Quote:
Originally Posted by YamahaFreak
Just to be safe, I'm asking once again where exactly my probes are supposed to go. :P (Across the cap, or across that string of resistors?)
EDIT: Checked Vscan voltage across resistors and got the correct 133 volts. I also pulled the buffer boards (XL, XR, XC) and checked visually for anything fried/burnt, and all looked good. (Are the buffer ICs the rectangular units embedded in the ribbon cables? There are 16 of them, one for each cable!)
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Those resistors are the bleeders for that capacitor and are connected in parallel with it so measuring across either the capacitor or the resistor chain is the same thing.
I can't see any ICs in the ribbon cables in your photos. [Edit: oh, I see you were talking about the X address boards, left, centre and right. They should be fine, I'm talking about the ones along the lefthand side, top to bottom.)
The Y buffer chips (actually they are high voltage shift registers) are the 10 small square chips (they look like they have heatsinks on them in your photo) along the lefthand side of the TV from top to bottom on the two narrow boards (upper and lower Y-Drive boards in LG parlance) that connect to the Y-Sustain board and then connect to the flex cables that go to the panel. Their purpose is to take the Yout signal generated by the Y-Sustain board and connect it in turn to each pixel of each row starting from the top down. The X axis columns are addressed by the boards along the bottom and aren't likely to be involved in this problem. (This is what the Va voltage is for, the "address" voltage that is used by the bottom boards to address each column using the row address electrodes in the panel.) The Yout signal blasts each pixel in turn up to 10 times (at something like 200A peak current!) to light each one up to the required brightness. (This is the 600Hz "subfield drive" that you hear about.)
If there are accessible solder points on all those traces or connector pins on the back of connectors that lead to the flex cables that run down the leftand side of the panel, put your multimeter in continuity beeper mode and put one probe to the Yout signal and run the other probe down all of those points where it runs to the flex cables to the panel in turn. There should be no continuity. (TV OFF, of course!) One trace goes to each row on the panel through the flex cables.
The Yout signal is on most of the connector pins that connect the big Y-Sustain board to the two narrow Y-Drive boards... Look for the pins that are all tied together on big fat traces. Remember, the Yout signal does up to about 200A pulses for each pixel so it needs lots of those connector pins to be able to do that current even in very short pulses so it should be very easy to see which ones connect together and are the actual Yout. There will only be a few pins that are NOT Yout, (basically just a few like 5V and GND, a clock line to shift the shift registers (buffers) and a reset line), so it should be easy to find the Yout since it is most of the pins.
If you don't get any shorts on any of those lines that run to the panel all the way down, your upper and lower buffers (the Y-Drive boards in LG-speak) are probably just fine. If there are any shorts (often just the very top row) don't put a repaired Y-Sustain board in there or you'll probably blow up the good Y-Sustain board...
LG buffer designs tend to be much less prone to failure than say, many Samsung models, but it is still good to check them anyway when something has fried on the Y-Sustain board just for good measure.
I would always also check any discrete transistor and diode looking devices for shorts also while I'm at it for good measure but in models using the IPMs generally all the faults will be contained within the faulty IPM. To check any for shorts anyway, simply use diode check mode on your meter between each combination of two of three pins forward and backward. (Two pins if it is a diode, obviously.) You should see a diode drop (.5-.7v) one way on most of those... Any that show low or 0 voltage drop are suspect for being shorted and should be removed and tested out of circuit unless there is an obvious circuit design like a low value resistor across it that would make it show 0. Generally if it looks shorted it is probably blown. Again, just good to check for obvious failures while you're at it.