electronics repair made easy: LG 15 INCH MODEL 15FG5MB-T2 DEAD NOW REPAIRED

I meet one of my neighbor on my way to work and wanted me to pass by his place to check out on his TV which had a problem.

I passed by over the weekend and since I was not prepared to repair on site I carried no tools with me.

Before taking the TV I did initial test by applying power to see if there is any sign of life, after applying the power and pressed the power on button I noticed the TV was dead as dodo.

Usually dead symptoms in electronics point to power supply and are easy fix; therefore I took the television to my workshop.

Back to my workshop I opened the set and scanned the primary side and I did not get any suspicious component.

I decided to do live test (voltage test), I started with the AC input pins on the PCB board and I found 220 VAc and therefore confirmed the power cable is not broken inside.

I tested the voltage across the main capacitor and I got 320 Vdc, Checked the secondary side and I got zero volts on all outlets and this lead to suspect the problem is on the primary side.

I checked the voltage at the pin 1 of the control I.C STR-6554 and got 320Volts and I was happy with this, I went straight to pin4 of the same IC and got Zero Volts.

From the circuit diagram above pin 4 is the VCC pin, also called startup VCC pin, for this I.C to work these VCC are very important and therefore if one is missing then the I.C will not work.

So I decided to follow up from this pin 4 backward looking for any open component or track and i did not get any.

At that point I remembered I had an experience in my previous repair with the power switch of this TV model,Usually TV switch are supposed to cut the live and the neutral line completely but his type of the switch only cut off one line and this explain why I had voltage at the main capacitor pins and seemed normal.

But remember this voltage is lacking the return path and therefore it is just hanging and therefore if by any chance you switch of the switch and forget to isolate the power cable from the power outlet then you will be in danger of electrocution.

This is because if by any chance you come in contact with this hanging voltage you are going to give the hanging power a very good return to the ground (earth) through your body and therefore get a nasty shock which can also be fatal.

From the figure above kindly note that the 240/volts, one path does not pass via the switch.

That is why I always advice technician that the only way to be sure you are safe from electrocution is by disconnecting the machine from the outlet otherwise switching OFF the switch is not good enough.

And when doing live testing of voltages then be sure you are using the isolation transformer, for those who have been in this field for long will agree with me that isolation transformer is not a good thing to have in your workshop but a MUST thing to have.

You can decide to buy one from the shop but if you don’t have the budget then you can easily make one yourself using the information I have shared in my eBook CRT TV Real Life Repair Experience Guide Vol.2. Bonus report Chapter 12

Solution: usually this switch has a tendency to wear the switch contact and therefore when you press the switch button ON, you hear a click but actually that does not mean the switch has connected the supply and therefore the TV will appear dead, the solution for this problem is to create link across the switch contact.
after doing a solder link across the switch contact the tv was now okay but the owner lost the power ON button facility and i told him to be using the outlet switch in the mean time since there is no spare part for this switch in my market.

Feedback from Justus Mwinzi-Kenya

Humphrey

Thanks to learn from this article how to take precautions on such Tvs with such power on switch. Secondly I would like to know from you are these isolation transformer in electronic shops here in Kenya?

Thanks Justus for your feedback, sorry i have not seen these transformer in our Kenya shops.

Myself i made one Juan kali way but serves the purpose.

Regards Humphrey

I know the ratio of primary and secondary winding should be 1:1 but how can I determine the swg of the wire.

In my case it is very simple, I just used two transformer connected back to back

Regards Humphrey

Here is some pictures of the do-it-yourself isolation transformer in my eBook