LG 60PV400 - Power but no picture

[97Kouki]

I have a 4 year old LG plasma 60PV400 that was working fine up until 4 months ago. I started having the green and pink snow on the display. A search on BadCaps.net told me to reflow the inductors on the YSUS and ZSUS boards. That fixed the colored snow and the TV worked fine for another 4 months.

Yesterday, my wife turned the TV on and after about 5 minutes of watching it she said the screen went dark. No POP, sizzle, smell or anything. The TV remained on, responded to the remote, and behaved normally except the plasma panel was off.

I tried the flashlight test and there is no display or backlighting; it's as if everything is working but nothing is getting to the panel. This TV was set up with a surround sound system so the TV speakers were shut off in the TV's menu so I am unable to tell if the set has sound. I hooked my laptop up to the TV via HDMI and the panel is initialized on the PC. With the back of the TV off the Control board has a blinking light (which I read from a similar TV's service manual means it's operating correctly).

I have removed the back panel and measured the VS, VA and M5v on both the YSUS and ZSUS boards and they all read within range of the voltages stated on the panel. All 5 fuses on the Power, YSUS, and ZSUS boards are good including the small green ones on the YSUS and ZSUS.

I have tried to research as much as possible into Plasma troublshooting but I am at a loss as to what to try next. I have no problem buying replacement YSUS and buffer boards (which I think is the problem) but I don't want to just throw parts at the TV and it still not work.

The only abnormal thing I have noticed is that when powered on both the YSUS and ZSUS produce an electrical hum that I'm sure is normal. But, the YSUS board is producing a faint whistling sound also (think of the sound that a bomb being dropped in a cartoon makes), about 5-7 seconds long then repeats. Its very faint but doesn't sound normal and I never noticed it before even though I would have been able to hear it with the cover on.

I searched the forum and havn't found anyone having a similar problem with this model. Any help you could provide would be greatly appreciated. I'm willing to do whatever is asked of me to diagnose this issue. I'm just not well versed on the terminology for TV's or Plasmas for that matter but I want to learn.

I have attached pictures of the inside of the TV. Thanks for your assistance.

[97Kouki]

Re: LG 60PV400 - Power but no picture

Just an update, I purchased the service manual for this model. I'm going through it now to see about diagnostic steps.

[dick_barton]

Re: LG 60PV400 - Power but no picture

There's a label at the top of the panel giving voltages. Va, Vsc, VE . Have you checked that they are present on the circuit boards.

[97Kouki]

Re: LG 60PV400 - Power but no picture

I did. The voltages are Va:55v and Vs:201v. I checked the voltages on the power board and where they connect to both sustain boards, power is getting to both boards. It may be my multimeter but I read Va:59v and VS:218v. I may go purchase another DMM just to make sure. I'm thinking about trying the pattern jumper but I'm not sure that would help as the panel doesn't power on at all. Would the pattern test power on the panel in a situation like this?

[97Kouki]

Re: LG 60PV400 - Power but no picture

I just noticed you mentioned VE as well. I don't see a sticker with VE on it. Only M5v which is supposed to be 5v but I read about 5.5v.

[Hamie]

Re: LG 60PV400 - Power but no picture

This is an LG so the VE is called z-bias.

You should notice test points on the z-sus labeled as such.

Vsc and vy test points will be on the y-sustain.

Without seeing the panel sticker a rough estimate would be:

Vsc should Roughly 150v

-Vy should be roughly -200

Zbias should be anything from 70-150v

[Hamie]

Re: LG 60PV400 - Power but no picture

Zbias is labeled VZB on your board.

[97Kouki]

Re: LG 60PV400 - Power but no picture

Thanks so much for your help. I went and bought a Fluke DMM to make sure I'm getting correct readings. Below is what I found.

M5v: 5.17v
Vs: 202v
Va: 55v
VZB: 110v
VSC: 0v

Again, thank you.

Edit: I just completely removed both Y buffer boards from the TV and measured VSC again. No change, VSC is still reading 0v. Am I correct in assuming the problem is isolated to the Ysus board? Where do I go from here?

[Hamie]

Re: LG 60PV400 - Power but no picture

Yeah so you have a blown YSUS.
Is the fuse open?

[Hamie]

Re: LG 60PV400 - Power but no picture

FYI as per your panel sticker.

-Vy = -178
Vsc = 150


VZB = 110

[97Kouki]

Re: LG 60PV400 - Power but no picture

I measured the only three I could find, fs101, fs102 and fs103. All show good. Would this board be worth attempting repair? While poking around I found that resistor r309 is shorted, reading .002 ohms from one leg to the other.

[Hamie]

Re: LG 60PV400 - Power but no picture

R309 will test short it's a 2.2 ohm resistor.
The problem is more likely to be a shorted igbt or diode attached to the heatsinks.

[97Kouki]

Re: LG 60PV400 - Power but no picture

I didn't find any diodes on the board and all transistors measured uniform with no shorts. Weird. Is there any way to easily test the buffers so I can be sure i don't need to replace them?

I don't know if this matters but the whistling sound the YSUS board was making disappears when either the buffer boards are completely removed or the ribbon from the control board is removed.

[97Kouki]

Re: LG 60PV400 - Power but no picture

Update: I'm determined not to give up on this board. About 2 inches from the vsc points there is a plug that goes into the lower buffer. Pins 9 and 10 say they are supposed to have vsc voltage... And they read 150v. But not the vsc points. I'm going to examine the traces and see why the vsc points aren't showing voltage but the circuitry is obviously working to produce the voltage.

[97Kouki]

Re: LG 60PV400 - Power but no picture

Ok, I'm pretty sure I have it solved now. I had removed a couple transistors from the YSUS board because they were showing open in circuit. Once I got them out and tested them I got a reading so I soldered them back in. While I was at it I touched up some very slightly questionable points and all of the other transistors. That is when I put it back together and found that the lower buffer connector on the YSUS board had VSC but not the VSC points.

Turns out that the lower buffer is what transfers VSC from that point to the VSC points. With the lower buffer in place the VSC points had 150v. I powered it off and waited to put the upper buffer back on. Once the upper buffer was put back on VSC read 0v again.

The weird thing is, removing the upper buffer was the first thing I did because it was suggested in a lot of other threads with similar problems but at the time VSC didn't have 150v. I am at a loss as to what I did to fix it.

Now that I am trying to figure out what is causing VSC to drop out with the upper buffer, I find that none of the ribbon connections are shorted to ground. But pins 1 and 2 are shorted to pins 5-12 in the lowest connector on the Buffer board where it connects to the YSUS board. The corresponding connections on the lower buffer have resistance but are not shorted. I'm not sure what pins 1 and 2 are for but with the upper buffer attached they are shorting to VSC. It's not the IC's, it's probably a simple short somewhere but I cant really get to anything because of all the glue.

Disconnecting the upper buffer gives me the bottom half of the screen, so I am off to buy an upper y buffer.

I appreciate everyone's help, especially you Hamie! You put me on the right track.

I'll update this thread with the results.

[97Kouki]

Re: LG 60PV400 - Power but no picture

Pins 1 and 2 are Va and they are shorting to ground through the buffer, not Vsc.

[SLK001]

Re: LG 60PV400 - Power but no picture

Just an update, I purchased the service manual for this model. I'm going through it now to see about diagnostic steps.

Where did you purchase the SM?

[97Kouki]

Re: LG 60PV400 - Power but no picture

Servicemanuals.pro

[97Kouki]

Re: LG 60PV400 - Power but no picture

An upper Y buffer fixed the tv. My only worry is that the YSUS caused the buffer to short. I'll update this thread if anything happens but so far so good.

Thanks everyone.