Thursday, September 30, 2010

Do it yourself television repair hit by high voltage from power surge caused by lightening or Generator.

Do it yourself television repair hit by high voltage from power surge caused by lightening or Generator.

I have received several emails from my blog followers on how to go about with electronics items hit by power surges. Mostly they have a problem of getting a technician near them and others just want to try their hand in this fascinating career.

Therefore in this article I am going to discuss how you can handle this problem at the comfort of your house.

But first you need to be prepared to do some investment of the basic tool of the trade.

The minimum you should have include:-


Screw driver(flat/star)


These you can get them cheaply from your nearest electronics shop which I believe they are readily available.

After you have the tools now we are ready to start the job but before you open the equipment safety first.

First be very sure you have disconnected the machine from the power socket, I mean not just switching off the power via the power switch but removing the power cable completely from the socket.

I don’t advice you to do any voltage testing here unless you some basic training on safety precaution around the power supply in electronics equipment.

From my experience I have noticed that problems of power surge usually affect the primary power supply components, especially semiconductors and capacitors.

Failure mode of these components is either short or open so your meter should be set to low resistance range/diode test is even better here for short and high range over 20 K ohm for open.

Get a comfortable place/table and open the machine, keep note of how you open the equipment because when closing it up you would be doing exactly the opposite.

After opening the machine go straight to the power supply section, ie components between the inputs of the power supply and the main (switching) transformer.

First check the fuse with the meter on diode test- it should beep indicating the fuse is ok, if not consider the fuse gone.

Next measure the resistance of the surge limiter (white rectangular in shape) this too should read low ohms on the diode range.

Now if the fuse or the surge limiter is gone, DON’T REPLACE THEM.

Expect a big short ahead and the most likely culprit is the switching transistor or I.C usually the one you will find bolted to a heat sink. You can directly replace them without even measuring them.

There is also another small transistor near that big one, one can change it too, it is recommended because sometimes its goes and other times it can survive.

Continue Scanning the area looking for the main capacitor (biggish) rated 400 Volts for countries using 240 vac or 200 volt on countries using 110 VAC like USA.

If you find this main Capacitor has top silvery plate swollen, even a slight sign consider it bad. Also note if at the button of that capacitor there is some liquid oozing. If none of the above symptoms then consider the capacitor is ok for now.


From my experience changing those components always make the power supply to come up again.

If you don’t want to invest on the multi meter then you need to directly replace the components I have listed above: i.e Main capacitor, chopper transistor, fuse, and surge limiter, diode/bridge are quite stable so you can leave them for now.


If you change them and still nothing happens then I advise you to take your machine to a trained technician


Note:

You can use this method to repair other electronics equipments using switch mode supply like DVDs, Monitors, etc so long as they have been hit by power surges.


Thanks you guys



Let meet in our next class

Saturday, September 25, 2010

Smile monitor shut down with a vertical line on the screen - chassis CA6415-CA6515

Smile monitor shut down with a vertical line on the screen - chassis CA6415-CA6515


Most of my customers come to me through referrals by others who I have maybe repaired their equipment and are happy with our service.

So I got this phone call from a customer who wanted me to look at his monitor which according to him was dead.


It was just near my residence and I decided to take a walk there.


When I arrived at the site I connected the monitor back to the CPU and applied power: Relay is heard clicking as the monitor try to come up and... The monitor came up and shut down after a few seconds, but before it shut down I was able to see on the screen a thin vertical line.


Facts at hand.
1. Monitor came up and shut down
2. Vertical line on the screen.


Probable cause: maybe the monitor is being shut down by the safety circuit because the fault is a serious one.


Vertical line on the screen- Usually vertical line is caused by failure in the horizontal circuit especially the deflection circuit.


For those who are new to electronics repair let me emphasis here that whenever you see a horizontal line on the screen, the fault is on the Vertical circuit, while vertical line on the screen point to fault on horizontal circuits.


Vertical line is caused by failure in the horizontal circuit especially the deflection circuit. Therefore check components for deflection circuit gone open (i.e. the horizontal pulses are not going back to the ground. The culprit could be an open capacitor(ceramic like the s-correction capacitor, linearity coil open, dry joint on the horizontal deflection yoke connection, also the yoke could be open etc.


So I opened the monitor and went straight to the deflection circuit to test my hypothesis. I found capacitor (c125) 0.39uF which is the same as no 394 was totally open.




I replaced it with capacitor no 474(0.47uF) which was readily available in the market and the monitor started smiling again.


Tip1: when measuring the components around the fly back in-circuit there is a tendency to get low resistance especially on capacitors (Ceramic) even if they are open because of having other low resistance components or path and hence never trust your reading when doing measurement in circuit. You must remove one leg in order to get correct reading. This applies also when using ESR meter. Sometimes if you notice a funny reading from your ESR meter it’s better to disconnect one leg from the circuit.


Tip 2: Observe carefully to see if there is any capacitor which has its body surface bulging out. Even slight swell on the capacitor body is enough evidence to suspect a faulty capacitor. (Observe carefully the body of all ceramic capacitors: if bad they become swollen and look like the back of a camel-or underfeed cat. i.e. backbone plus a big stomach) by the way sometimes you don't get the exact replacement for the s-cap. In this case you can always use a cap which is near that in capacitance but same working voltage.




Thank you guys




Let meet in our next Class


Monday, September 20, 2010

GLD TV FIELD COLLAPSE WITH A SINGLE HORIZOTAL LINE

GLD TV FIELD COLLAPSE WITH A SINGLE HORIZOTAL LINE across the picture (clear picture below the line but distorted picture above the line. Also white lines across the upper part of the screen. There is also blank part above and below the screen.


In my opinion a technician and a medical doctor have a lot in common, we deal with symptoms(facts) analyze them and figure out from the data at hand the most likely cause of those symptoms.


Fortunately for doctors they have added advantage because at least with human being they can speak out how they are feeling, add some additional test and there you go, Drug prescription is done. Ok?


Now for technician’s machines don’t speak, therefore we rely heavily on keen observations, testing critical test point, some history from the owner on how the problem started etc to be able to figure out the actual circuit causing the problem (symptoms) at hand.


This part is very critical, the more information you gather the more likely you are going to narrow down to the actual circuit which is causing the problem.


If the machine (Television or Monitor) has something on the screen, you can get a lot of clues on the screen behavior even before opening the machine.


Let take this GLD TV as a case study, the information on the headline of the article is what I was able to observe on the screen.


I have tried to reproduce the symptoms on the picture below




1. Screen height is narrow.


2. A SINGLE HORIZOTAL LINE across the picture


3. Clear picture below the line but distorted picture above the line.


4. Also lines across the upper part of the screen.


5. There is also blank part above and below the screen.




These are 5 strong facts all pointing to Vertical Circuit, am I right?




NOW LET OPEN THE TELEVISION AND TEST OUR ANALYSIS


After opening this particular set I noticed two capacitors 1000uf/35v which had their top silver part swollen. I put my finger on them and they were also getting hot. This was enough evidence that these two capacitors are faulty.


Both capacitors were on the secondary output source and after following that source it lead me to the vertical IC (LA78040). Even if this capacitor were not swollen I would have gotten them pretty easily because the first thing I could have done is testing the supply to the vertical I.C. If capacitors with filter functions develop problems (fail) usually causes the voltage to drop on that line. If the voltage is lower than normal there will be subsequent effects on that circuit may it be vertical, RGB, horizontal etc.


I replaced the two capacitors plus the vertical I.C to avoid call back because I figured out that because this IC was being feed with dirty dc (not clean dc) it might have developed some internal injury which might manifest after sometimes.


Conclusion
Whenever you see a single horizontal line across the screen always suspect problem on the vertical circuit regardless of other symptoms on the screen. Usually vertical problems manifest themselves in various ways with some being weird (funny). Also presence of lines on this circuit besides the single lines is a strong indication that a capacitor might have failed too on that circuit. I.e. vertical


After you have localized the faulty circuit open the set and go straight to the suspected Circuit. In this case vertical circuit was suspect. Do a thorough scan on all components around the vertical I.C. usually symptoms like lines on the top of the screen and sometimes a black stripe or no image at the bottom are pointer that one or more capacitors are faulty. Scan and check any bulging capacitor top or capacitor getting hot which are sighs of bad capacitor.


Tip
How to recognize related components on a particular circuit;


Use the numbering of component on that circuit.








Thank you guys
Let meet in the next class

Tuesday, September 14, 2010

No disc symptoms- JVC DVD

No disc symptoms- JVC DVD

No disc problems in all types of DVD is a common phenomenon, before a DVD start reading the disc, there are several test it does just like the booting of the computer (cpu) called start up process.

If by any chance any of the stage is not working the DVD will display an error on the screen, most of the time it is disc error, no disc etc.

Among the components which are first suspected is the LENS but from my experience this is one of the most stable component with a relatively very low failure rate.

Top suspect is the Spindle motor (the motor that spins the disc), over 75% no disc error is caused by this small motor where the disc sit on.




How to measure spindle motor with a DMM, set your meter to low resistance test (200 ohm) range, if it is okay should read between 5 and 20 ohms.

On analogue meter, this is my favorite set the meter to X1 and measure across the two wires coming from the motor(red and black) if the motor is okay, it will spins at very high speed indicating to you that it is up to the task.

A word of advice- I have found several of these motors which are not working right from the shop and therefore I advise you to ask the store keeper to give you the meter and test on the counter before taking it home only to realize that it’s not working. This is also true with other components make it a routine to test components before replacing them in circuit.

Back to my dead JVC DVD

As usual my first suspect- Spindle motor. Measuring this motor, YES it was dead short.

I replaced with a new one, alas still no change..Still no disc symptoms.

Looking at the logic circuit (motherboard) I noticed a swollen cap on +5 volt line. A good hint what might be pulling the spindle motor power supply down!

I replaced the capacitor (470uf/10v) with 470uf/16v…still no change but this time I noticed that the lens reader is trying to focus, I mean moving up and down.

But as the lens move up I noticed it is hitting the spindle motor table when trying to focus.

Another clue, that the disc table (spindle platter) is not adjusted properly, meaning when I replaced the spindle motor I failed to set the high correctly.

Hence no disc error. I adjusted the spindle motor table up slightly and the DVD came back to life.

BIG PROBLEM --- SMALL SOLUTION

Conclusion;

Most of DVD problems are mechanical and very inexpensive to repair and doesn’t require sophisticated machinery to test just observation using the best tools I have come to realize in my troubleshooting experience which are absolutely free, YOUR SET OF EYES.

Thank you guys..

Let met in the next class



Thursday, September 9, 2010

JVC Television Dead 21”

One day I was brought a dead JVC TV to repair; when I pressed the power ON button, I could hear the degaussing coil engage as usual on power on.





 I opened the set and I measured the voltage across the main capacitor. No voltage was found-absolutely zero volts. Alas what is the problem now? All over a sudden I heard a loud crack sound and noticed a resistor has broken down—see resistor R904 above across the degaussing coil- what was it doing there in the first place(anyway in most set its written optional but in this case it was there)

I never had a chance to know its actual value to date. It was also very hot and broke into pieces.

After that the voltage was able to flow up to the control chip (STR-F6653). Now pin 1 is 318V dc which is normal in this case but there is a funny reading on pin4( start up VCC pin) it should be 19 volts but on my meter it was reading less than 12volt and pulsating-usually when you find lower voltage than expected and pulsating means there is a shorts on that line.


So I lifted up the VCC pin 4 and the voltage increase to 25volts. With that I concluded that that I.C was shorted on that pin.

I replaced the I.C together with Posistor and the unknown resistor which burst into flames instantly on power on. The resistor value which I figured was low resistance almost caught fire.

So I decided to leave this resistor completely and only replaced the I.C and the Posistor and the TV started running on like new. To date I have not replaced the resistor and the customer has never come with any complain. Isn’t funny sometimes we remove components to make a machine to work again.

What was the use of the resistor in the first Place. Remember in all this the fuse never shorted. Could it be even the Posistor was good also? Because there is no way a Posistor can short and the fuse does not blow....

A word of Advice:


Never disable the x-ray protection circuit because this can cause serious x-ray radiation which can harm the user- Always think safety first in whatever you do.


Wednesday, September 8, 2010

Hum-tech electronics tip of the Month- Tool and the user


In the field of electronics just like any other fields may it be farming, defense, teaching etc requires particular tools to do the work.
Sometimes you notice a fellow with only the basic tools is able to do more than the one with whole lot of the equipment at his disposal.
So what makes the difference: the difference is that as is true with all tools, it is only as effective as the user.
A sword is only as good as the worrier wielding it.
A car is only as good as the driver.
This is a very important concept to understand… that the performance and effectiveness of a tool depend a whole lot on the user.
That’s why hum-tech electronics has introduced this forum to sharpen each other skills in quick troubleshooting ideas.

Wednesday, September 1, 2010

IBM 14” MONITOR DEAD

First I measured the voltage at the main Capacitor and found it was normal i.e. around 300 Vdc on 220 Vac) for countries using 110 Vac expect around 155 Vdc across the same Capacitor).

I also measured the voltage at the Chopper transistor (S.O.T) and found the same voltage at its Collector leg (middle).

I checked the output voltages and none has voltage.

Measuring the B-E voltage of the S.O.T was zero meaning the transistor is not switching.

I measured the Vcc pin of ic3842 no 7 and found it has voltage of between 12v to 14v but pulsing. The voltage was within the normal expected so I ruled out any short on the I.C other legs of the ic3842 has some voltage but also pulsating.

I also disconnected the B+ line and used a load to see if there is short on that line, still there was no output. I checked all the output diodes and all were ok. So I suspected there could be still a problem on the primary side.

I checked all the capacitors on the primary and the all tested good. Then I started checking the resistors, usually when testing components I usually disconnect one leg to be sure I am only measuring the component in question.

After measuring a few resistors I bumped on one high value resistor which was open completely i.e. 160 K resistor now total open. This was a new experience to me and I was very happy to experience such a behavior. After replacing the resistor the machine worked again like new.

Please see the resistor which was open in the circuit below: One of the two resistors from the 300 Volts rail to pin 3 of the ic3842 and the Source of the S.O.T transistor (F.E.T) was open.

I think if I had used the voltage check from the 300v rail to the I.C or transistor I could have gotten the culprit pretty easily than removing all the components from the circuit and measuring them out of circuit. This is especially easy when one has the circuit diagram to guide you on the major voltage paths.