Samsung PS50C91HD buffer chip issues

[chrismurphy]

I have 3 of these TV's and all have the same problem.
All have the top buffer ,top chip faulty (shorted)
I have replaced all three buffer chips and all TV's are now working , all three Y-SUS boards all were ok . It seems that when these chips go, it does not cause any issues with the Y-SUS boards
Does anyone have any idea why the first chip on these TV's always goes. My mate has repaired many of these all with the same fault. Is there anything on the Y-SUS board i should look for ,or is this just a design fault. Any info would be gratefully appreciated.

[ReeceyBurger123]

Re: Samsung PS50C91HD buffer chip issues

The first IC fails because the Ymain, so you probably have 3 bad Ymains have you even checked them ? The first IC fails because it's addressed first which means if the Ymain is faulty the top Ic blows. Check the Ymain for S/C and get back to us !

[chrismurphy]

Re: Samsung PS50C91HD buffer chip issues

All 3 ysus boards have been checked and no issues found
they are in the 3 sets and working ok
The question was, is there something on the ysus that may cause issues because i cant believe that all these sets have the same buffer problem unless something i have overlooked is caused the buffers to fail ,they dont run hot so my guess is voltage is taking them out. Between my service mate and myself we have repaired around 7 of this particular set and ALL had the first buffer chip failure but no ysus failures ....very strange ,i think it has to be a design fault

[tw2005]

Re: Samsung PS50C91HD buffer chip issues

I could be wrong but i think TOM66 had an idea as to maybe why and was suggesting the address voltage was very close to the thresholds at startup. but to find that particular post or thread would be challenging. No idea what keyword would help and the guy has over 30 000 posts!

But I do stress it's a very fuzzy memory i have

Interesting to hear all the ymains were ok. quite often they go with it with the 88N30W FETS go, + bad caps I think 680uf? some can have a lot more but I think I did 2 or 3 and all needed the ymain worked.

[ReeceyBurger123]

Re: Samsung PS50C91HD buffer chip issues

Normally what happens is the Vset up ramp is 400+ volts which is almost double what the buffer Ic's can take however as they are essentially switches inside and are separately addressed, it relies on them breaking over internally and going to the next very fast before they dissipate enough energy to explode.

Samsung messed up here as they wait for the second side IGBTS to be switched after the Vset up ramp is applied but if they short which normally the 88N30W type do as they fail to switch fast enough causing S/C between the S G D on them all kills them all. Then the 400v is sent to the buffer and it dies, the reason why it doesn't shutdown is because the Smps doesn't detect a short until after its tried to load the Vset up but the output is shorted to the failed fets. Some times the buffer does short which also creates a S/C between the IGBTs but normally they only ever fail in my experience.

Samsung shitty engineering basically, they should have added fans to these TV's as they run too hot which fatigues the fets overtime and then they fail causing hell.

[chrismurphy]

Re: Samsung PS50C91HD buffer chip issues

Thanks for that ,i might add a fan to the ysus board
I have added a heat sink to the failed buffer just in case
Thank for all ,i thought it may have been a design fault ,hopefully they last a few years