Servicing LG Monitor Model 500G

By on August 15, 2017
lg crt monitor repairing









servicing lg crt monitor

This LG CRT Monitor was brought to me with the complaint that occasionally there was firing noise inside the set for some time and now it is dead. As usual, I opened the set, discharged the anode of CRT and did a thorough cleaning. I do not spare any part and even the back cover will be made as clean as possible!

fix crt monitor

Downloaded the service manual from web.

On checking the board at my desktop, I noticed that one capacitor was bulgy, in the secondary side of power supply, 1000/10V.

repair crt monitor

Replaced it and checked further. Noticed that the B+ supply section was shorting, when checked with analogue Multimeter.

Quickly traced it to a mosfet, IRFS630A, which was short between Gate, Source and Drain:


Replaced it. Did a thorough dry solder patch up. I had to remove the shield from the CRT PCB. Look at the shining board after cleaning it:

how to repair crt monitor

What I like in this set was the very clear markings on both sides of the board! Even in the microcontroller, the pin functions are written! Voltages are also written! Very useful for trouble shooting! This is a class design, I can certainly vouch!

crt monitor repairing

After this, I disconnected the jumper wire to LOT (FBT) and connected a 60W bulb as load. Switched on. All B+ voltages were ok. So connected the jumper wire, fit the PCB properly. Switched on the set, the display came, along with firing noise and arcing at the anode wire, near the joint where it goes inside the FBT. I could clearly see it! Switched off the set immediately. When I checked with my friend, who is an expert in doing CRT Monitor, he told me that if it was LG, the problem of arcing at that spot is common and the only solution is to replace the FBT. So discharged the anode and once again dismantled the board to remove it. (You can see the number clearly)

lg flyback transformer

Got an exact replacement (See the third picture above). Fixed it, and fit the board once again. I would like to share one interesting information on this FBT. This has a wire wound on the FBT and both ends are taken to SMPS hot end for feeding to PIN 4 of 3842 PWM Controller IC through components for oscillation check. In this, the one end coming from the left has a black sleeve on it. That goes to hole marked S+ and the other to S-. I think we need to be careful on this connection, otherwise the designer would not have marked these on the board. I have also given the picture of the new FBT, where this is not done. But while connecting, I inserted the left end in S+ and right end to S-.

how to fix lg crt monitor

I had also replaced the following capacitors on the board, as a precaution since the problem was high voltage arcing. My practice is to remove these at a stretch, after checking its value, using a special mirror, whenever necessary (See picture of these two tools that I use, when we cannot directly see the reading. One is telescopic and can be extended up to one meter) If the surface of the component has collected dust, I use another tool to clean it up, which has cotton bud at its tip (See the third picture below)

how to fix lg monitor

Switched on, green LED started blinking and suddenly smoke came out of another 1000/16V Cap in the secondary power supply! Switched off! Discharged the anode and removed the board once again! Removing the PCB is not an easy job. The PCB sits inside a metal tray fixed underneath, in which the rotating stand is fixed. Without this, the CRT cannot be kept upright. The bottom of the PCB is blocked by this tray. The top portion is not easily accessible. Four earth wires are inserted onto the shield of CRT PCB. These are to be removed followed by the CRT PCB, Yoke connector, Degaussing coil connector, anode connector etc.

We should also remember to discharge the anode at the cap side, if we do not want to ‘enjoy’ a free nasty shock! Checked the bottom of the PCB to locate where the capacitor was connected. It was in the 5V path. Since a capacitor will burst normally only when higher voltage passes through it than the limit specified, I suspected that there could be a solder overlap from some other path. The next path that goes near to it is 80V. Checked with a lens and found a hair line solder overlap from that to 5V. Removed it. Checked for any component failure on both B+ lines. Could not find any. Reconnected the board and switched on. The filament of CRT started glowing. Since when replacing the FBT, I had kept the Focus and Screen Controls at minimum, slowly increased the screen voltage to get a display and adjusted the focus properly.

monitor osd menu

As you will notice, the display travels on the surface from top to bottom in Red, Green and Blue (RGB) in sequence. The display showed ‘Self Diagnosis /No Signal’ in some other language. Kept it running for a few hours. As there was no problem, connected it to my computer and checked. It was perfect:

lg crt monitor servicing

Here is the monitor ready for delivery, kept on my sit out, after I wiped it clean once again, with a caring ‘motherly love/touch’, and wishing it ‘long life’ after recovering from a serious ‘illness’! This is not a joke; I almost fall in love with the sets that I service! I feel ‘pity’ on its condition when ‘admitted’ and feel so happy when ‘discharged’ back to normalcy! Probably I am a sort of some mental (LOL), but I am very comfortable with these feelings! A job done to my fullest satisfaction!

how to repair lg crt monitor

(I think I may have to remain content with servicing CRT Monitors or small size CRT TVs in future, as I had a sudden lumbar sprain, while trying to pick up a screw driver from the sofa, in an absolutely normal and my usual way! But fortunately, recovered fully by Ayurvedic Treatment within a fortnight. But Doctor has cautioned me not to handle heavy things, though I have returned back to normal condition! So, sadly, I had to decline accepting CRT TVs for repair to a number of customers! What else to do!? )

This article was prepared for you by Parasuraman Subramanian from India. He is 68 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done  graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.

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You may check on his previous repair article below:




  1. Robert Calk

    August 15, 2017 at 1:21 pm

    Good job, Parasuraman. Nice article.

  2. James

    August 15, 2017 at 2:06 pm

    You Rock man'' 🙂 LoL and you are very dedicated, wich is a good thing and may even give others the will to aspire to be like you. Thanks for sharing your very good repair jobs.

  3. Colin

    August 15, 2017 at 3:39 pm

    The wire that you mention that goes from the FBT back to the power supply switching IC is normally used to synchronize the line output and the PSU frequencies together. This technique reduces noise on the picture due to the otherwise free running frequency of the PSU. The PSU will normally function correctly without this wire connected but the display quality will be degraded.

    • Parasuraman

      August 16, 2017 at 11:06 am

      Thanks for this input!

  4. Suranga Electronics

    August 15, 2017 at 7:03 pm

    Hi. Mr parasuraman , well done ...nice repair job.

    • ANGELO

      August 16, 2017 at 12:15 am

      hello sir suranga
      good evening to you

  5. Gary Gemmell

    August 15, 2017 at 11:37 pm

    Very comprehensive and expert repair.
    I love the attention to detail so lacking in todays modern board swapping and throwaway society!


    August 16, 2017 at 12:14 am

    Dear sir

    when more than 5-7 caps are replaced i simply assume
    that this is none other than sir "Parasuraman Subramanian"
    cause i have noticed that you always try to provide
    a new life to the device that you work on..
    i appreciate your hard work sir
    you all the seniors are our inspiration "May God bless u all "

    thanks to sir "jestine"
    for this wonderful site

    what happens when the "FBT" goes defective ?
    i mean what are the symptoms?
    thank you

    • Parasuraman

      August 16, 2017 at 11:01 am

      Almost all the secondary power supplies generated by FBT will be missing, as a result of which the set will be dead with either LED blinking error codes (if programmed that way) and/or totally dead, if the Horizontal Output Transistor (HOT) has failed and fuse blown. In a common circuit only preliminary voltages required are generated by the SMPS. The one word answer to your question (assuming that by 'defective' you mean total failure) is dead!

      • ANGELO

        August 17, 2017 at 7:21 am

        Dear sir
        thank you very much
        yes i mean if the fbt is totally dead what will happen then?

  7. Yogesh Panchal

    August 16, 2017 at 12:25 am

    Repairing a CRT TV and monitors is really a FUN!!
    thanks for sharing your experience.

  8. Henrique Jorge Guimarães Ulbrich

    August 16, 2017 at 3:05 am

    Thanks for this nice article.

  9. Albert

    August 16, 2017 at 10:55 am

    First let me say to please make sure that you place your health above any device that you repair and clean into extreme and profound ways, Parasuraman Subramanian! And we can't save any device that breaks down. After all we are not getting any younger either and Ayurvedic Treatment sounds serious! (if it was not just a probably wonderful Ayurvedic massage? LOL). Great article of course! I still do own several old CRT multisync monitors myself I do not want to depart off. Like a Targa TM1480 and a Microvitec Amiga M1438S monitor. Because unlike todays LCD monitors they are capable of handling much higher frequencies above 15KHz upto 38KHz horizontal and upto 90Hz vertical! Which makes them ideal for my Amiga's that use other frequencies than normal VGA monitors are able to show.

  10. moshe jacobson

    August 16, 2017 at 2:11 pm

    This was a pleasure to read.
    Thank you.

  11. Bibhas

    August 17, 2017 at 5:56 am

    Thank u for shearing a nice article. Thanks again. I wish to JESUS, that u must be recovered from illness as soon as possible. u are great. u are inspiration to us. thanks, long live sir.

  12. Tito

    August 19, 2017 at 4:55 pm

    Thanks for the comprehensive explanation of the problem solving Mr. Parasuraman. It's quite a class.


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