A friend of mine referred to me a client who wanted her TV checked by a technician. I usually carry with me a small digital meter and a screw driver because most of my job are done in a freelance kind of setup.
This is because i have another full time job very different from electronics repair which i do as an hobby and also my plan B.
My friend it is good to have plan B in this life because even if you are employed one can be retrenched or loss your job but no one can sack you from using your hands(talents).
For this particular day i forgot to pack these tools of trade. So after the request i almost gave the lady the following day appointment but since she wanted to have her TV back due to high demand by the children,i inquired from her if she has a screw driver in her house and promised me that her husband is a computer guru and therefore he has all manner of screw driver sets in her house.
With that i decided to attend her case on the same day. When i reached on site, true to her words she went into one of the rooms and she came out with a bunch of screw driver(all sizes) and i pulled out which i thought was best for the job.
Before opening the TV she connected me to her husband via cell phone for some briefing.The husband told me he suspect the switch because some times the tv had problem to power on when the power button is pressed.
I took that information seriously and to confirm the switch is okay or not i pressed it till home and still nothing happened on the screen.
I pulled my voltage tester stick and it lighted when brought near the tv. This confirmed to me that the tv is getting powered and therefore there could be something else blocking the power.
So i undressed the TV and took with me the motherboard to my workshop for further testing.
When the lady was packing the board into a plastic bag, i heard her scream and told me that she got a shock from the board but i didn't take her very serious.
Back to my workshop, i pulled my meter and tested across the main capacitor(220uF/400V) and to my surprise i got 242 Vdc just hanging there.
Then i remembered her words of getting a shock, with that i realized how lucky she was because if she could have touched directly across the two pins of this capacitor it could have been a different story all together.
Lucky was i too for packing it in a polythene bag (poor conductor of electricity) otherwise i could also be in trouble, thank God non of the above happened.
After discharging this capacitor using the series bulb, i started to troubleshoot it.
Because the complain was set dead, i started scanning the primary side of the power supply and looking at the power switch it reminded me of another TV of the same model which had a intermittent switch problem.
I pressed the power ON switch and clicked how they do when fully engaged but testing across the neutral line of the switch i noticed the switch was open and because i had such an experience i just soldered a jump wire between the two pins of the switch.
I thought the problem is now solved but to my surprise the TV has no voltage on the secondary side.
So i decided to test components on the primary side, just by coincidence i started with the capacitor which filter the Vcc pin voltage supply to the control ic which was rated 22uF/50V.
I was doing this from the bottom side of the TV board and when i tested across this capacitor my ESR meter registered an abnormal ESR reading as you can see here.
For your information a capacitor with ESR of 5 ohms can completely cripple a whole 47 inch LCD tv and here we are talking 91.80 ohms.
To be sure i was testing the correct component i decided to solder it out of the circuit completely and test it again and truly my fears were confirmed as you can see here.
I salvaged another capacitor from the junk and after testing and finding it to be in good health condition i replaced it.
I applied power again to the board and this time i got a voltage at the secondary side and i concluded that the issue is now fully solved.
I went back to the owner the following day and after re-assembling the board back to the tube the tv came back to life.
I strongly advice technician to be on lookout for this kind of switch because even if it is switched off still there is power on the live line all the way to the main capacitor but no return, therefore if you touch on this line by coincidence assuming the power switch is off then you will provide a very good ground and hence get electrocuted
This also explains why the voltage tester pen was lighting when brought near the TV.
Author CRT Television troubleshooting guide
For more information about this book click the book cover.
Thank you guys
Hello Mr KimathiReplyDelete
Thanks very much for the useful article it remember me again how to discharge a big capacitor otherwise we can get shock which it is not good to be hit several times by shocks you never know how much cells you lost by the one shock,the article also show us importance of having modern tool like Esr meter,digital multimeters and other which speed up repair works withiout investing on these tools our repair jobs will be tough and for those who have permanent or government job we must look for plan B job so that when government job end plan B as Electronic repair take place and life continue.
Well put Amos.Delete
thank you so much for this very happy ending repair .
Hi Beh, Thank for passing by Mr.Beh.Delete