32” LG LCD TV (32 LG 5000-ZA), no starting repair
This set belongs to our building manager who is a good friend of mine.
About one month ago he told me that he had starting problems with it and some days after that he told me again that the trouble disappeared by itself… I answered him right from the first time I heard his complaint about it that his set had problems with its PSU and this will worsen over time no matter if it seemed to be cured by itself.
Well, one week ago he told me again that the set refuses to start at all. He could only see the standby red LED light flashing, but no further change of it in a steady blue color which is a sign that the unit started normally. He brought the set to me and asked me to see what the problem was with it and then repair it. The model number is shown below
I opened the set for a quick optical check and before doing anything else I tried to start it. Its overall internal view is shown below
I saw the same effect: only the red color LEDs flashing at the front panel and nothing more. The PSU this set uses has its big and heavy components on its top side, as you can see below,
and its SMD components at its back side, as shown below:
Then I removed the power and left it for a while waiting for the filter capacitors to discharge. I started then with static measurements on the various power components. No defect found. Also, at least optically, there was no apparent problem with bulged capacitors. Everything was in order.
I removed afterwards the PSU for an extensive check of it. I measured all the semiconductors on it. No defect found. The situation seemed already weird …
I re-connected the unit again on the chassis and tried to start it, using a hair dryer and carefully heating its components. Heating a specific spot, around this big cap shown below, the unit started normally.
I had already tested the electrolytic caps for excessive ESR and I found this cap along with another one being in marginal but nevertheless accepted condition.Anyway I removed the PSU again and replaced them both. The one you see above is the new replacement cap (C512, 100µF/50V).
Then I reconnected the PSU and tried to start the unit again. No result. After reheating it, the test led me to a MOSFET nearby. I happened to have a readily available spare of the same type and thus I replaced it. Then I retried again. The same effect…No starting without the heating aid…
I thought then that I had a problem with some SMD components located exactly on the back side of the PCB, at the spot I was heating it from upside. So I removed the PSU again and tried to resolder all the suspect circuit in that area, removing as much quantity of that problematic lead-free modern solder it had on it as I could and replacing it with ordinary one.
Upon completion I tried it again. This time the result was disappointing indeed… No effect at all. No red light flashing…Absolutely nothing! I rechecked the rework I did using a magnifier lens and searching for possible shorts when resoldering (no matter that I already did this beforehand). No problem at all. All joints were as they should be.
Now what? As usually I had no schematic to go on. I thought to start with the basic information I could get through the web by searching for the relevant data sheets of the SMD ICs used in the back side of this PSU. It was really weird that I lost the standby power supply after my rework on it, having at the same time improved the quality of its solder joints.
After searching in the web I found all the basic info I needed and in order for me not to lose much time for the repair I ordered the three types of ICs this PSU makes use of, to a local shop which sells electronic parts. When I got them I tried to go on with it starting first with the standby supply. The driver IC of this stage, is a NCP 1207A. You can see this stage below
I checked its power supply voltage and I found it O.K but there were no trigger pulses at its output pin to drive the relevant MOSFET. After I replaced it, the stand by power supply was restored and I had back the 5.1V d.c, the 21V d.c for the regulated and switched 15V supplying the rest two I.Cs along with the 16V d.c used for the production of the switched 12V, but nothing else was working.
The situation now was very difficult for me in order to go on without a schematic in my hands. I decided to perform an overall rework on the solder joints… After finishing, checking for shorts and cleaning the PCB with flux removing spray I tried it again. No improvement.
Then I stopped there and searched in the web for a servicing manual. Finally I found a manual which I downloaded and started searching in it in order to find some helpful info.
This ended in another disappointment for me because the only info therein for my case was a simple logic diagram ending with the usual expression “replace the PSU”…There was no PSU schematic in it at all!!! Only parts of it located in the main board, the T-con etc.
I called a friend of mine who owns a repairs’ lab far away from Athens and works officially as an LG service center and asked him if he had something in his mind about this failure. His answer was even more disappointing:“I know that there is no schematic in there” he said. “I am also working without schematics”. And how do you manage the repairs I asked him. The answer was of course as expected…”Don’t bother yourself by performing component level troubleshooting. Replace the PSU. It costs 75€”. Then we stopped the discussion there.
The point is that I had in my hand a PSU unit in perfect condition with all its power components intact and with a mysterious problem which I considered as minor right from the first moment I spent in it.
Once again I was forced to analyze the circuit in order to locate the problem…
I started with the original observation I made, namely that if the standby PSU wouldn’t start, then nothing else could happen as the power supply for the control circuit of the motherboard and the basic 15V d.c supply needed for the rest two I.Cs was coming from the standby supply.
Now I had this voltage of 15V d.c available, but nothing else was working. Having the PSU completely disconnected and removed from the set, I started sketching the switching circuit which energizes the active PFC correction circuit. The circuit is shown below
And the driver I.C of the active PFC stage is shown below:
Following the PCB tracks, I spotted the transistor switch stage for “power on”. So I knew now that if I apply a 5V d.c artificial signal to the base circuit of this transistor by hand, this would equal to the “power on” signal of the main processor. This point on the other hand ends in the connector terminal marked as “power on”.
In the same way, following the PCB traces each time, I spotted the PNP SMD transistor switch (Q103) which had a marking of 2F on it and was responsible for powering the P.F.C stage of the PSU (which rises the voltage after the rectifier filter cap from its 320V of nominal magnitude, to 390V in order for this stage to function correctly). This stage was dead so far.
It’s the one just above the black squared painting shown in the second sequentially photo above, depicting the overall SMD side view of the PSU. Its driver I.C which performs the active P.F.C function is a SG 6961SZ as shown just above.
And the last IC for the switched auxiliary voltages of 24V d.c and 16V d.c for the supply of the display inverters etc (NCP 1396 AG), although it was getting the switched 15V, had no other reaction obviously due to the action of some protection signal, of course when I was testing the PSU before removing it. This I.C is shown below:
So it was a good starting point to begin troubleshooting by searching for the reason that this Q103 couldn’t switch those 15V to energize the PFC stage.
This transistor had its base controlled by an SMD I.C (U101) having the markings E1 on it. One of its terminals was connected to the cathode of one of the four optocouplers of this unit. Based on my experience I realized that this IC was the SMD version of the well known TL 431 used in such cases for regulation purposes. Later on I verified this. It was the same chip indeed.
Now I had all the info I needed in order to go on. You can see this stage below. The U 101 is located at the down-right side (corner) of the photo. It is the last component shown there. The Q103 is located at the same horizontal line of components, at the middle of the picture.
I connected the PSU to the line power having connected in advance a 40W lamp in series with it for safety reasons. I verified again all the available voltages. Then I connected a small piece of cable at one point I was measuring the standby 5V d.c, with its other end free for the moment.
Then I connected my multimeter to the switched 15 V d.c for the operation of the auxiliary supply and giving a “power on” signal to the transistor I spotted beforehand, I saw that I had the voltage output feeding the NCP 1396 I.C. I verified that this voltage reaches the relevant Vcc pin of this chip, but again possibly due to an acting protection signal there was no further function of this I.C.
Then the remaining key question was why that Q103 was not operating at all in order to power the active P.F.C stage. Of course I had already realized that the PSU would work normally only if this stage would be in operation.
Following the sketched circuit I had done, I was measuring the unregulated 21V d.c. supplying the anode of that optocoupler, but at its cathode I was measuring only one volt less which (by experience again) is a very high voltage for a stage like this one. I measured the current through the circuit by measuring the voltage drop across the terminals of the 10KΩ resistor feeding the circuit and measured zero volts.
Then I measured the voltage at the reference terminal of U101, expecting to see 2,5V there. This was also measuring zero volts! Now I had a vital info…
I followed the foil trace and stopped to the point of R109. As you can also see in the above picture the place of this resistor is empty. It is the one at the left side of the grey SMD cap shown in the bottom horizontal line of components, just above the wire bridge J21.
Its lower end was measuring 11,7V, which was also too high in comparison to 2,5V I expected to see there.
I followed the foil trace again and verified the values of all the resistors I saw up to the power supply point where they were finally connected. They were all O.K as shown in the sketch above. Then I decided to bridge the empty terminals of R109, but before doing that I removed the power again and observed this empty place again, using a magnifying lens and supplying a lot of light in this spot. There were visible remains of that red glue which is used in order to hold the SMD components in their place before their hot air soldering during the production process. I checked some other empty places of SMD components and there were no signs of red glue on those. Now I was absolutely sure that a resistor was put there which (inexplicably) was missing now.
According to the sketch I did, this should be a zero Ω bridging resistor, like the rest of them in the circuitry having the marking 000 on them. So I removed a resistor of this type from a salvaged PCB I had and put it there. (The solder remains you will see on the empty terminals of R109 in the above picture are there from the bridging tests I did).
Then I reconnected the PSU to the power line and gave it the artificial “power on” signal. All the missing voltages were now there, the active P.F.C stage started normally rising the voltage at the terminals of the bulk capacitor to 390V and the 11,7V I was measuring previously at the node before the free reference terminal of the U103 dropped now to its normal 2,5V value. The repair was over and the normal operation of this set was restored, as you can see below.
You can also see the defective components I replaced in this PSU unit below,
Then, trying to conclude with this puzzle, connecting all the events together, I remembered again the sayings of the owner of this set when he was telling me that “the TV starts once per each fifty attempts”. Apparently, when this R109 resistor was making a good contact with its connection terminals (depending obviously on the room temperature at the time of trying to start it) the set was starting quite normally. On the contrary, when this good contact was absent, the set could not start and only the standby part of its PSU was in normal operation, trying unsuccessfully to start the set…
What really happened with this set? Here is the sequence of events.
This resistor was so very badly soldered therein that over time the set worked “when it liked to work” as its owner told me…!! During my first rework on this PCB, apparently it dropped from its place and disappeared without myself to notice that event. Besides I didn’t find it anywhere although I searched the entire area very thoroughly for finding it… On the other hand, during my rework I was very careful and focused on the job and didn’t notice any accidental removal of this component when resoldering. I only made two accidental solder shorts which I noticed and removed immediately during the rework process.
Then, after the rework I did and for some unknown to me reason, the standby controller chip suddenly stopped working. Meanwhile this resistor was dropped somewhere, probably during the heating tests I did, and when I finally replaced it the proper function of the unit was restored.
A minor cause indeed, as I originally had estimated during the aforesaid heating tests since the PSU was starting normally by heating it, which nevertheless caused me so much trouble. Anyway it ended in a successful repair which I hope that you also enjoyed reading about it!
After this final verification which was free of any other problems, I reinstalled its top cover closing the unit. The repair was successfully over and this UPS was ready to be put in normal operation again making my good friend Makis very glad with its survival…
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
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Note: You can check out his previous repair article below: