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 382.7
24v - reading 24.13v (Inverter supply)
*13.5v - reading 13.92
16v - reading 16.65v
5v - reading 5.11v
3.3v - reading 3.28v
LCD POW - reads 3.27v
*connector pin says 12v, regulator voltage is stated as 13.5v
The 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.