LG 50PA4500-ZM.BPFYLJP (PD23A) Plasma TV - Page 1
Forum section: electronics / repair

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 02, 2021, 10:36:49 am
content: I found this not working on the pavement.Power supply was reading low, removing connectors showed the Y-Sus to be defective.The original problem was Q501 (TK8A60DA MOS on Set_Down) shorted, likely due to very weak solders all over the PCB.Replacing this lead to Q403, Q404, D562 shorting on the next power-up. I gave it another try but this time Q401, Q402, Q403, Q404 shorted. I checked the floating gate drive power supplies in an attempt to understand why all this destruction but the problem wasn't there.At this point considering the cost, extra time searching and uncertain outcome I turned to look for a cheap replacement Y-Sus PCB.The original EBR75416301 wasn't available at a decent price but I found a EBR73747601 looking almost identical, listed as new and unused.On receiving the new PCB I spent a while checking the differences with the original one and found:-The large film capacitors in the middle between heatsinks total to 4µF on original PCB, on replacement they add to 6.6µF.-On P101 control FFC connector #7 "OC1_B" maps to #8 "OC1_B" on P151 connector to Y drive boards, on original PCB it's unused.-On P101 #24 "D_VY_EN" on old PCB becomes "PT_RST" on new one which is actually unused and tied to GND through a 4k7 resistor.On the new PCB what used to be "D_VY_EN" seems to be derived from "ER_DN" and "ER_UP" control inputs before being fed to a 393 comparator same as "D_VY_EN" was on the original PCB.So far I've only swapped the previously mentioned capacitors down to 4µF.While comparing boards I found that on the apparently new one R408, R410, IC402, IC404 were defective (Gate drive to Q403, Q404) the IGBT's were still in good condition, I swapped these components from the old board before testing.Testing the TV with the new (repaired) PCB shows a dark screen with the very rare lit pixels on the brightest picture contents. (At least nothing popped...)The panel PDP50T40121 is labelled:5.0V/ Va:55V/ Vs:205VN.A. / -190V / 150V / N.A. / 105VI have readjusted Va and Vs spot-on.Other measured voltages are:VZB: 105V (Z-Sus)VY: 150V (Y-Sus)VSC: 106V (Y-Sus)[EDITED and corrected]:VY: 191V (Y-Sus)VSC: 148V (Y-Sus)I'm not experienced enough to remember what the second line of voltages on the PDP label should match.Is my VSC low? Should that be reading 190V? I can't seem to find a negative 190V anywhere.Most of my PDP experience dates back to the LG 42V7 models so it's a little rusty. Help appreciated!

author: PKTKS
author_rank: Super Contributor
timestamp: March 02, 2021, 10:41:00 am
content: I haven't dealt w/PLASMA for a loong time...But you must find your 190 and -190V readingssomewhere inside the RED marked area..On the YSUS board.. need to trail the PCB to findthe regulators and which supervisor is there...Paul

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 02, 2021, 11:36:07 am
content: It seems I had the wrong reference point for the VY and VSC measurements their floating ground is used as reference point rather than frame.New measurements are:VY: 191VVSC: 148VSo that could be close enough to panel label if considering absolute values, no negative 190V to be found though.The larger transformer supplies the floating VY and VSC voltages, the smaller one the floating gate drive voltages.So assuming that all the PDP related voltages are close enough, what waveforms should I be looking at to decide whether it's a timing issue possibly related to "D_VY_EN" difference (which after double checking comes from one of the 393 comparator's outputs) or a dead panel?

author: PKTKS
author_rank: Super Contributor
timestamp: March 02, 2021, 12:07:47 pm
content: very hard to answer these questions..Usually you will need to place your scope on the Ysusboard **WITH GREAT CARE WHERE YOU HOOK YOUR GND...***And with a slow time base 5ms or 10ms check if theYsus high voltage PSUs are really soft starting...they seem to have a problem on starting..After carefully checking each and every one..soft starting .. why the nominal value is wrong..most likely to my experience are the supervisor faultyof just another damn capacitor.. leaking..Paul