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Old 06-13-2020, 07:46 AM   #1
angelos
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Default LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

Hi Guys.
A week or so ago i asked how to get this plasma set into Auto Gen mode to see whether my drive panels are ok and now i know they are not. The set starts up by hearing a clicking and there is a slight pre charge flash, i have to cup my hands with my eyes against the screen to see it.
If i pull the AC mains plug i get an image on the screen. It's the exact image that is being auto generated by the logic board and then the tv dies off as power drains. I think the image is the for about 0.2 seconds before disappearing. I wrote down the voltages that i could find on the board but i don't know... on the sticker which label is for which voltage. Can anybody clear up for me the what the sticker indicates, the format of Vscan, Vy Vbias etc....
Also i really suspect the Y-sus is to blame hear but hope to repair it since replacement part is quite expensive.
I have noticed that someone has already replaced the fuse on the Y-sus board but this fuse is ok now.
I have checked on the IPM that there are no shorts between its outputs and ground. Like i said no fuses are popping.

How do i isolate where this issue is?

Kind regards
Angelos
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File Type: jpg IMG_20200613_151941.jpg (229.3 KB, 15 views)
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Old 06-13-2020, 08:17 AM   #2
dick_barton
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Default Re: LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

As well as the shorting of the two points on the logic board did you also isolate the ribbon cable between the mainboard and the logic board and then power up?

Although you have no image on the screen do you have sound and does the set respond to the remote?

I see one of the fuses is a glass cartridge fuse that really needs to be changed to a 20mm ceramic fuse. Glass has a tendancy to shatter when there's a short circuit.
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Old 06-13-2020, 08:50 AM   #3
Biruslapio
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Default Re: LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

Hi, this image is from an LG plasma 50PQ30R, 2009, newer than yours but the information on the sticker is the same, -VY/VSCAN/ZBIAS.
There seems to be some disparity between your VSCAN reading and what the sticker says, are you measuring it correctly as the boards indicates the + and - reading points?
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Old 06-16-2020, 03:30 AM   #4
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Default Re: LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

As well as the shorting of the two points on the logic board did you also isolate the ribbon cable between the mainboard and the logic board and then power up?

Yes, i removed the cable going from the main board to the logic board, The LVDS cable?
I also removed all power to the main board. There were two plugs.

Although you have no image on the screen do you have sound and does the set respond to the remote?

I was not supplied the remote but will ask for it and get back to you on this one.

I see one of the fuses is a glass cartridge fuse that really needs to be changed to a 20mm ceramic fuse. Glass has a tendancy to shatter when there's a short circuit.

Yes, the glass fuse was inserted by the customer. In outr country finding ceramic fuses is like chicken teeth. Wearing our protective glasses is all we can do.

Regards
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Old 06-16-2020, 05:22 AM   #5
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Default Re: LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

Quote:
Originally Posted by Biruslapio View Post
Hi, this image is from an LG plasma 50PQ30R, 2009, newer than yours but the information on the sticker is the same, -VY/VSCAN/ZBIAS.
There seems to be some disparity between your VSCAN reading and what the sticker says, are you measuring it correctly as the boards indicates the + and - reading points?
There is a marking on the Y-sus board showing Vsc, i assuem that is the Vscan? This measurement is on the plug going to the buffer boards.
I must add that upon switch on this voltage slowly climbs to 188V taking about 6 seconds to get there. Not sure but perhpas the IPM IC is faulty. After all the client has replaced the fuse that feeds the IPM's SUS_OUT pin. I suspect this chip is not switching anymore. I did ESR all electrolytics and did not find any at fault.
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Old 06-16-2020, 09:48 AM   #6
dick_barton
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Default Re: LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

When you have powered up the set does the led on the logic board blink 1sec On, 1 sec OFF?
Strange that you can see the picture as you shut down. Can you turn Vscan down to match the panel @ 180V
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Old 06-16-2020, 10:10 AM   #7
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Default Re: LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

Quote:
Originally Posted by Biruslapio View Post
There seems to be some disparity between your VSCAN reading and what the sticker says, are you measuring it correctly as the boards indicates the + and - reading points?
I think Biruslapio may well have noticed an issue.
Your panel label indicates a Vsc of 110V but you give a reading of 188V in post #1?
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Old 06-16-2020, 03:54 PM   #8
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Default Re: LG Plasma Model 42PC7RVH Chassis PDP42V8 - No picture!

I have MSPAINTed your image with points of interest that may have to do with VSCAN/VSCN/VSC (they mean the same in this case).

That 1 ohm resistor, is it measuring correctly?
The L8 inductor looks darkened, may be dust or something, but it looks different.
VR1 may be able to lower or rise VSC, make note of it's original position and try turning it counter-clockwise to lower VSC, if it doesn't change, it is lowering something else, like -VY, try then VR2.
IC20 or IC17, one of them are what generates VSC, from the looks of this board IC17 is much closer to the VSC checkpoint, as well as VR1, to me that area is what generates VSC, I cannot confirm that without the board in hands.
Those golden pins at the top Y-Buffer can be used to measure VSC.
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