Samsung PS50C91HD - No picture, not sure which board is faulty

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Not quite the result I was looking for. D = diode . It should give short beep and a voltage reading in one direction, not the other. What the part number on the diode, I still have an Xmain here left over which may have the same diode.
Were any of these
Q5500,5302,5303,5304,5306 showing short? If not that's going to make it hard for me to guess where to next since I'm basing eveything on my experience with this board. I've only ever had bad FETs but obviously other thing could blow.

Post a picture if you can.

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Have a look at post#28 at the pictures you can see where I've removed the FETs, thats where I'm talking about.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Looks exactly like this



Two diodes are under long heatsink. I put multimeter across two outer pins = Reads as short.

Going to grab lunch, back in an hour.

Thanks very much for the help, I'm new at this level of diagnostic and repair, but a quick and keen leaner.

Cheers

[tom66]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Two outer pins are typically shorted on a diode, but you would need to look carefully at the PCB to be sure.

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Yep, That brings back memories, forget about those diodes, 3 pin double diode outer pins are anode and center is cathode so yes it will be short be Anode to Anode- Normal

Head for the smallest heatsink, bottom right in shot. I've got things to do too, so if you find FETs shorted where I'm saying, you then need to remove them one at a time and test out of cct. You may have only one which will short the rest or all will be bad.

I've had Qty 1, 2 & 4 shorted. If the one pulled is short , then recheck the in cct ones and if still short pul another out, and etc etc. They're fairly cheap to buy online so replace the lot or just the faulty ones. I've no dramas just doing the bad ones but probably good practice to replace the set.

Assuming you find the ymain as bad, you then have the pleasure of tryin to locate good buffer boards. On the 3 sets I've done, all uppers bad, lower boards were ok but best to replace both or confirm which are bad.

I don't know if they're still available from Samsung but I do remember they're scarce to find. I think some places may be refurbing them with new buffer ICs.

This is where it may get expensive and you'll have to decide if it's economic given how cheap new sets are now.

I have used the buffer boards from the 50Q7 model which are slightly different but will work.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

No short on any FETs on any of the heatsinks

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Well crap, that puts me in a corner. I have had them go open or test incorrectly. I had a couple bad one here but threw them last week, could have used them to see how they read.

I don't know if it's possible for the buffer to go bad and the ymain to survive. I still believe the issue is on the y side of things.

Hopefully Tom can guide us further on this. You could still remove those fets and look up how to check them out of cct it may be a goose chase but you need to start somewhere and the fact the logic performs normal with the Y disconnected it's a signal from the Y that's is shutting the panel down.

I'm stumped .

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Thanks mate.

Calling Tom......

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Just another thing I remember, The panel won't fire up without the upper buffer connected, unlike some where you can remove the top and get half a picture.

So let's assume the ymain is ok, a bad upper buffer potentially could also produce the same symptoms, I just can't believe all 3 of mine had the same fault (dead Ymain&upper buffer) with those sympoms and yours is ok. But I have seen posts with different components fail on the same sustain board.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

I found this diode which tests as short (ZD5806) other ZDxxxx diodes were fine. I assume ZD is zener diode?

What do I replace it with? I've googled the 24b79 that is on the diode = no luck on determining its value.

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Zeners can be tricky in cct, in the picture it looks like it sits across R5803 which if it is a low value resistor could give you a false indication . I'm thinking it could be 24V zener.

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

http://www.youtube.com/watch?v=WB238KaaHVA have a look at this as it's the same buffer and ymain you have. he states it will fire up with one board, obviously so when you watch it, but note when he does it's the upper that's connected. So I know with no upper it won't but in this with an upper but no lower it will.

Note also the upper was obviously bad and he's managed to locate the one buffer IC that's bad, removed it to clear the short and it fired up.

He runs through how to test for shorts. How about you leave the ymain for now and check the buffer boards. I'd start with the top one, it's a very common fault in this model.

I've seen some of his before but worth viewing especially it's the same boards. he may have some follow up videos on this as well

[jdnz]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Hey I have a PS50C91HDX with similar symptoms.

So far I have identified a blown upper Y-buffer and afew shorted zeners on the Y-Main.

I see you had trouble identifying the 24B-79 zener.
I eventually tracked it down to a Rohm PTZTE2524B

Did you have any luck getting yours going?

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Still sitting on the bench in the garage, awaiting attention...

Too busy lately to get back to it after getting stuck.
Will have another crack next week hopefully

[veliki]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

hi i have the same problem i change fets and caps but my PSU not have Vs.I have PSU outside whan i put gnd to ps_on relay open and have 5.3V but cant power up Vs_on.If i put to gnd or to 5V nothing.So how to power on vs_on?i cant find schematic for PSU.
b.r veliki

[jwarnes]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Edan what did yours turn out to be?

[Amraks]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

same problem here, got a set sitting here.

The top buffer board has one IC gone which is the second one.
Will be removing it after I find this short.

trying to work out what is wrong with the y-main.

anyone got any idea's?

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

same problem here, got a set sitting here.

The top buffer board has one IC gone which is the second one.
Will be removing it after I find this short.

trying to work out what is wrong with the y-main.

anyone got any idea's?

How much of this thread have you read? plenty of ideas given. Start with checking the bank of FETS where the 88N30W are and also in front of those there's a surface mounted fet B20NK50Z see if that is shorted. You'll have to pull those 88N30W out to find out how many are bad, the K2837 could also be bad.

You have to check all combinations of the 3 legs, sometimes only 2 legs will show short.

Other parts could be bad, I did 3 of these where the 88N30w were the only bad bits but I have one now which has more wrong with it.

[Amraks]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Hey mate will sus them out tomorrow out them fets tomorrow.

The 2nd ic which is u5402(top buffer board) has failed after running the meter over it and has a visual burn mark in the top. when I opened it up I suspected that chip, so it was no surprise really.

I get the green light constantly on on the y-main, I read some where this is meant to be flashing.

We will see where I am tomorrow when I pull them fets and test.

()
These sets are wonderful when they are going, Its the misses parents tv.
they bought a new one after this happened, then run into trouble with the new one(her old man accidentally bought a display tv, they got that fixed under warranty luckily.

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Hey mate will sus them out tomorrow out them fets tomorrow.

The 2nd ic which is u5402(top buffer board) has failed after running the meter over it and has a visual burn mark in the top. when I opened it up I suspected that chip, so it was no surprise really.

I get the green light constantly on on the y-main, I read some where this is meant to be flashing.

We will see where I am tomorrow when I pull them fets and test.

()
These sets are wonderful when they are going, Its the misses parents tv.
they bought a new one after this happened, then run into trouble with the new one(her old man accidentally bought a display tv, they got that fixed under warranty luckily.

I never knew our version had the LED on the Ymain, but hey had plent of versions . constant lit on the ymain from what i've read means the ymain has a fault, no lit is good, the logic ctrl board though has a blinking led when it's running about every sec i think.

If you think these are good, crossover to a panasonic and see how good a plasma can be.

I'm biased I prefer the Pana and done more on them, better manuals and complete schematics plus built in diagnostic codes to boot.

Only touch other stuff when i'm really bored or it's free. the one i've got was free and going to be dumped.

be interested to see how you go with the buffer board repair. That's the expensive and hard bit to find,

maybe you can give me some tips because if i have to find a complete upper board, i'll probably ditch it. not wore much these days to sell and i have 2 spare panasonic plasmas so it'll be something I touch if i can do it for about 450 -60 bucks and all component level stuff. This one is a mess, PSU is blown too.

Might be worth your time to reflow all the heatsinked fets, they commonly get fractures and the psus can randomly shut down with heat or blowup.

Caps can be an issue too, check the 680uf on the sustain boards, they can go bad too.