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

[psmedley]

Hi All,

Considering bidding on the subject TV - description of fault is:
- No Longer Working, (Clicks when turning on and off)

From what I can find searching - this could either be a bad sustain board, or a problem with the power supply itself, but should definitely be repairable.

Thoughts?

Cheers,

Paul

[tom66]

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

Sustain board, probably. I had a PS42C450 which clicked on and off - shorted X-main board - cost £16 for a replacement.

You've been bitten by the repair bug!

[tw2005]

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

Hi All,

Considering bidding on the subject TV - description of fault is:
- No Longer Working, (Clicks when turning on and off)

From what I can find searching - this could either be a bad sustain board, or a problem with the power supply itself, but should definitely be repairable.

Thoughts?

Cheers,

Paul

I'll go with pwr board. Fixed one of these recently with bad solder joints on a row of transistors attached to long heatsink. Symptoms were randomly shuts off, repeated cycling of the pwr board relays with unit not powering on. seems to be a very common fault with this series from what I'm seen with the broken sets advertised.

[psmedley]

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

Got the TV - probably paid too much ($A101), but Tom66 is right - I've been bitten!!!

Pictures of board layout attached.

Will get closeup photos of the boards once I can get some decent lighting.

Initial observations:
- when connecting power, clicking noise from the PSU, repeats every 10-15 seconds.
- disconnect Y-main board from PSU - same
- disconnect X-main board from PSU - same
- disconnect board behind main board (Z-main?) from PSU - same
- disconnect board that runs across bottom of TV - same
- disconnect all boards except main board - same

Suggests a PSU issue?

I do note that there are 2 25v 68uF Samwha's on the Y-main, and 1 x 25v 680uf Samwha on the X-main that are clearly bulged.

PSU caps are Samyoung

More to come - but I'll probably test all caps on the PSU with my ESR meter and check for dry joints next. Might not be until the weekend tho.

[psmedley]

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

Have found several badcaps on the PSU: - photo attached - will try get a clearer one tomorrow in daylight
50v 47uf with 0.6 ohms ESR vs reference value of 0.5
25v 200uf with 0.34 ohms ESR vs reference value of 0.23
50v 100uf with 0.84 ohms ESR vs reference value of 0.31
50v 47uf with 0.84 ohms ESR vs reference value of 0.5
50v 47uf with 1.01 ohms ESR vs reference value of 0.5
25v 1000uf with 0.21 ohms ESR vs reference value of 0.08
25v 1000uf with 0.17 ohms ESR vs reference value of 0.08
50v 47uf with 0.91 ohms ESR vs reference value of 0.5
25v 220uf with 0.56 ohms ESR vs reference value of 0.23

Not sure about the following as I don't have reference values:
100v 820uf with 0.05 ohms ESR vs reference value of ??? - guessing from table between 470 & 1000 uF I'd say reference should be 0.05-0.06 so borderline??
10v 2200uf with 0.06 ohms ESR vs reference value of ??? - guessing OK? 10v 1000uF has ref of 0.12, 4700uF has reference of 0.23 - this is lower than both
10v 2200uf with 0.06 ohms ESR vs reference value of ??? - guessing OK?

big caps:
250v 820uf with 0.13 ohms ESR vs reference value of ???
450v 270uf with 0.18 ohms ESR vs reference value of ???
450v 270uf with 0.17 ohms ESR vs reference value of ???

[tom66]

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

All caps look okay (typically ESR is higher for small can sizes) but if you want it to last, I'd replace most of the >470uF ones. The 450V, 250V and 100V can stay.

The problem is probably the cold joints as tw2005 mentions... http://www.youtube.com/watch?v=-Ya8zJMShcs

Check though if it powers on with either sustain disconnected.

[tw2005]

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

I've marked the heatsink that I reflowed on the one I fixed. I could get it to work/fail by applying pressure with a wooden spoon. It could be live! Can't remember now.

I'd reflow that first. I was at a lady's house picking up another broken set whan she mentioned she also had the same model doing the same thing but she had 5yr wty. The PSU literally went up in smoke whilst she showed me when it cycled a couple of times!

[psmedley]

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

Check though if it powers on with either sustain disconnected.

I disconnected all boards other than the input/main board and the behaviour was the same. So either the input/main board is the problem or the PSU (IMHO).

Will try reflowing the joints tw2005 suggests tonight - off to work now!

[psmedley]

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

OK reflowed joints that tw2005 suggested - also the joints for the parts attached to the LH heatsink - and no change in behaviour....

[tw2005]

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

OK reflowed joints that tw2005 suggested - also the joints for the parts attached to the LH heatsink - and no change in behaviour....

Well that's a bugger. Maybe it's blown then. The only other PSU I've had to fix in a plasma was a Panasonic with a blown MOSFET. It continuously clicked because obviously the PSU was not starting up due to the dead switching transistor.

Time to follow the circuit and fault find the components I guess or find a cheap PSU.

Not cheap new.

http://www.electronicsmarket.com.au/...c1-p-6698.html

[tom66]

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.

[psmedley]

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

Startup caps would all be on the hot side of the PSU?

I also see some repair kits on ebay, that suggest checking QX801 or QX802 S/C. (if these are okay then check QS801 and QS802 for S/C. I will look into this later today.

Will also try and check voltages - unfortunately there don't see to be too many voltages labelled in order to verify they are correct

[tom66]

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

Startup caps would all be on the hot side of the PSU?

I also see some repair kits on ebay, that suggest checking QX801 or QX802 S/C. (if these are okay then check QS801 and QS802 for S/C. I will look into this later today.

Will also try and check voltages - unfortunately there don't see to be too many voltages labelled in order to verify they are correct

Yep startup caps on the primary. The voltages are usually 5.2/5.3V standby, 5.2-5.4V run and 18V audio plus normal plasma voltages of 50-70Va and 170 - 210Vs.

[psmedley]

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

found this too: http://img249.*************/img249/8...10127at184.png
which shows voltages.. not where to check them but at least its a start

[tom66]

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

SM:

Pin outs for PSU connectors are on page 40.

[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

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

[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

[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

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?