Sunday, June 5, 2016

LG 21” CRT Television intermittent dead symptoms

This TV belong to a friend of my friend, from the word go my friend informed me that his friend suspect the switch is faulty because the TV would stay on so long as the finger is pressed against the Power on button.

So I prepared myself with a limited number of tools because I considered it a small assignment, in any case the problem has been isolated and mine is to confirm the same and do whatever is needed and go back to my house.

On reaching the site the owner give me a little background and the story was the same with the previous one and even pressed the power on button to confirm what she was saying.





After that initial test I concluded straight away that maybe the circuit board has moved a bit to the back and therefore the switch is not able to go home completely so that the switch will remain closed.

So I opened the set and tried to move the board a bit in front and because I was very sure it was the only problem I re-assembled the set and even returned in all the bolts.

I pushed the power button in and I could hear it locking but alas nothing else happens…after several attempt doing the same thing I remembered that doing the same thing usually give the same results and therefore I abandoned the idea of pushing the motherboard in and I kindly requested the owner to let me go with the board to do further testing.

Back to my workbench I attached the dummy load (bulb) across the collector pin of the Hot and the ground and nothing happen. The bulb remained off.

This confirmed to me that there is no power on the secondary side of the board. So I took my meter (digital) and tested the voltage across the main capacitor.

Here I got 304 VDC and I was happy with the reading, now I had to confirm if this voltage is reaching the main DC pin of the control IC (this TV use control I.C STR-6554)


To my surprise the voltage at pin 1(was 304 VDC) and zero (0 v) on all other pins. So I googgled this I.C STR-6554 and I got the above circuit. I followed pin 4(Start up Vcc pin) looking for whatever is blocking this important voltage.

I went past the start-up resistor R602 (200K 1W) and still no voltage whatsoever.
Finally I measured across the bridge diode inputs pins (here I was expecting 220 VAC) but I got zero. How can this be and I am getting 304 VDC across the main capacitor.

To me this was a misery and I wanted to find out what is really happening here!…just by coincidence when pressing the power on button I heard the set fire and also because I was working with the dummy load bulb (60Watts) connected between the collector (middle) leg of the H.O.T and the turner ground, I noticed the bulb light briefly.

I remembered the owner talked of TV staying on so long as the power switch was pressed in, with that I concluded that the culprit is the main power switch.

On keen observation I noticed the switch was very poor quality and also instead of being double Pole (Double Pole, Single Throw) it was a single pole switch.
This explains why I was getting voltage at the main capacitor and nothing at the bridge inputs. 
 

Using the jumper wire across the switch contact solved the power problem temporally before getting the exact switch which was not in the market at the time and therefore I advised the customer to be using outlet power switch to switch the TV on and off before I get the exact switch for this TV.

She has not called me of late to hear the progress and therefore I assume she has gotten used to the new mode of switching off her TV.
Advice: 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 article is cut and paste from my CRT television repair real life experience Vol.1 currently on offer at only $6.95 USA dollars.

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2 comments:

  1. Very informative article as I always look forward to reading your articles and wish you post more frequent articles for us to enjoy,thank you and please more :).

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    Replies
    1. Thanks and welcome Marco, all the best from the desk of Humphrey Kimathi

      Regards Humphrey

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