LG 60PZ950T Z-sustain repair

[hartonoh]

Re: LG 60PZ950T Z-sustain repair

I've explained this before but the floating ground is only used on the Y-sustain. It is the output voltage generated by the Y-sustain board. It's what all the IGBTs on the board are working to make.

So if you ever see an oscilloscope waveform like this:
http://4.bp.blogspot.com/-8RDK8j1TqP...600/fig-11.jpg

then that is the floating ground output.

Hmm, you say, that doesn't look very "ground" to me...!

That is because it is not a true ground but rather the ground reference for the scan driver ICs (buffer board ICs). These scan drivers select between the VFG (floating ground) and the VFG + VH voltage (which is floating ground + 150V). But as far as these ICs know, it's ground. They are fully isolated digitally and electrically from the normal ground. Though a path to ground from the sustain board output will cause a failure of the sustain board or buffer boards. *THE ONLY PURPOSE* of the scan driver ICs is to select a line to scan on the panel. They apply the 150V VH voltage to select a line and the address / data drivers which run along the bottom of the TV screen program the line with the appropriate pixel data in the given subfield. In this sense, the scan driver ICs are similar to the LCD gate driver ICs, though failure symptoms do vary.

The VH is often called Vsc, or Vscan... And the floating ground is the same ground reference as used for -Vy or -Vad voltages... Though Samsung calls -Vy the -Vsc voltage just to be confusing... But in general the negative ramp voltage/floor voltage -Vy is around -180V to -220V. And the positive line scan voltage Vsc is +140V to +160V.

Plasma TVs before 2005 used some different designs that were more expensive to implement but from roughly 2006 onwards all manufacturers standardised on floating ground designs. Hitachi panels are the only exception because they use TERES and ALiS technology which is...strange... to say the least.

I have Bad and Good experience to this similar problem. After searching and found shorted 30F124 and change both 30F124 in he neighborhood, and check that no other short, I power up the panel, fail again, and found another 30J124 short. Waiting for the spare to arrive, I check all of the solder side, and found alot of solder joint are bad. So, if you found some shorted MOSFET or IGBT, check also the solder joints for Thermal crack.