50" Samsung Plasma (model number PS50Q91HD) - possible purchase

[keats11]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

hey guys, I have a Samsung Plasma hp-t5054 with catastrophic failure (several FETs, transistors shorted, the main fuse blown) and the top IC on the upper buffer board shorted. I ordered a kit and replaced the transistors on the Ysus, plugged in just the lower buffer board and turned on the TV. The lower half of the screen lights up but there is no picture. I can hear the sound. My question to you guys is, does anyone know if this is still a Ysus problem or do I need the upper buffer to create any picture at all?

[scokljat]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Vsc is 2 - 3 volts - not 190... what are the most likely causes?

I suppose Vsc circuitry is on the YSUS board near Vsc trimmer. Vsc should usually be measured across a resistor. There are some transistors, fets, diodes around Vsc trimmer that you should check. ...or maybe is it possible that YSUS shuts off Vsc when buffers are not connected?! Hmm..

Suad

[scokljat]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Thanks, Tom. :-)

[tom66]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Functions of the voltages (since you asked):

Vs: Sustain voltage. Typically 170 to 210V. An AC signal consisting of this amplitude is applied to the panel at approximately 200kHz causing the cells which have accumulated a charge to light. Majority of PDP power consumption is in this rail. Usually 1.5Arms for a big 42" panel with pulses up to 50 amps during the sustain cycle. Failure leads to no picture. Dropping-out of Vs leads to picture with cloudyness/dark spots or flickering pixels.

Va: Horizontal address voltage, used to energize a column for addressing. Typically 50 to 70V. Used only by the X buffer COFs (NOT the X-sustain.) About 10% of PDP power consumption on this rail. Generated separately. Usually 600mArms for a big 42" panel. Failure mode - Never seen it but would presume similar to Vs.

Vset/Vw: Write voltage. Typically 180 to 200V. This is used to accumulate the wall charge on a cell. Generated from Vs BUT it is isolated from the Vs ground, allowing addition onto the Vs rail giving up to 400V. Usually <100mA draw. Failure of write voltage is rare, but it would lead to no picture or a poor picture.

Vrst/-Vy: Erase voltage. Typically -180 to -200V, not necessarily the same as write voltage but generally close. This is used to clear the wall charge accumulated on each cell, and sometimes used in the Vset_dn ramp. Generated from Vs. Usually <100mA draw. The whole panel is erased on each subframe, unlike the write voltage which is used on each pixel individually. The failure of erase voltage is common, leading to sparklies and stuck images. (Note - poor timing can also cause both of these symptoms.)

Ve/Vzb: Z-bias - used to drive the X/Z board components. Not particularly important. Around 60V to 120V. Usually <50mA draw.

--

Plasma panels work only with cells set to off or on, so the cells are modulated on a per-subframe basis to generate 8 or 10-bit colour. Plasma panels are pulsed at many hundreds of kilohertz to generate moving imagery. Also, the plasma panel can remember an image if the erase is not working (hence sparklies or image retention), because it has integral memory.

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Vsc is 2 - 3 volts - not 190... what are the most likely causes?

[scokljat]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

reconnected ribbon cable, with lower buffer only connected. Turned on power, got humming from ysus but no picture, waited a short while after sound came on, and got a slight puff of smoke from the lower buffer board, so ripped the power cord out.

Wow! What a nightmare! Where did the smoke come from exactly? Did you see? Is there something visibly burned, scan IC or maybe some cap? Tantalum caps like to burn... When you measure lower buffer for shorts, did they appear now? Relief is that scan IC's are quite cheap so even replacing all of them is affordable, although it's lot of precise work...

I think I see some test points for Vsc and Ve which are labelled on the panel as -190 and 94 respectively - I guess I should check these when I get a moment.

Yes. I thought you did all measurement and adjustment before when you got YSUS up and running without buffers...

My plasma has two voltage controls on the YSUS itself. Vsc and -Vy. -Vy is negative.

It would be great if someone describe in detail what are all plasma voltages for, where each of them flow and what are consequences when some voltage is missing and/or to low or to high or ripple...

Cheers.

Suad

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

one of the buffer chips... looking at the other buffer chips, they all have slight upwards lumps/bumps on the surface of the chip, so I guess they're all on their way out...

[tom66]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Seems like this set is being a real bugger to you. What went up on the buffer?

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

reconnected ribbon cable, with lower buffer only connected. Turned on power, got humming from ysus but no picture, waited a short while after sound came on, and got a slight puff of smoke from the lower buffer board, so ripped the power cord out.

Looks like the lower buffer board is toast too now

No smoke from the ysus though, so hopefully that's still ok.

Is there anything I should check on the ysus to confirm it's OK?

I think I see some test points for Vsc and Ve which are labelled on the panel as -190 and 94 respectively - I guess I should check these when I get a moment.

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

idiot alert... lying in bed last night, realised I forgot to connect the ribbon cable from the main board to the ysus - no no surprise there is no picture. will reconnect when I get home from work and try again

[scokljat]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

OK ysus is back together.
Resistances between the bottom two pins and the remaining pins are all opencircuit or megaohms - so I think I'm ok now with the one buffer chip removed. I just need some reassurance :/

So, after power up with lower buffer only all FET's on YSUS and PSU and diodes didn't blow again? No white smoke, no pops, no burns? Well, this is kind of reassuring ;-)

It sucks when you got no picture with lower buffer installed. Not even few wokring bars? Hmm. Than try with upper buffer only. Wish you luck...

What did you say, you don't have an oscilloscope?

Cheers,

Suad

[tw2005]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

OK ysus is back together.

Sound with no picture with no buffer board installed (no surprise).

Installed the lower buffer board only - still no picture which was a bit of a surprise although tw2005 mentioned the same thing with his similar model Samsung.

Not brave enough right now to reinstall the upper buffer... I have removed the melted upper buffer chip, and the shorts to the three bottom pins are now gone...

checking the pins tom66 suggested earlier - the top 12 pins all 'short to each other' I assume these are the earths.With the way tom highlighted the top half and bottom half separately, I assume this is normal.

Similarly, the bottom three pins are all shorted to each other - I assume these are the power - although the Coppell post suggests 'On the TOP buffer board, the power lines are the top two lins of the black connector and ground are the most of the remaining connetors' This seems the opposite to what I'm seeing?

Resistances between the bottom two pins and the remaining pins are all opencircuit or megaohms - so I think I'm ok now with the one buffer chip removed. I just need some reassurance :/

Here's mine. Patience! Like I said in my PM, I've killed my panel surging the f-buffer which I expect has blown the COF. All because of my impatience and leaving that power connector off for the E,F buffers. Residual current in the PSU meant it arced when re-connected.

Anyway i reconnected my bad upper after I got the ysus going and no smoke but I turned it off straight away. From what I saw the logic board detected a fault and never gave the panel on command because the blinking LED quit and the ysus and xsus never energised.

But I'm not saying this to encourage you to whack it in fingers crossed. Hate to see you go backwards with all the time and $$. I'm still sulking over mine

Makes me wonder why it could not do that in time to save the ysus with the original fault

[tom66]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Maybe you can power up Y sus with a light bulb in series like you did for the power supply.

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

OK ysus is back together.

Sound with no picture with no buffer board installed (no surprise).

Installed the lower buffer board only - still no picture which was a bit of a surprise although tw2005 mentioned the same thing with his similar model Samsung.

Not brave enough right now to reinstall the upper buffer... I have removed the melted upper buffer chip, and the shorts to the three bottom pins are now gone...

checking the pins tom66 suggested earlier - the top 12 pins all 'short to each other' I assume these are the earths.With the way tom highlighted the top half and bottom half separately, I assume this is normal.

Similarly, the bottom three pins are all shorted to each other - I assume these are the power - although the Coppell post suggests 'On the TOP buffer board, the power lines are the top two lins of the black connector and ground are the most of the remaining connetors' This seems the opposite to what I'm seeing?

Resistances between the bottom two pins and the remaining pins are all opencircuit or megaohms - so I think I'm ok now with the one buffer chip removed. I just need some reassurance :/

[tom66]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

For the tab/tag pin of the FET, use solder paste and a soldering iron, or flow solder under it while pressing it down. (Solder the other two pins first.)

[tom66]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

The included 680uf 25v caps were Jackcon - they'll be going straight in the bin Already replaced them with Rubycons Will try finish off the repairs tomorrow evening.

Keep the Jackcons - they make good firecrackers.

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Ysus repair kit arrived today (6 days from UK - impressive). 3 pin mosfets installed ready to solder, still need to install the surface mount mosfet and work out how best to solder the base to the PCB.

Also need to test the resistors that are included in the repair kit R5508, R5509 and zener diode ZD5502 - although the zener appears visually to be ok.

The included 680uf 25v caps were Jackcon - they'll be going straight in the bin Already replaced them with Rubycons Will try finish off the repairs tomorrow evening.

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

Well I was going to do initial power up with repaired ysus with NO buffer boards connected

Then I'll try lower buffer... I'll also retest upper board for shorts once blown 3407F chip is removed to confirm no shorts...

[scokljat]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

No - ysus repair kit ordered from the UK - but not expected until sometime next week.

My hot air gun arrived yesterday, so I'll remove the buffer chip from the upper y buffer and confirm that that eliminates the short when I get an opportunity.

Other than that, it's a waiting game

When you install new YSUS FET's maybe you should try without whole upper y buffer board. Topmost IC on upper buffer board is visibly blown and should be replaced, but maybe there are more shorted IC's on the same board but not visibly blown. Even if you remove topmost IC other possibly shorted IC's can blow Y FET's again. Testing with new FET's and lower buffer only (which you confirmed has no shorts) would ideally produce normal picture on the lower half of the screen. If that happens, you can start celebrating ;-)

Cheers!

Suad

[psmedley]

Re: 50" Samsung Plasma (model number PS50Q91HD) - possible purchase

No - ysus repair kit ordered from the UK - but not expected until sometime next week.

My hot air gun arrived yesterday, so I'll remove the buffer chip from the upper y buffer and confirm that that eliminates the short when I get an opportunity.

Other than that, it's a waiting game