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Old plasma screen Y_SUS anomalies

Started by legg October 5, 2021
An older and poorly-documented plasma screen (LG 42PC3DV) 
apparently went 'pop' and developed dark screen.

It had the usual bad 5V rail caps (x9) of the era (mfrd 6 months 
in 2006), and an overloaded -VY rail. I fixed those issues, to 
discover that Y_SUS waveform had no 'set-down' ramp.

The mosfet generating that ramp can and does produce a -ramp, but 
only at the termination of the panel drive waveform, not within 
it. There is no gate drive signal, at the right time, to produce 
this ramp.

It's complicated by the fact that models subsequent to this put 
zero volts across the panel outside of the drive period - up to 
that time the panel rested at -VY in most models. 

There are, of course, no schematics. The service manual includes 
only a module disassembly diagram and some vague flow charts.

There are 'Troubleshooting' and 'Training' manuals for different 
models of the era, none of which include schematics of the 
power train or drivers, just the signal processing cctry.
None have the same connectorization, harnessing, test point 
nomenclature/position.

I'm counting on a 50PC1DR or 60PC1D training manual to give 
display waveforms for a 'return to -VY' system.
There's a 42PC5DC, that exhibits the 'return to zero' system.

None of the other power semiconductors are capable of pulling 
Y_SUS low - but there is no gate signal at the right time to 
do so. There also seems to be no factory reset procedure for 
these  dinosaurs' firmware, that doesn't involve a special harness 
and PC software.

Any ideas on getting this thing to perform?

It's not for me, but seems to have sentimental value for the 
codger who put out kilobucks, in 2006, to own it.

Assumed config of major switches-
http://ve3ute.ca/query/42PC3D_Panel_Drive.jpg

expected waveforms of similarly functioning models-
http://ve3ute.ca/query/42PC5DC_Y_SUS_waveform.jpg
http://ve3ute.ca/query/50PC1DR_Y_SUS_waveform.jpg

Y_SUS of sick puppy-
http://ve3ute.ca/query/42PC3D_Y_SUS_issue_211005a.jpg

The scan buffer panel has been replaced once. It ~stores 
the SUS_UP peak, but does not follow the Y_SUS drive 
low, even outside of the drive period - simply bleeds 
down to 0V, as illustrated.

RL
Hi RL, 
Sorry I have no specific fix advice.  I hope you're one of the
"journey is the reward" types, since the reward at the end
of this one, if you get it working, is a very outdated video
display.  According to  my records, this model released
in April 2006, is an "EDTV" (852x480p), and MSRP was
$2100 (US)..
I would rather tell the owner to buy a new model (not
plasma - but an OLED. But even many LCDs will out-
do his/her/their old PDP.) If cost is an issue, then
even a used LCD HDTV model from 10 yrs ago would
be likely better then his old beast.
cheers, Rich S.





On Tuesday, October 5, 2021 at 11:43:28 PM UTC, legg wrote:
> An older and poorly-documented plasma screen (LG 42PC3DV) > apparently went 'pop' and developed dark screen. >
On Mon, 11 Oct 2021 18:58:49 -0700 (PDT), Rich S
<richsulinengineer@gmail.com> wrote:

>Hi RL, >Sorry I have no specific fix advice. I hope you're one of the >"journey is the reward" types, since the reward at the end >of this one, if you get it working, is a very outdated video >display. According to my records, this model released >in April 2006, is an "EDTV" (852x480p), and MSRP was >$2100 (US).. >I would rather tell the owner to buy a new model (not >plasma - but an OLED. But even many LCDs will out- >do his/her/their old PDP.) If cost is an issue, then >even a used LCD HDTV model from 10 yrs ago would >be likely better then his old beast. >cheers, Rich S. > > > > > >On Tuesday, October 5, 2021 at 11:43:28 PM UTC, legg wrote: >> An older and poorly-documented plasma screen (LG 42PC3DV) >> apparently went 'pop' and developed dark screen. >>
I'm told that plasma screen tv repair is best tackled by masochists. Anyways, you can get a blow by blow report in a (solo) thread at: https://www.coppelltvrepair.com/boards/topic/27/lg-42pc3d-fix#560 I think its open for anyone to read. At present this is just something to scratch the kibble under my hood, whenever rare parts arrive for the next symptom. I have good gate control signals out of the control board, finally, and a bit of screen illumination, if only for short periods. Still trying to get a good negative-going Y_SUS out of the buffer (it's good going in). Nearest thing I could get to a schematic for this area was from the patent office. ( ! ) RL