JVC LT-42DR9BJ TV repair - Page 1
Forum section: electronics / repair

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 15, 2018, 02:18:10 pm
content: Some time ago I repaired a computer monitor with a little bit of help from the forums. The problem turned out to be a faulty inverter transformer which was replaced and the monitor brought back to life.Last week, my step son told me his 42in TV had failed with no picture and asked me to have a look. Confident after repairing the monitor I figured I could probably identify the problem with the TV as well, but unfortunately I have come to something of an impasse.The TV will power on and can be switched in and out of standby using either the control on the TV or the remote, but there is no image on the screen at all. It did flash up with a plain blue background at one point, but the display switched off almost immediately. Armed with the circuit diagram, I have checked the power supply and I am seeing all of the expected voltages, with one or two caveats:375v - reading 382.724v - reading 24.13v (Inverter supply)*13.5v - reading 13.9216v - reading 16.65v5v - reading 5.11v3.3v - reading 3.28vLCD POW - reads 3.27v*connector pin says 12v, regulator voltage is stated as 13.5vThe voltage for the line marked LCD POW is unmarked and I have no idea whether 3.3v is correct but if that's a high signal then it probably is what should be expected when the display is on?So in summary, I think that the power board is working Ok, but since there is some confusion on the diagram with voltages I cannot not be 100% sure. What I am seeing does seem reasonable though.I have checked the transformers and capacitors on the left and right (master/slave) inverter boards. Capacitors look Ok on visual inspection and all seem within expected capacitance and ESR parameters when removed from the board and checked with an LCR/ESR meter. All of the transformers seem OK. In-circuit readings on primaries varied between 0.13 and 0.21 ohms, and 998ohms to 1032ohms on the secondaries. The pairs of secondaries were at a variance of no more than 8ohms from each other on any individual transformer so there were no obvious imbalances.I have tried shining a torch at the LCD hoping to see a faint trace of an image, but am seeing nothing at all. It may just be difficult to see and I am working blind so its difficult to tell whether I have activated the correct settings on the remote, but even if I couldn't get as far switching to the correct HDMI input to displaying an image from a laptop, I had hoped at least to see maybe an outline of the main Menu or the EPG,I have one of these cheap tube testers and was hoping to test the tubes, but since there is a pair of connectors at each side of the display, I am wondering whether there are two tubes, one inserted from each side and half the wideth of the display, or one long tube covering the whole 40in width of the set with tubes arranged in pairs on each set of connectors. The hot on one side is matched by a cold on the opposite side, although HT transformers appear to supply hot and cold pairs per each side. In any case, I tried hooking up both ways but I am not seeing any light. This might be a problem to see anyway if the LCD is not turning on.Any ideas as to what the next steps to take to diagnose the issue would be appreciated.

author: Rasz
author_rank: Super Contributor
timestamp: September 15, 2018, 03:51:48 pm
content: try harder with the flashlight, plug some signal so it displays moving picture instead of blue backgroundmaster/slave inverter boards are like 5-10 pounds each in UK, you could just shotgun it if you value time

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 15, 2018, 06:54:14 pm
content: Yup, those inverter boards can be obtained quite cheap on eBay, however, I'm not convinced at the moment that this is down to the inverters. I am aware how difficult it is to see anything on an unlit LCD, but I am seeing zilch. I might have another go under different lighting conditions. Still, swapping the boards might be one way to rule out those inverters.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 16, 2018, 01:17:28 pm
content: I had a look at the display today in more subdued lighting conditions and although I am still working "blind" with the remote, I thought at one point I could make out a box in the top left hand corner. After a while I clearly picked up the text 'PR LIST EMPTY' in the centre of the screen. The lighting angle from the torch was very critical and there was still plenty of smudging visible under close up light despite me having cleaned the display.So it seems that the LCD and display controller are Ok. My next question is how to test the CFL tubes?I don't know whether it is significant, but before the TV failed, it had developed a fault where a shaded patch appeared on the screen after a few minutes warm-up.I will order a master board, but I just want to check one thing. The sellers say that the part number of the boardas well as the model of the TV setmust match. The boards I am looking at have exactly the same part number I420H1-20B-Master, but is the controller programmed for different brands and models, or will the board work in any set? I don't see a JVC branded one available. I have been stung with this kind of thing with auto electronics parts before which were coded for a specific vehicle. Is it the same true with TV parts?

author: Rasz
author_rank: Super Contributor
timestamp: September 16, 2018, 03:46:25 pm
content: same inverter pair goes into a list of TVs using one CMO panel, so I would say no as long as you get matching setI have no idea why they work in master slave configuration, and how its all wired.

author: PKTKS
author_rank: Super Contributor
timestamp: September 17, 2018, 12:53:46 pm
content: All models in this CHASSI line enters SERVICE MODE via "remote"(dont ask my opinion about it... )So as you still have the remote working try hard entering SERVICE MODEusing the remote to make the adjusts following service manualEach voltage reading should match cf mode **IF** the panel isstil working - or you may be lucky and got a service error code...

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 18, 2018, 09:30:55 am
content: Quote from: PKTKS on September 17, 2018, 12:53:46 pmThank you for this information and the extract from the service manual. I was not aware that this was possible.Quote from: PKTKS on September 17, 2018, 12:53:46 pmOk, I will bear this in mind.After quite a bit of searching I finally located the correct service manual for this specific model which may be of some help. I have also ordered a replacement master board as I suspect this to be the culprit. We will see how it goes when it arrives.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 19, 2018, 12:40:40 pm
content: The Master board arrived today. It is identical to the existing board so I gave it a try. Unfortunately it has not solved the problem. A couple of the capacitors on the original board were getting close to tollerance although still well within it so it seemed worth a try. replacing capacitors on the original board did not seem to work either.I suppose I could swap the slave board although that seems even less likely to be at fault.Not sure what steps to take next.

author: Rasz
author_rank: Super Contributor
timestamp: September 19, 2018, 01:05:48 pm
content: shouldnt you always replace those in pairs unless you can diagnose one being bad?

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 19, 2018, 03:26:40 pm
content: That's quite possible and to be fair you did say to get a matching set although you didn't sound too sure either. There wasn't one available for JVC and this specific model range so I guess my purchase was a bit of a shot in the dark, although at £6 it would not break the bank. A matching set has just come up today for £15 so I have the option to buy that I guess.In the meantime, with the help of the service manual, I have been able to get into the service menu and run a self check.It was a bit tricky, but this is what I was able to read:Code:LOB0MEM0AIO0TUN9HDM9TOS0HOUR082442SYNOKCDN01ADS00ADH00PPL600LPF137INT1FMT00FMDFFAPL00If that hour count is accurate, this LCD has seen 3435 days, or 9.5 years, of running time! TUN and HDM have a max failure count. TUN is the analog tuner and HDM relates to the HDMI ACK signal. I'm not sure whether the later is a source for concern? The other noteworthy items are INT and CDN both which have a failure count of 1. The remaining items seem normal and the good news is that none are NG (Abnormal).Of course, this does not appear to show the status of the inverters or the power supply so I still don't know for sure whether these have failed. I guess it will cost me a further £15 to find out for sure....

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 22, 2018, 12:56:04 pm
content: I eventually found a pair of inverter boards for this specific make and model and have now replaced both boards together. This has brought the TV back to life. After a bit of experimentation I found that replacing just one of the boards does not work. So it would seem that perhaps both are bad, or that you need a matched pair as you suggested.What I now find is that the background is very unevenly lit with one very large dark patch and several smaller ones visible on the screen. There were spread over a far larger area that could be accounted for by any single tube. The set is now working, so thanks for that, but I would appreciate any feedback from anyone regarding what these dark patches might be and how to get rid of them.

author: Rasz
author_rank: Super Contributor
timestamp: September 22, 2018, 02:59:14 pm
content: post couple pictures