Friday, February 14, 2020
Friday, January 31, 2020
For those who have been doing LCD/LED repairs will agree with me that screen issues are very common and are almost always caused by the small circuit usually located on the top of LCD panel as the major cause of these problems.
These issues are quite common and therefore a tech without adequate knowledge on how this circuit works will have it tough on his workbench.
Besides having it rough will also part with less profit since most of the television will be send back to the sender for lack of repair.
The most effective way of troubleshooting is voltage testing, in this article I am going to look at critical t-con voltages and what happen if they fail.
Monday, January 27, 2020
14-inch China made CRT television screen colored with a black slip at the bottom of the screen now repaired:
I received this television from a customer who wanted it checked and the screen had spots and the bottom of the screen had a blank slip across the screen which made the height narrow.
Usually if there is something on the screen it can help the technician locate where the problem is even without opening the set.
In the case at hand since the screen height is affected we can conclude that the vertical circuit has a problem.
Therefore, after opening the set I will proceed straight to the vertical circuit.
This television was using the popular ic LA78040 which is attached to the heatsink using a bolt as you can see below.
Saturday, January 18, 2020
After opening the set, I went straight to check on the safety components:,I found the surge limiter Resistor (dropper) open circuit but the fuse ok.
I replaced the dropper and powered on but it blows again instantly, I wish I had used the series bulb but it was too late.
I replaced the dropper again and used the series bulb and I notice d the bulb was lighting with lots of brightness indicating a heavy short. I measured the voltage across the man capacitor and found around 65 volts.
Now back to basic electronics I remembered that if the diode bridge has a problem usually cause the voltage at the main capacitor to fall.
I tested across each of the four (4) with my digital meter on diode test and I found one diode was shorted (I got reading both ways)
I replaced the four diode (bridge)...Usually whenever you find one of these diode shorted it is important to change all the diode because even if they test okay it will be a matter of time before they succumb also and therefore to avoid call back I usually change all of them.
Now after changing the diodes I again applied power via series bulb and still the bulb remained bright even after staying for over one minute, with I concluded there is another shorted component which I have not yet picked.
My next suspect was the switching transistor (S.O.T), so I switched of the power and removed the power cable from the outlet and cut the middle leg (collector of the switching transistor) with a side cutter, this effectively removed this transistor from the circuit…I usually do this to avoid wasting time soldering out the whole component. If the transistor is found to be okay I apply a small solder to the cut leg only.
After cutting the middle leg (collector) of the S.OT I again applied power via the series bulb and this time the bulb brightness was first bright as the main capacitor was charging and then went down.
With this I concluded the switching transistor was shorted and need replacement.
I replaced the transistor and re-tested again, this time I was lucky and I even saw the dummy load I normally used while doing power supply troubleshooting lights up indicating that there is power on the secondary side.
I re-assembled the television and everything was well.
Whenever you find a fuse or dropper gone never replace them and apply power without using the series bulb-there is always a cause why it blow i.e. expect a heavy short on the primary area-like the diodes bridge, Chopper transistor / control chip, degauss Posistor) or component that lie across the power rail (one leg on the live and the other on the return or ground) see the figure below.
You can relate to the actual components on the picture below (diagram used only for identification purposes only, from a different model) most of the power supply doesn’t have varistor on the power supply.
Monday, January 13, 2020
A client contacted me via my Facebook ad with an issue of faulty Samsung television.
Since the advert was broadcasted in the city of Nairobi, I expected him to be from within the city but it turned out he was from quite far place from where my workshop is.
I pointed to him my willingness to repair his television but distance does not favor us.
He promised to bring the machine up to my shop when coming to Nairobi and I left it at that.
Monday, December 30, 2019
As a technician you have come across many transformers, First on the list is step down transformer which work is basically to step down voltages like 110Ac to 12Vac or 240Vac to 12Vac etc.
We also have step up transformer which steps up the voltage, a good example is fly-back transformer.
Another transformer available in the market is isolation transformer (this transformer has the ratio 1:1) which means if you give it 110Vac on the primary you will get 110Vac on the secondary side. Give it 240Vac on the primary and you get 240Vac on the secondary side.
So what is the purpose of using this transformer and get the same voltage (results)
This is where isolation (separation) comes in. this means even if the secondary voltage is still the same, it is not connected directly to the main supply but done in isolation (magnetically) so to speak.
Monday, November 25, 2019
This television came from a friend with the complaints above, I tested it at his place and I confirmed the symptoms.
So I took it with me to the workshop and since I confirmed the symptoms at the owners place I went straight to open the panel and accessed the backlight and after testing with my backlight tester I did not get any fault led and I was frustrated because I expected it to be easy fix.