Defective Samsung LJ92-01200A Y Sustain Board

[electech]

I am repairing a defective Y sustain board (Samsung LJ92-01200A) from a Philips 42" (model 42PF7220A) plasma TV.  The Y sustain board has a shorted diode (reference designator D5018) which is shorted.  The diode is is a SMD marked F4 followed by 5D (probably date code).  I need help in identifying the part. Can anyone help??

Also I suspect the upper Y buffer board may have a problem or needs to be checked. The shorted diode provides power to the upper Y buffer.  Does anyone have a technique for testing the Y buffer board??
I have located a source (datasheet and spares) for the Y buffer.   

[Bobf645]

Hello,

I'm repairing a Sanyo TV with the same board. I've identified the problem as the Y sustain board, but have not yet figured out what went wrong with the board itself. How did you idenfify the diode as the problem? Were you able to find out how to test the upper/lower Y buffers?

Would it be possible to share your source for the Y boards?

Thank you very much for your help!

Bob

[a002074t]

I have 3 of these boards and all 3 have gone bad.  I would love to see a schematic if anyone has got one.  I'm pretty sure all 3 boards have the same problem - just haven't been able to pinpoint it yet.  The last of the 3 went bad before my eyes.  I plugged in a bad y-buffer board and power up the TV for 10 sec to see which of the 6 ICs will heat up (indicating a short) and after that the Y-sustain board went out for good.  I ran my fingers across the components to see if anything had overheated but the entire board was cool to the touch.
This seems to be the most common problem with these plasmas - y-buffer IC shorts out from the high voltage it carries and takes out the y-sustain board within seconds.  I don't think the problem is with the diode mentioned earlier (D5018) as it reads short when mounted on the y-sustain board.  This diode is reading short on all 3 of my boards (even the one that was working - it read short on a functional board).
If I had a schematic to look at, maybe I can make some progress but without it or any insight, I'm just flying blind...

[electech]

Thanks for the tip.  I removed diode D5018 from the circuit to confirm it is OK (there is a low value resistor across D5018).  Without a schematic it is hard to toubleshoot!

There are several companies that repair/refurbish these boards.   However, the cost $100 each board (up to 3 may need to be replaced) is hard now hard to justify for an older Plasma.  I have seen new Y buffers boards for $100 each.  There is also no guarantee that all the problems will be corrected as the Plasma screen could also be defective (not as likely).  It would be preferable to buy a new Plasma with prices as low as they are now.

It is reported from some other repair tech posts that the FETs or HV capacitors on the Y sustain board can short out .  The Y buffer boards have scan drivers which can breakdown. Often one board will take the other.

This Y buffer board uses the ST Microelectronics STV-7617 scan driver.  Data sheet is available online.

My Y sustain does not have a dead shorted FET. I may try to troubleshoot without the Y buffers attached.  Otherwise the Vscan voltage is loaded down and the TV goes into protection mode.

The Samsung PDP service manual for this Plasma has an adjustment section which shows the waveforms that the Y sustain/Y buffer boards can generate.

[electech]

Just wanted to post this update.  There was nothing wrong with my Y sustain board or Y buffer boards.  The problem turned out to be cracked solder joints on the power supply board.  When the TV heated up for 15-20 minutes the brittle lead free solder joints would open up and cause a shutdown of the Vscan voltage. 

I was first fooled into believing there was a load (Y sustain/Y buffer short).  After checking the boards I could not detect a short so I brought the TV up slowly and got a picture.  The power supply board shut down again after 15 minutes.  The third time around the power supply would not boot up without the Y sustain power cable hooked up (so I knew problem was contained in the power supply board).

The brittle lead free solder joints and normally dull and cracks are very hard to detect.  I solder sucked numerous joint before using a leaded 60/40 based solder.  Now the TV works fine without shutting down.

Electech   

[Jay Abed]

When I changed diods.. open resistors.. I get most to work. but sometimes.. that dont do it.. when I test the output to the buffers... the power that should remain at or about -48 volts.. keeps fluctuating (changing from about 140 volts to some -39 volts) this voltage should stop at about a -48 when no buffers plugged in. if any experianced tech who knows what I'm talking about can help.. I surelly appricate that. thanks

[Turnip]

Nice one, electech,

Had PS listed as a dry joint problem but original guy never said how and why. He didn't say who made the PS either - but, we're getting there - Chris.