Samsung PN50C450 YSUS Board Diagnosing Help

[jimg1948]

I have a Samsung PN50C450B1D that would not power up. The LED on logic board blinked 5 seconds on and 2 off. This indicates that the problem is on YSUS or XSUS board. Y Buffer board tested fine. Ordered used YSUS(LJ92-01728C) and Y Buffer Board(LJ92-01729A). Installed both and TV works fine. While waiting for boards, I started to diagnose the problem on YSUS board. I would like to fix and keep as a spare in case the board I bought craps out. IGBTs and power diodes all seem to check out fine. While probing resistances on the board I found a low resistance (4 Ohms) between pins 4(VCC) and 6(Float GND) on two opto isolators which are the IGBT drivers for YR and YPS signals. Both these opto isolators get voltage from a multi tap tranformer through a diode and electrolytic capacitor to floating ground. I have attached a picture of the board highlighting the areas of concern and a hand drawn circuit diagram to help understand what I am talking about. I removed the electrolytic capacitor thinking that it was the most likely candidate. It did not solve the problem. Before I pull more parts, I thought I would post here for any help. The questions I have are:

1. Has anyone encountered issues with the opto isolator IGBT driver circuits on Samsung TVs? If so what was the culprit( i.e. capacitor, opto isolator, dual NPN/PNP transistor or diode?

2. Is there a method I can use to further diagnose the issue or should I randomly remove parts until I hit the bad one? Is there an order I should follow (i.e. caps first, optos second, dual transistors third?

3. I noticed possible damage around the YR circuit (see CloseYR picture). Near the heatsink the circuit path is copper color instead of green like the rest. There is also white stain in same area. Should I focus here? Is this kind of damage typical? If so what does it represent?

[Biruslapio]

Re: Samsung PN50C450 YSUS Board Diagnosing Help

I have an LG 50PA4500,the lower buffer shorted itself(i'm hoping that's not a panel issue),and damaged the Y-SUS,checking it,i found 3 IGBTs shorted,replaced them,also another IGBT and a diode just for safety,powered on,vscan and -vy appeared,but no display.
checked the Y-SUS again,measuring for shorts between ceramic capacitors,found one measuring 1.6 OHMs,it was in parallel with an optoisolator(155E),since it wasn't an exact short i bet on the 155E,removed it from the board,and it was shorted between emitter,collector and output,it wasn't near the shorted IGBTs,but died anyway,and also an open 2r2 resistor,i haven't found a replacement for the 155E,i'll buy a defective board,it's a safe bet that i'll find a good one.
when you get low resistance but not exactly zero OHM it's probably a transistor,and these optoisolators have at least one equivalent of a transistor inside them,they usualy "drive" an IGBT,which could also be shorted.

[R_J]

Re: Samsung PN50C450 YSUS Board Diagnosing Help

LTV-155E is available from most suppliers, here is the pdf in case you need it.

[Biruslapio]

Re: Samsung PN50C450 YSUS Board Diagnosing Help

LTV-155E is available from most suppliers, here is the pdf in case you need it.

I also have this datasheet,but to get an 155E in Brazil is a whole different story,2 months shipping from china,a new one here is more expensive than a scrap board that has 7 of them,used and all but i risk it.

[jimg1948]

Re: Samsung PN50C450 YSUS Board Diagnosing Help

Just to help others that may encounter a short across the opto-isolaters I thought I would provide an update on my progress. I took a brute force method an started cutting each part out until I found the bad part. I started with the opto's, transistors, and then caps. Murphy's law took place and it was the last part selected. It was a blue ceramic capacitor the was causing a 4 ohm resistance across the opto. It was a 10uf 25v capacitor with board marking c5403. Hope this might help others with the same problem.

[tw2005]

Re: Samsung PN50C450 YSUS Board Diagnosing Help

Just to help others that may encounter a short across the opto-isolaters I thought I would provide an update on my progress. I took a brute force method an started cutting each part out until I found the bad part. I started with the opto's, transistors, and then caps. Murphy's law took place and it was the last part selected. It was a blue ceramic capacitor the was causing a 4 ohm resistance across the opto. It was a 10uf 25v capacitor with board marking c5403. Hope this might help others with the same problem.

Interesting feedback, I've had trouble with the Xmain for one of these, kept failing so I've now shotgun a whole bunch of caps and it's already run longer than previous, hoping a failing cap was it.

[jimg1948]

Re: Samsung PN50C450 YSUS Board Diagnosing Help

Interesting feedback, I've had trouble with the Xmain for one of these, kept failing so I've now shotgun a whole bunch of caps and it's already run longer than previous, hoping a failing cap was it.

Do you know where I can get a replacement part for cheap. The part is not expensive but shipping it sure is. Do you have a part number for a replacement. I have been looking at the TDK FG26X7R1E106KRT00. I am assuming a 25V cap is ok.