LG42PC3D -UD originally with green lamp but no display or audio.
Owner says was working, then went 'pop'.
All 5V caps bulging. replaced.
All PSU output voltages within spec but -Vy is low on YSUS.
One of three parallel fets between SUS_OUT and SUS_DN is zenering,
eventually shorts DG. Replaced all 3 - -Vy now OK.
Y scan buffer test point does not follow YSUS output below -50V
Note: This unit seems to expect YSUS output to park at -Vy in between
drive signal pattern periods. A replacement drive signal board confirms this.
This shows up in a training manual for early 60PC1D, as being normal. Later
models will park at 0V . . .
Also, YSUS waveforms sometimes show a double setup ramp before the
single setdown ramp at pattern termination. This is only shown in training
manuals for dual-scan types. It might be because I'm not supplying a white-
screen input signal (no input source is present).
A second buffer board produced first screen illumination, for a short period,
before R15 popped, leaving only a top horizontal band illuminated (greenish
random). R14 is 47R 2W smd - looks to be in an RCD snubber network
configuration. Replacing the C and popped R allowed the same behavior to
be repeated, with popped R15 again.
I replaced the 74AC540S supplying signals to the buffer board and a second
replacement buffer board gave full screen illumination without anything
popping - but only running it long enough to get waveforms and images.
There is a random greenish pattern, with a redish-yellow vertical band shifting
randomly back and forth across the screen.
The only similar random illumination images I've seen in plasma screen
diagnostic literature refer generally to either loose connectors or corrupted
memory.
Reprogramming seems unlikely for this model - distributed for 6 months only,
in 2006, even though it was subject to an industry-wide bug fault requiring
widespread reprogramming, at that time. You'd expect programming tools to
still be floating around somewhere.
This unit was a revision within a revision - no PFC in power supply as with 42PC1x.
Its YSUS board had 74AC540S and 74AC541D (different body sizes) on
the same board . . . .
Anyone with ideas what's next?
Y buffer test point still does not follow YSUS output lower than -50V
RL
Owner says was working, then went 'pop'.
All 5V caps bulging. replaced.
All PSU output voltages within spec but -Vy is low on YSUS.
One of three parallel fets between SUS_OUT and SUS_DN is zenering,
eventually shorts DG. Replaced all 3 - -Vy now OK.
Y scan buffer test point does not follow YSUS output below -50V
Note: This unit seems to expect YSUS output to park at -Vy in between
drive signal pattern periods. A replacement drive signal board confirms this.
This shows up in a training manual for early 60PC1D, as being normal. Later
models will park at 0V . . .
Also, YSUS waveforms sometimes show a double setup ramp before the
single setdown ramp at pattern termination. This is only shown in training
manuals for dual-scan types. It might be because I'm not supplying a white-
screen input signal (no input source is present).
A second buffer board produced first screen illumination, for a short period,
before R15 popped, leaving only a top horizontal band illuminated (greenish
random). R14 is 47R 2W smd - looks to be in an RCD snubber network
configuration. Replacing the C and popped R allowed the same behavior to
be repeated, with popped R15 again.
I replaced the 74AC540S supplying signals to the buffer board and a second
replacement buffer board gave full screen illumination without anything
popping - but only running it long enough to get waveforms and images.
There is a random greenish pattern, with a redish-yellow vertical band shifting
randomly back and forth across the screen.
The only similar random illumination images I've seen in plasma screen
diagnostic literature refer generally to either loose connectors or corrupted
memory.
Reprogramming seems unlikely for this model - distributed for 6 months only,
in 2006, even though it was subject to an industry-wide bug fault requiring
widespread reprogramming, at that time. You'd expect programming tools to
still be floating around somewhere.
This unit was a revision within a revision - no PFC in power supply as with 42PC1x.
Its YSUS board had 74AC540S and 74AC541D (different body sizes) on
the same board . . . .
Anyone with ideas what's next?
Y buffer test point still does not follow YSUS output lower than -50V
RL
Attached Files