Monday, August 24, 2015

21 inch LG CRT television model 21FS6RG-T3 dead now repaired

I received a phone call from a client who wanted me to pass by his place to check on his TV which had a problem.

He told me he was referred to me by a gentleman who thought I can help him out of the problem, so I arranged a date and we meet at a specified location within my estate and off to his residence.

Upon arriving, the television which was already packed in the store was brought over to me and before opening I decided to apply power to see if there was any sign of life on the TV.

After applying power I got no symptom of power getting on and therefore decided to open the set…before opening the set the owner give me a disclaimer that the TV was from another technician.


I continued to open the set and after brief scanning on the board I noted the control I.C was missing, not even the bolt was spared.

Off to the horizontal circuit and again I noted the HOT transistor was also missing and the PCB where it is installed was like someone was trying to sink a borehole as you can see below.
Also absent from the board was the Posistor and the fuse…
After seeing all this injustices done to this television I told the owner I cannot promise that I can repair the set and to my opinion the chances of its survival was 50/50 since I don’t know what else was interfered with.

He asked me if I can try and if it doesn’t work so be it. With that I decided to take with me the motherboard to my shop and the first thing to do was to Google that model so that I get the missing part numbers.

I was not able to get the exact model and the nearest I got was LG 21FUILR.



Looking at the components on the horizontal circuit on the circuit diagram above almost resembled the one pictured below and therefore decided to use this model as my guide for the missing components.


With that I decided to go to the shop to buy at least these two major components (i.e. STR W6554A and HOT 2SC6093)

Going to the shop I did not get the HOT 2SC6093 and therefore I bought transistor TT2170 which has the same specification.

Back to the shop with my two components, I installed the control I.C STR W6554 and applied power using the series bulb and the bulb was bright then low.

This confirmed to me that there is no short on the primary supply too, Alas! Looking at the power On Led was also on...

This again confirmed to me that the power supply was in good health and there is also voltage on the secondary side.

With that my focus now was on the secondary side, the best place to start troubleshooting the secondary side is testing the B+ voltage.

I started by testing  across the cathode leg of the B+ rectifier diode and I noted the voltage was over 114 volts but pulsating, this is not a good sign since this line is highly regulated.

Then I tested across the middle pin of the HOT transistor I found less than 3 Vdc.

So what is blocking this important voltage, to find out I decided to check backward and I bumped on a big resistor.

Testing across this resistor I got one side 3 volts and the other side I got over 100 volts and still pulsating.
I lifted one leg of this resistor and testing with my meter set to the highest ohm setting i found it open.


Looking at the circuit board I noted it was labeled FR403, which means it was a Fusible Resistor and hence its main work on that line is acting as fuse for the B+ line.

This lead me to conclude that for this fuse to open there must be a shorted component(s) on that line and the main suspect would be the HOT transistor, snubber capacitor or the fly-back itself.

I did a ring test with one probe of my FBT tester on the B+ pin and the other probe on  the ground(emitter) and I noticed the tester LED were ON and therefore ruled out any problem with the fly back transformer.

Before installing the new HOT I decided to do thorough testing on the components of the horizontal circuit. I decided to lift one leg up of each component for best results.


All diodes were found okay but I was not happy with the snubber capacitor which was rated 183J/2000V...this means the capacitor was rated 18000pf same as 18nF or 0.018uF. By 2000 volts.

To learn more about capacitor body marking meaning click the link below.


Testing with multi tester this is what I got…..


As you know this capacitor is also called safety capacitor and its work  is to suppress (snub) induced high voltages by the collapsing magnetic field in the fly-back transformer primary winding which could otherwise lead to destruction of the Horizontal output transistor (HOT).
With this I concluded that even if I change the HOT 100 times it will still short since the snubber was faulty.

So I went to the shop and bought another snubber and before installing it I tested and I was happy with the results.

Note: Never assume every component from the shop is good and therefore it is always good practice to confirm before replacing.



I replaced the snubber together with the HOT transistor and now it was time to apply power and see if the Television was firing.

After applying power I noted the TV was going on and shutting down immediately.

I decided to do thorough testing on all capacitors on the secondary side and most of them were found to be out of range and I replaced them all.

Among the capacitors I changed one of them caught my attention because it has very high ESR reading as you can see here below.

 

This capacitor C847 was rated 1uF/250volts had an ESR reading of 49 ohms, a good one should be less than 10 ohms.
 

I believe even if I could have replaced all the component on this TV and failed to pick this one capacitor this TV could still have ended in the dust bin…that is why I always advice technician to invest in an ESR meter.

I replaced the capacitor and having no other suspect component I decided to go back to the owner to do live testing with the tube installed.

I re-installed the motherboard and applied power using my famous series bulb and I was happy with the results and therefore plugged the TV directly from the power source and the TV came up beautifully in front the owner and the entire family as witness.

It was at this point that the owner told me that he wish he had known me earlier, he told me the previous technician who came to repair were two and spend 3 days  8.00 am to 6.00pm trying to repair before giving up.

That explain why they took off with the vital components maybe thinking the TV has no hope of working again and therefore was only good for spares.

Thanks guys
Stay safe

Humphrey Kimathi






11 comments:

  1. Well done. Well persevered. Also, why don't you simply take the whole set back to your shop and then you can test it fully before the customer see's it, especially as it had already been messed with by someone else?

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    1. Hi Martin, Actually it was to my advantage because if the customer had not seen the mess done and the missing parts maybe he could have concluded the set has a minor problem. usually i like confirming what the customer said was the problem in his present, i have seen a set taken to a technician with color problem but when back from the tech it dead...most customers will conclude that are the cause and therefore it good practice confirm before taking away the equipment..otherwise thanks for your input Martin.
      Regards Humphrey

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  2. Hi Mr Humphrey, thanks for this article which made my day because I like how you go step by step on your diagnosis of the circuit and the parts.looking forward for more articles and thanks for this one.

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    1. Hi Marco, Thanks for your feedback and i am glad to see you here..sure i will be right back...watch this space!
      Regards Humphrey

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  3. Hi Humphrey, I really like your courage when it comes to handling very badly handled TV sets by other technicians. Imagine how you take up a job which other Techs have started and messed up to the extent of stealing parts leaving one with no trace of the exact part numbers to use for replacement. I have learnt here that one must not give up easily but accept such challenges to become more experienced in the repair business. I like your systematic approach in troubleshooting and have learnt to be patient even when initial test seem not to be fruitful. Thanks once again for sharing with us from your wealth of experience. God bless you Humphrey. Regards, Moses Olaki, Juba, South Sudan

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    1. Hi Moses, Thank you once again your feedback and passing by. God bless you too my brother.
      regards Humphrey

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    2. Oh my God, i admire to be like HUMPHREY,
      ''SINCERE, ACCURATE, INDEPTH UNDERSTANDING AND TRUSTMORTHY''

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    3. Oh my Oh my! Glory to God for the talent
      Regards Humphrey

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  4. Sir h0w to trace vertical problem becouse of arching of fbt regards

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  5. sir what is the posible reason ...my tv wont turn on during night time..i mean peak hours...but during day time it operate smoothly... pls answer..im jay from philippines..thank you

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    1. Check faulty capacitors and dryjoints Mr Jay

      Regard Humphrey

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