EBR73712701 YSUS No Vsc

[mmartell]

From an LG 60PA6500...

The PO stated the set would lose video soon after turning on and after leaving it unplugged for some time the video would come back and the process would repeat.

This was verified by me. After taking measurements I found that -Vy and Vsc were missing - all other voltages present and on spec.

After touching up some dry joints on the transformers -Vy has returned to -185v and appears to be stable. Still no 150v Vsc. Obviously it was present at least temporarily for me to see video but now I can't get it back.

All voltages were measured with the buffers disconnected, though I've run the buffer test and they appear fine but I think it best for the time being to remove them from the equation.

I have 149v at both sides of the 10 ohm power resistor at the top left of the board which I think my be the Vsc but it doesn't make it to the test point. Having a hard time tracing the circuit.

Any help appreciated.

[mmartell]

Re: EBR73712701 YSUS No Vsc

Some pics...

[mmartell]

Re: EBR73712701 YSUS No Vsc

Better pics courtesy of SJ

[mmartell]

Re: EBR73712701 YSUS No Vsc

Anyone have any insight on this ?

[tom66]

Re: EBR73712701 YSUS No Vsc

Both -Vy and Vsc are generated by the same transformer. If -Vy is good then Vsc should be too, so it must be a cold joint or a blown fuse somewhere, or something like that

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

Attach lower buffer only. Check vscan.

[mmartell]

Re: EBR73712701 YSUS No Vsc

I've read that but I have no vscan with NO buffers connected. Does Vscan need a load on it to come up ??

[mmartell]

Re: EBR73712701 YSUS No Vsc

Both -Vy and Vsc are generated by the same transformer. If -Vy is good then Vsc should be too, so it must be a cold joint or a blown fuse somewhere, or something like that

So the smaller transformer is for generating the 18v for the X sustain then I take it.

I think the joints are good now. It's difficult for me to trace the circuit but that information might help me trace it back some.

[mmartell]

Re: EBR73712701 YSUS No Vsc

Hey tom how would I check the output of the transformer, I mean since it's floating where would I put my negative lead ?

[freakaftr8]

Re: EBR73712701 YSUS No Vsc

I've read that but I have no vscan with NO buffers connected. Does Vscan need a load on it to come up ??

That was my thought. This model and it's 50 inch brother are known for plastic frame ribbing the panel y ribbons shorting a top buffer IC and killing vscan.

You could try chassis ground for lead negative, but on the buffer ther e should be a test point for vscan + and -

[mmartell]

Re: EBR73712701 YSUS No Vsc

There's a test point on the ysus behind one of the connectors but the voltage never reaches there. I can try using one of the test points for the negative lead. Trying to find out if the voltage is coming out of the transformer or where it stops.

[R_J]

Re: EBR73712701 YSUS No Vsc

It looks like vsc uses chassis for ground, the + connects to both upper & lower buffers at P155 & p153. You will need to trace that point on the buffers to see where it comes from. tell us which plug and pin, then trace it back on the ysus
You can see where the vsc adjustment is, so that is part of the vsc circuit

I will guess based on where the VSC adjustment is, that this could be the path???

[mmartell]

Re: EBR73712701 YSUS No Vsc

Where you're line ends is the last place I see the voltage. It gets lost between there and the test points behind connector P153. Basically in the area behind connectors P154, P155 and P156.

[mmartell]

Re: EBR73712701 YSUS No Vsc

Looking at the solder side I'm not entirely sure how one could read Vsc without having the buffers installed. The circuit seems to complete through the upper buffer board and then continue through the lower. I must be wrong.

Also chassis ground is not continuous with the negative test point. You can see where the negative test point connects to chassis ground through some poly caps between connectors P153 and P155.

[tw2005]

Re: EBR73712701 YSUS No Vsc

Could there be a chance a winding could be open cct in that large transformer?

I've had several in the series prior both 50 and 60 inch as they use the same transformer have defective windings which go open.

[tom66]

Re: EBR73712701 YSUS No Vsc

Remember that the entire output of a Y-sustain board is based on a floating ground. The sustain output which swings 200V-0V and ramps to -190V has Vsc added to it for the Vsch output, or the buffer positive. All the buffer ICs do is select between the Y-sustain output and Y-sustain+Vsc, the higher voltage acts as a select for writing individual lines on the panel.

So when you are tracing Vsc remember it is not relative to the chassis but in fact it is relative to the Ysus FG (floating ground) output

[tom66]

Re: EBR73712701 YSUS No Vsc

Smaller transformer generates the F18V, which floats above the FG, think that's used to generate FG5V (for Y-buffers) and FG15V (for gate drivers on Y-sustain)

[R_J]

Re: EBR73712701 YSUS No Vsc

I was wrong about the (-) for the VSC, Its hard to tell what is ground and what is not just going by pictures,

I see a small pico fuse near the 10 ohm/5w resistor, or is that a small cap?

[mmartell]

Re: EBR73712701 YSUS No Vsc

Will check that shortly...

[mmartell]

Re: EBR73712701 YSUS No Vsc

It's a pico fuse - I have 110v on either side of it with respect to Vsc- test point.

I'm still stumped by the fact I see no path for Vsc from the upper buffer top connector to the Vsc+ test point

Do the buffers need to plugged in to get Vsc ? If so then I would guess I have a failed upper buffer.