Samsung PS-42S4H flashing pixelated screen

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

Yeah I don't know if that's too low for this panel or not unfortunately. Any sticker on the panel indicating Vsch?

[wrx884]

Re: Samsung PS-42S4H flashing pixelated screen

The only sticker with any set readings is this one.

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

.

[wrx884]

Re: Samsung PS-42S4H flashing pixelated screen

.

Where's VSCH?

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

Where's VSCH?

VSCH is Vscan as far as I understand ,marked in the pic. Unless I'm offtrack in which case, yeah , what is Vsch?

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

just did a search for vsch, looks like something other than Vscan so it's not something I understand fully, sorry. I guess whatever voltages that are listed on that label you've found them and they're correct

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

Vsch is the row selection voltage. Around 120 to 160V. It is applied above the FGND on the Y-sustain board. It's only used by the buffer ICs, they either output the FGND current (peak 100A+), or the FGND+Vsch (very low current.)

The Y-sus output is effectively ground for the ICs -- all the signals for the ICs are referenced to FGND.

When you check for shorts on a buffer, you're checking between FGND and Vsch, the primary node, and FGND and all the logic ICs, and between FGND/Vsch and the outputs.

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

Vsch is the row selection voltage. Around 120 to 160V. It is applied above the FGND on the Y-sustain board. It's only used by the buffer ICs, they either output the FGND current (peak 100A+), or the FGND+Vsch (very low current.)

The Y-sus output is effectively ground for the ICs -- all the signals for the ICs are referenced to FGND.

When you check for shorts on a buffer, you're checking between FGND and Vsch, the primary node, and FGND and all the logic ICs, and between FGND/Vsch and the outputs.

so is it something you'd check with a cro more than a meter?
if we had to translate this back to a Panny would this be the MID voltage?

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

MID is a different voltage, that's the energy recovery voltage.

Hang on, I'll draw something up on this. It's hard to explain in words.

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

MID is a different voltage, that's the energy recovery voltage.

Hang on, I'll draw something up on this. It's hard to explain in words.

cool,

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

Ok... here's a rough overview.

The buffer ICs switch between FGND+Vsch (I'll call it VH) and FGND (I'll call it VL.)

FGND can vary from -190V in -Vad portion up to +200V during Vs in modern designs, some older sets would peak at 350V for Vset but this has been largely eliminated now as panel design has improved.

HOWEVER... effectively the buffer IC only "sees" the Vsch voltage, nothing else.

Each buffer IC switch is about ~20ohm IGBT and there are ones from VH to the panel output, and ones from the output to VL, the ground. For a 1080p panel that's 2,160 different IGBTs inside ~12 chips. Pretty impressive stuff, when you consider each IGBT stands off about ~160V max and has to sustain several amps peak.



As for the rest of the sustain board layout...

The various voltages are generated by one or two transformers, which generate isolated voltages, or in the case of older Panny sets, an isolated boost converter does the job.

Most sets only use Vsch during the scan period, when most panel outputs are at -Vy, so some of them turn off Vsch or lower it during non-scan periods.

The panel scanning works in tandem with the address buffers (Vda/Va/Vad etc) but is an independent system. The buffer ICs only select rows - they do not scan in individual lines of data.

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

Ok... here's a rough overview.

The buffer ICs switch between FGND+Vsch (I'll call it VH) and FGND (I'll call it VL.)

FGND can vary from -190V in -Vad portion up to +200V during Vs in modern designs, some older sets would peak at 350V for Vset but this has been largely eliminated now as panel design has improved.

HOWEVER... effectively the buffer IC only "sees" the Vsch voltage, nothing else.

Each buffer IC switch is about ~20ohm IGBT and there are ones from VH to the panel output, and ones from the output to VL, the ground. For a 1080p panel that's 2,160 different IGBTs inside ~12 chips. Pretty impressive stuff, when you consider each IGBT stands off about ~160V max and has to sustain several amps peak.



As for the rest of the sustain board layout...

The various voltages are generated by one or two transformers, which generate isolated voltages, or in the case of older Panny sets, an isolated boost converter does the job.

Most sets only use Vsch during the scan period, when most panel outputs are at -Vy, so some of them turn off Vsch or lower it during non-scan periods.

The panel scanning works in tandem with the address buffers (Vda/Va/Vad etc) but is an independent system. The buffer ICs only select rows - they do not scan in individual lines of data.

so a low voltage there would mean problem like either susup or dn not working?

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

can these be run with only one buffer? I'm thinking lower only, remove the Top board

I was looking at this video, same model and not as bad but that no signal box distortion looks the same.

Could it be that there's bad buffer ic still but not shorted?

https://www.youtube.com/watch?v=EhRDSY4RCt4

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

Generally not as the buffer ICs are blocking devices but if the ICs are bad yeah this could cause the Vsch to collapse through SUSDN.

But that would require the buffer ICs to fail first...

This is one possible path for which the sustain board can be damaged by the buffer ICs, but it can only happen while the set is running, not when the board is installed.

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

Typical fault path if buffer IC burns. There would need to be a path from FGND to GND and up thru Vsch perhaps thru transformer to kill SUS DN. It's pretty unlikely.

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

Could it be that there's bad buffer ic still but not shorted?

https://www.youtube.com/watch?v=EhRDSY4RCt4

Yeah it could be a bad IC but not shorted, running hot is one clue. Perhaps leaky, or the logic on each IC is glitchy. It's really hard to call without having a look at the scan buffer outputs. You can see if the Vsch is firing at the right time.

[tw2005]

Re: Samsung PS-42S4H flashing pixelated screen

Yeah it could be a bad IC but not shorted, running hot is one clue. Perhaps leaky, or the logic on each IC is glitchy. It's really hard to call without having a look at the scan buffer outputs. You can see if the Vsch is firing at the right time.

well if I read the thread right I get the impression only 1 chip has been replaced? Since it was on the top board I was thinking if it's possible isolate that and run just the bottom board but I don't know if that would be risky or not.

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

Maybe if it was pulling down one of the logic signals.
I still don't understand what they are, OCL, OCH, CLK, LE, etc.

I mean I know OC=Output Control, CLK=Clock, LE=Latch Enable but not sure what each does specifically.

[wrx884]

Re: Samsung PS-42S4H flashing pixelated screen

Yeah it could be a bad IC but not shorted, running hot is one clue. Perhaps leaky, or the logic on each IC is glitchy. It's really hard to call without having a look at the scan buffer outputs. You can see if the Vsch is firing at the right time.

well I did notice the replacement IC did feel cooler to touch than all the others IC's so would it be best to just bite the bullet and replace both upper and lower buffer boards? then again these did have heat sinks on them so I have no idea if the heat sinks are needed at all times, do u have an idea of how hot these should run roughly? I can measure these temps using my BGA rework machines TC to get an accurate reading.

[tom66]

Re: Samsung PS-42S4H flashing pixelated screen

They should only run warm, maybe 40-50 degrees tops.