Thursday, August 26, 2010

Chanwong tv-china- see circuit below- No Power

Motherboard
Actual Voltages -Circuit

This TV has no power symptoms-first I checked the fuse and it was blown but not blacked inside meaning there was no heavy shot in the supply.


Remember a fuse can tell you a lot about the last moment of any equipment before it died (in medical terms postmortem)

1. Fuse blown, dark inside means the machine died due to heavy shot. Please be sure to use the light serious Lamp to bring up the machine after replacing the fuse.

2. Fuse blown, clear inside means the fuse might have died through natural dead.

After I replaced the fuse it didn’t blown again confirming to me that there was no serious shorted component (I mean component shorted which is taking the supply directly to the ground.

I measured the output voltages and there was none: absolutely zero volts. Then I checked the voltage at the main capacitor and it was 320 v dc which is okay. The voltage was also reaching the s.o.t collector same voltage.


I measured the S.O.T base voltage and it was 0.02 volts. I followed the start up circuit divider and the voltage was ok and no resistor was open.

But from the above voltage measurement you find the Zero volts at the base of the s.o.t is suspect.

Either the S.O.T (v513)is sorted base-emitter because its taking the voltage to the ground. Or V512(c3807) is shorted collector-emitter because it’s also taking the base of the s.o.t to the ground. Or capacitor c517 across the collector emitter of v512(c3807).

I removed the two transistors i.e. s.o.t(v513) and driver v512 and measured them out of circuit together with cap c517.

To my surprise the S.O.T v513 (C4429) was shorted Base-Emitter junction i.e. was 82 ohm both ways on diode test on my Dmm meter. It should have been high and low ( I mean the B-E junction has two reading meaning the transistor is shorted B-E junction which is very rare.

Conclusion

If there is zero voltage at the output then most probably the primary section is the cause of the problem.

Narrow down your faulty circuit (area) by use of voltage testing only. This one is a very fast and valuable method I have come to learn.


Thanks guys

Let meet in the next class

Monday, August 23, 2010

Do it yourself Flyback Tester


As a technician there are test equipment that one requires to get started in electronics which are assumed to be obvious in order to get started in your repair work may it be for hobbyist or professional purposes.

Among them they are others which are not taken seriously as a starter kit but as you walk down the road you find it necessary to add them on your work bench.

When this need arises the technician has an option to go shopping for one or decide to do it by him or herself.

I would like to address the latter group which decides to do it themselves. Although most people would prefer to go this route, you realize that very few people can access the printed circuit board facility around them and hence become difficult for them to get started.

In this article I am going to show you how you can go round this problem by using readily available vero board and achieve the same result like the machine printed circuit board.

As a starter kit (D.I.Y) I have done a template using strip board of the flyback tester based on circuit diagram found on the internet and its working wonderfully for me.



Component values are recorded on their body. Resistors, capacitor, ics , transistors etc for ceramic capacitors the number is written on the body. For ics I advice to use ic sockets instead of soldering them directly to the circuit board.














Testing Degausing Coil-Only 3 LEDs ON.



What is this simple tester capable of doing:-

1. Testing primary winding of flyback transformer in circuit.

2. Testing horizontal yoke coil in crt television /monitors.

3. Testing primary winding of switch mode power supply(all) in circuit

4. Testing the degaussing coil winding in crt television/monitor

5. Testing small DVD motors like the spindle motor ie 2 led lights.

Challenges: as you can see i had a problem of getting a good housing for my project, electronics shops here in my town don't seem to store them therefore i decided to use one of the junk analogue meter housing to serve the purpose.

Feed back: i would love to hear feedback from you fellow technician about your experience on this simple but very important workbench tool.


Additional information for the do it yourself fly back tester

For those who like to use the 9 Volt battery for the power supply please use this adapter to get the 6 Volts for the circuits.




Friday, August 20, 2010

ORION DVD DEAD

ORION DVD DEAD

On pressing the power on button the set was dead as dodo. No sign of life anywhere so i suspected power supply. As usual i scanned through the power supply board carefully both sides to check for any burnt areas or burnt components and alas i noticed a capacitor which was bulging on the top silvery side. This is usually an indication of capacitor failure due to high voltage hence it’s important to consider that source peak working voltage of the capacitor when replacing. Higher voltage is better but same capacitance

Culprit: capacitor 1000uF/10v on the secondary output of switch mode power supply. I replaced by 1000uF/16V which i figured was the best for that output source. This was a design error by the manufacturer. Very commons with china made equipment.

After replacement of that one capacitor the DVD started working like new:

Below is the DVD power supply Board

DVD Power Supply
The DVD Power supply unit is Just like any Power supply unit using Switch mode power supply (chopper), so use normal PSU fault finding techniques: Start with, any power from secondary of chopper transformer, at the diodes? Usual PSU voltages are +5v,-12, +12v.If yes the power supply unit is okay, if no check for 320v across mains smoothing Capacitor. If 320 Volts is okay but no output, check for shorts on the output lines diodes, capacitors ESR gone high

Thanks

Stay safe

Tuesday, August 17, 2010

Sony TV 1902 GE..NO PICTURE NO SOUND only screen display ie channel Number.

Sony TV 1902 GE..NO PICTURE NO SOUND only screen display ie channel Number.

This is quite old machine over 10 years. The customer complains was that the machine has intermittent picture on and off. Usually if the picture misses they would tap slightly at the back of the TV at the RF inputs. Also the TV used to have a drift on the channel when off (every time they put off the TV they must tune again because they find the channel is not clear).

First suspect – dry joint. I did a thorough check for dry joint and I did all that I could but to no avail. Next is voltage checking around the CRT circuit and I noticed that the RGB collector voltage is high..ie 200 Volts before and after the 15 k resistor. All the three cathode. I measured the RGB BIAS VOLTAGE and I found 0.76 Volts in all three. Hence the reason why the collector voltage was not dropping. ie its like the RGB TRANSISTOR ARE OPEN BECAUSE THERE IS NO BIAS VOLTAGE.



I followed that circuit up the main ic processor and the voltage was the same.. I suspected the ic could be faulty…but before I embark on that mission I was to be very sure it is that ic because it is quite expensive to change.

By the way all other low signal voltage were ok..the 9 v, 12 volts were ok meaning the ic and the turner is getting powered.

I did esr test around the processor ic and changed whatever capacitor I was not happy with but still no change.

Then when fixing the Ariel into its sockets I noticed that it has some effect like if there is some disconnection inside the turner.

I opened the turner and did a thorough re-soldering. Both the inside of the turner and the inside of agc, if, sif, circuit(like the turner) and when I powered the machine. Everything is now perfect.

Conclusion.

I don’t know why this particular TV could cut the RGB bias voltage from the processor ic when there is not input from the turner. Usually in new machine the bias voltage I guess should be ok 3v each but the screen would be snow…no sound and picture though.

Anyway I learnt the hard way that whenever you experience no picture and no sound symptoms the best place to start is the turner circuits(inside and out the turner)…especially dry joints inside the turner and the VCC for the turner( 5 volts-turner, 0 to 33 v- turning voltage and 12 volts-band switching in some model)

The tuner components are usually quite reliable unless the antenna experiences a lightning strike. However, it seems that even after 40+ years of solid state TVs, manufacturers still cannot reliably solder the tuner connectors and shields so that bad solder connections in these areas are common even in new sets.

Also whenever you have a issue with any electronics gadget please take your time to think logically using flow chart to narrow down the problem to the actual circuit…very important even before you open the machine.

That’s all for today

Thank you and stay safe

Monday, August 9, 2010

Hum-tech technical tip for the month:-how to save a mobile phone that has fallen into water

This month tip is about how to save a mobile phone that has fallen into water.


Most mobile phone which happen to fall into water end up being thrown into the dust bin by the owner just because of lack of proper information on how to handle such a case.

This month tip of the month, we at Hum-tech feel we should enlighten our customers on how to handle a case where a phone drops into the water.

Most important is to remove the phone battery…this prevent your phone from shorting both the motherboard and the battery itself.

Please don’t be tempted to power it up even for a moment because that will surely cause a lot of damages even if not immediately noticed.

Then take your phone to a qualified technician and explain that you have a phone which has fallen in water…so that they can open the phone and clean it with medicinal spirit and blow it up with hot air (by the way even a hair blow drier can do it very well but be sure the phone is cleaned first to avoid rust on the inside.

That’s all for this month tip….see you next month for some more…



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