EBR73712701 YSUS No Vsc

[tom66]

Re: EBR73712701 YSUS No Vsc

-Vsc = VFG, the Y-sustain board output
+Vsc = the VFG+VSC going to Y-sustain

Vsc is passed along the buffer boards, so the Y-main just connects the two together
The source is around the 10 ohm resistor and diode pair which form the impulse reduction network (Prevents buffer IC destruction due to sudden picture load change.)

[mmartell]

Re: EBR73712701 YSUS No Vsc

Vsc is passed along the buffer boards, so the Y-main just connects the two together

This is what I'm getting at - without an upper buffer connected there's no way to read a working Vsc at the test points. So since I have 150v at the resistor you're talking about I have a problem in my buffers or panel.

Correct ?

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

You need to have at least one buffer connected upper or lower to properly load VSCAN. However with not having the top buffer installed only the lower buffer installed your picture will not be proper due to data drive signals not being carried from the upper to the lower. But you should still have VSCAN no matter what.

I mentioned only trying the lower buffer because like I said before this particular series of plasma TV's have a problem with the plastic frame rubbing into the top buffer output to the panel causing a short loading down VSCAN.

What I have found in my previous years of working on plasma, the VSCAN will operate with or without buffers installed. Sometimes you'll have a floating VSCAN voltage due to the fact that it's not loaded down but it should still be there

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

This is your VSCAN test points? Red to +, Black to -.. do you have voltage there?

[mmartell]

Re: EBR73712701 YSUS No Vsc

This is your VSCAN test points? Red to +, Black to -.. do you have voltage there?

No I do not but that is exactly my point. If you look at the solder side of the board it looks like Vsc+ never makes it to the test point except when it is passed through the upper buffer.

I've also read about the ribbon being damaged, I did a quick inspection on the second from the top ribbon and I see nothing.

Testing with the bottom buffer properly... disconnect upper buffer from the Ysus and leave the data ribbon connecting the two buffers connected ? Or remove upper buffer completely ?

[tom66]

Re: EBR73712701 YSUS No Vsc

On this particular set it appears that Vsc is passed between the buffer boards by the Y-sustain. So indeed with a missing upper buffer you will not read Vsc. This is NOT the case for every plasma. They do things differently on different sets.

You need, in this case, to check Vsc with all boards in. If the voltage on the big 10R resistor is LOW after this, you may have a shorted buffer

[mmartell]

Re: EBR73712701 YSUS No Vsc

Ok guys much thanks for the input will report back with the results.

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

O yeah I just looked at the bottom side the board lol you guys are absolutely spot on. What a cheaply designed passive system. I remember LG built TVs that were robust without making cheap single layer boards that crack from heat.

It's been a few years since I looked at my PA 5500. That was operating without a top IC because of the ribbon short in my shop running news 8hrs a day.

This particular series shorts the IC if it has one or more missing output lines.

Make sure to check that very top IC on the upper buffer board for shorts. I noticed you mentioned 2nd from the top ribbon for rubbing it is not it is the very top ribbon you want to look at closely.

[mmartell]

Re: EBR73712701 YSUS No Vsc

D'oh! I read somewhere it was second from the top will have a look. Is the failure mode to have intermittent video ? That's what was reported by the previous owner and that's what I saw when I first turned it on at home. Since then, no video. That would be an incredible coincidence for it to fail in my possession and right away at that.

Are you saying running the set with only one buffer will short it out ?

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

I suppose it's always possible considering on the movement and transport of the tv could have caused it to you get worse

[mmartell]

Re: EBR73712701 YSUS No Vsc

Ok so reassembled and no prime and no Vsc and no -Vy. Well there is a slight buzz from the ysus about every second and the -Vy goes from about 13v to around 30v. Vsc is essentially zero.

Either the ysus is not repaired properly or I have a failed buffer dragging down my ysus.

Is it safe do you think to run the set with just the lower buffer to see if I get prime and good voltages ?

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

I suppose you could always try it wouldn't hurt.

[mmartell]

Re: EBR73712701 YSUS No Vsc

The buzzing about once per second is coming from the Vy/Vsc transformer. Probably normal against whatever is causing the voltages not to come up.

I looked at the upper panel ribbon - looks ok too.

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

You had -vy before. Was this with buffers disconnected or connected because if you tested it disconnected before then buffers are loading down the y board since connected no -vy

[mmartell]

Re: EBR73712701 YSUS No Vsc

Correct. I have both of those voltages without buffers connected. When connected they are gone or pulsing at a much lower voltage.

I've forgotten how to bench test buffers properly. Will look it up to verify a short in one or both.

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

Most likely a buffer short. Most likely vscan does not come up without buffers connected and connected dead short considering the pulsing ysus. In this case you will not have vscan either way.

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

I believe in diode check mode black lead to any of the aarows on the input connectors, red lead run it down the outputs with the clips up. Look at the readings. Maybe a beep on even one line output.

[mmartell]

Re: EBR73712701 YSUS No Vsc

Confirmed upper buff short in either the top one or top two ic's. I've replaced these scan ic's on Samsung buffers anyone have the same success on these LG buffers ?

By the way there is some artifacting going on with only the lower buffer installed. Nothing you can make out but big blocks of things probably where the on screen display is.

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

That's normal with no top buffer installed. Data doesnt drive to lower without upper installed. Now that you mention top buffer IC short I would really scrutinize those ribbon cables going into the panel. Hopefully you're lucky and it's just a shorted IC, However this is how this model represent itself when there is a problem with the ribbon cable. Look for any micro fractures or cracks from the plastic bezel rubbing into the ribbon before replacing buffer board.

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

And yes you can replace a bad IC. Same as Samsung if you've done it before. 1080p is more of a pain in the ass since there's more pins then 720p I've learned.