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Servicing LG Monitor Model 500G
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!
Downloaded the service manual from web. https://www.manualslib.com/manual/700188/Lg-Studioworks-500e.html
On checking the board at my desktop, I noticed that one capacitor was bulgy, in the secondary side of power supply, 1000/10V.
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:
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!
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)
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-.
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)
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.
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:
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!
(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: