in electronics we deal with facts, in our case at hand the facts are voltage measurements and being voltages they are represented by figures and as you know figures never tell lies and therefore when need to deal with this truth.

From the figures and from what my electronics teacher told me about the RGB circuit I was not happy with the figures for the BLUE cathode at the CRT pins connector which was at 65 volts…as a rule of thumb the voltage at this point for the three cathode should be nearly the same with an margin of plus or minus 5 volts between the three cathodes and therefore the 65 volts on the blue cathode is first suspect.

So I tried to figure out why this voltage is very low compared to others , looking at the voltages on the B-C-E of the Blue transistor amplifier I noted that it has 4 Volts at the BASE and 4 volts at the EMITTER..

Again I am not happy with this because the Voltage drop across the base-emitter of a transistor should be between 0.3volts to 0.7 volts for silicon transistors but in this case was zero.

Did I say zero? Yes this means the voltage entering the base is 4 volts and coming out at emitter is 4 volts
again…how can it be?

The only reasoning is that the transistor is shorted base-emitter junction and hence behaving like a jumper across the base and emitter.

So I decided to remove this transistor out of the circuit completely and actually I was able to get a picture but missing the blue color and therefore the screen was yellowish..THE PRODUCT OF RED AND GREEN.

With that I concluded that the blue amplifier transistor is actually the culprit and having a few of these transistor at all times I rushed to the workshop and back with two pieces of the type and replaced it.

I replaced the transistor and the picture was back to normal.




Thank you guys

Let meet in the next class.

Tuesday, December 7, 2010


Friday, December 3, 2010

Goldstar TV vertical partially closed

This Goldstar television came in with the customer complains the picture has a problem…so I powered the television to confirm his report.

As a technician it is very important to confirm what the customer has said in his presence otherwise the set might have gone to another technician with no color symptoms and when the customer picked it from technician A he/she didn’t bother to check the condition after staying at this technician A for six months with the usual excuse of lack of spares.. And was just returned to the box and brought to technician B who in this case happens to be you.

No color symptoms you have figured it is not a big deal and therefore you will hold it a bit to attend more serious repairs...so two days later you decide to work on that set with no color.

So you apply power with your eyes fixed on the screen to confirm what the customer told you two days ago. Pressed POWER ON button and nothing happens.

Checking on the power outlet it working well and therefore you decide to open the set to see if the fuse is gone and when you finally open the television you notice the fuse is okay but a lot of the circuit has been tampered with and components even missing including the microprocessor, you know how much it cost in your local currency.

Just when you have confirmed the above here comes the customer coming to see the progress of the set and as you begin to thank him for coming your way so that you can show him how his set was abused by the previous repairers he tells you that he only knows you and if you are not able to fix his set just bring it back the way it was with” NO COLOR SYMPTOMS’. Or he calls 911.

These things happen and if they have not happened to you yet please take my advice otherwise history does repeat itself.

So I applied the rule above and I switched on the Goldstar television and I observed the following:-

1. The set came up beautifully and I noticed the picture was partially closed with a black slip below and on top of the screen.



2. Sound was okay.

As a technician you can make a lot of conclusion just by watching the screen if you have a good knowledge on how that equipment circuit works.

Let do some conclusions from the above observations.

A: There is something on the screen in our case CRT television. This means the primary power supply is okay, horizontal drive circuits is okay, horizontal output circuit(fly back, EHT, screen, focus, heater, RGB circuit) are doing their work well.

Beside the picture is covering the whole screen width(horizontal) and therefore we can conclude that the horizontal circuit, primary power supply and the audio circuit are okay and therefore not considered for trouble shooting.

B: The picture is height is low…then have a problem here because we expected the picture height to cover the whole height and the circuit which is supposed to open the picture upwards and down seems to have a problem.

So my conclusion was that there must be a problem with vertical circuit of that television.
I open the set looking for the vertical circuit and after scanning a bit I noticed three capacitors which have their top swollen a bit as you can see in the picture below.


I decided to confirm my eyes by doing an ESR on the three capacitors and they were as follows C564 1000uF/25 was 35 ohms, capacitor C562 470uF/35V was 20 ohms and C457 1000uF/25V was 8 ohms.
All of them were way too far from the good ones which should have been less than 0.5 ohms and therefore I replaced the three and that solved the low height symptoms.

Latter I checked my circuit diagrams achieve and I got this circuit which has a circuit near that television because I could not get the exact circuit and I observed the following.



In the above diagram notice the two filter capacitor for the secondary source B4 27 Volts and B6 10 Volts. Both supplies for the vertical circuit with ic LA78040 shown below.
Also I hope it is good to point out here that almost all ics are feed with two or more power sources as seen below on this ic and many others.





On the circuit above you can capacitor C457 1000/25V which was also swollen has also some effect on the vertical circuit because as you can see the vertical pulses from the ic passes through the vertical yoke through this capacitor down to the ground via 1 ohm resistor R459.

As you know capacitors does not drive direct current therefore only vertical pulses will pass to the ground. Also note that if the resistor 459 (1ohm/1watt) open it will also have some effect on the vertical circuit…this resistor has a high failure rate and therefore be on look out whenever you have a vertical problem.


Thank you guys

Let meet in the next class