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

[tw2005]

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

This is what I ended up ordering:
http://australia.rs-online.com/web/p/products/671-4777/

Just ordered the 2x FETS and replacement fuse for now - will order caps later once this issue is resolved.

Good thing about RS is they have free delivery and no minimum order value - farnell (element14) is only free delivery if you order over $A45 - otherwise it's $12.95

Found a Fairchild drawing of their 18N50 - the parts I removed are definitely TO-3P

Hope it all goes smooth, I've only ordered once from RS but when I found out the part was coming from the UK I cancelled because I did'nt feel like waiting 2 weeks for a fuse. One of the older more experienced blokes at work has and no issues.

Interested in how this progresses for you, glad you sorted out what package you need. I mentioned it just in case.

[psmedley]

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

This is what I ended up ordering:
http://australia.rs-online.com/web/p/products/671-4777/

Just ordered the 2x FETS and replacement fuse for now - will order caps later once this issue is resolved.

Good thing about RS is they have free delivery and no minimum order value - farnell (element14) is only free delivery if you order over $A45 - otherwise it's $12.95

Found a Fairchild drawing of their 18N50 - the parts I removed are definitely TO-3P

[tom66]

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

TO-3P is a bigger brother of TO-220.

Do not get TO-3 and TO-3P mixed up... TO-3P is the same as TO-247. TO-3 is a metal cased power transistor, completely different form factor.

You can't really substitute them anyway as a TO-3P has a lower thermal resistance, essentially this means because it is bigger it can conduct more of the heat away.

I had a similar blown MOSFET problem with a Pioneer plasma, but I used a compatible FET and it worked fine... (only to find it had a vertical bar on the display, so unrepairable.) I used one which was better than the original. Remember like most consumer electronics it comes down to cost, so they'll choose the minimum cost components.

In a quick search I found https://cdn.badcaps-static.com/pdfs/...004f1f60b9.pdf

On-resistance virtually the same, higher breakdown voltage and better Qg (lower.) It's only rated for 13A -but- I doubt your set would pull that much even with full white.

Or,
https://cdn.badcaps-static.com/pdfs/...5a4135ad6d.pdf - high Qg, good current rating
https://cdn.badcaps-static.com/pdfs/...2bb90d52a3.pdf - okay Qg
https://cdn.badcaps-static.com/pdfs/...7bf7bf2936.pdf - okay Qg, higher current rating, lower on-resistance
https://cdn.badcaps-static.com/pdfs/...4a721cfa78.pdf - lower Qg, but limited to 16A, lower on-resistance

Seems like Samsung really liked to choose FETs made of Unobtanium!

[psmedley]

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

trying to find information now on mosfet packaging - an ebay UK listing suggests replacement fets that are TO-3P packaging, but I can only find TO-220 in stock not sure what the differences - I'd imagine it's physical size/shape

bugger it - RS has the TO-3P's on backorder - I'd rather wait and get the right size, than order the wrong size from farnell and have to root around making it fit...

[tom66]

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

You can buy small quantities of ICs as "commercial samples" from many manufacturers. Just shoot off an email and you'll usually get a discounted price too and decent shipping rates, sometimes free if you can find a sales office in Oz.

I had someone contact me about a similar 50" plasma "clicking on and off" - but I think I low balled them too low at £40.

[psmedley]

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

I think there's a good chance they'll need to be insulated from that heatsink. On that Panasonic I fixed they use a thin silicon pad. In fact the guy I got the TV from at work had lost it and used a cut down TO3 mica but it arced through a hole and spot welded the mosfet. He blew that set up so many times he gave it to me in disgust.LOL

We laugh about it now but careless mistake on our part. Maybe you could take a photo of the setup and get more opinions from some of the other experts here.

Just had a quick look at prices for those FETs. Pretty cheap at RS. Where are you getting your bits from?

I know we had a little trouble finding the one for the PANASONIC and certainly not as cheap as these.

Looks like there are a couple of different package designs too.

OK sounds like the pad is important - will see if I can find replacements, or may even reuse the old ones.

Looking at FETs from element14 - http://au.element14.com/fairchild-se...220/dp/1495258 $4.30 ea

hmmmmm I see they're quite a bit cheaper at rs - glad I didn't click the order button last night

Planning on ordering replacement caps too to get rid of the Samyoungs, so will reprice my order from RS and order today.

edit: RS out of stock/discontinued on all 5 possible 18N50's

[tw2005]

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

I think there's a good chance they'll need to be insulated from that heatsink. On that Panasonic I fixed they use a thin silicon pad. In fact the guy I got the TV from at work had lost it and used a cut down TO3 mica but it arced through a hole and spot welded the mosfet. He blew that set up so many times he gave it to me in disgust.LOL

We laugh about it now but careless mistake on our part. Maybe you could take a photo of the setup and get more opinions from some of the other experts here.

Just had a quick look at prices for those FETs. Pretty cheap at RS. Where are you getting your bits from?

I know we had a little trouble finding the one for the PANASONIC and certainly not as cheap as these.

Looks like there are a couple of different package designs too.

[tom66]

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

If it is not electrically conductive (is it?) and you spread enough to ensure isolation, then yes, but a mica/rubber pad is probably a safer option.

[psmedley]

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

another question - the existing 18N50's had thermal tape between them and the heatsink - can I use Arctic Silver CPU compound or what do people recommend?

[tom66]

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

In addition, the top branded ones will actually have a high-end endurance (for example, they will do 12k hours at maximum temperature and ripple current), but the low end ones will struggle to do 5k under ideal conditions!

I wouldn't worry about replacing any of the 100V, 200/250V or 450V caps.

[tw2005]

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

Well I was going to suggest checking those ones out first. That smoked PSU I saw when it cycled came out directly in line with that heatsink I marked. I don't have enough experience so I refrain. It's hard enough fault finding let alone giving someone bad advice. Did you compare the cap datasheets? Notice the lifespan 5000hrs compared to 10 - 12000hrs for those top branded ones.

[psmedley]

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

Yup the two transistors that are shorted are the top two ones that you highlighted. Need to figure out how to remove them too - going to be difficult to access the screw without removing all 4x mosfets and the heatsink. (Edit: a set of pliers worked to loosen the screws)

I'm not overly fussed if I don't replace the 22uf 450v if people think it will be OK based on the ESR of it. My reference table suggests 3 ohms for a 22uf 250v, so the 1.42 should definitely be ok for a 450v

[tw2005]

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

Planning on ordering parts tonight, will also order caps to replace the ones on board.

Looking for suggested replacements for the Samyoung KMX series 22uf 450v cap - datasheet at https://cdn.badcaps-static.com/pdfs/...0d392625c3.pdf

http://au.element14.com/ doesn't have a huge range of 22uf 450v - and none seem to be low ESR.

ESR of the cap that's currently on the board is 1.42 Ohms

How about RS Components? Were those Transistors part of that heatsink arrangement out of curiousity?

I always thought generally those larger caps rarely gave troubles?
Sounds like you'll have this running again soon.

http://australia.rs-online.com/web/c...ons=4294881167, 4294885866, 4294884262, 4294884421,4294722838,4294726026&lastAttributeSelectedBlock=4294960519

[psmedley]

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

Planning on ordering parts tonight, will also order caps to replace the ones on board.

Looking for suggested replacements for the Samyoung KMX series 22uf 450v cap - datasheet at https://cdn.badcaps-static.com/pdfs/...0d392625c3.pdf

http://au.element14.com/ doesn't have a huge range of 22uf 450v - and none seem to be low ESR.

ESR of the cap that's currently on the board is 1.42 Ohms

[psmedley]

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

Photos of front/back of PSU board - hopefully there was enough light left that these are sharp enough!

Also just confirmed that fuse F803 is blown, but there is still continuity through the terminals - I guess because QS801/802 are shorted?

[psmedley]

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

I read that if a mosfet is dead, they often short from drain to gate.

This seems to be the case here - I have continuity (< 3 Ohms) between drain and gate on QS801 & QS802, but not with QX801 and QX802

[psmedley]

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

I would begin by checking all supply voltages across the board, seeing if any are obviously wrong. Replace all startup caps too.

On CN801 - the connector to the input board, I measure the following:
STDBY 5v = 5.09V
5v = 0.69v
12v = 0.83v
18V = 0.83v

This is with CN801 unplugged. In this condition there is no clicking from the PSU

[tom66]

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

Just check all pins relative to each other; should measure >200 ohms.

[psmedley]

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

Thanks Tom - will investigate more and report back later after dog walking, etc

Also, given all the repair kits mentioning shorted mosfets, looking for good ways to test them for shorts - preferably in-circuit (if possible).

[tom66]

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

SM:

Pin outs for PSU connectors are on page 40.