Samsung PN51F8500AF continually clicks

[mtnickel]

I acquired a pn51f8500AF from a local guy. He had troubles with it and sent it out to have the Power supply fixed. put the power supply in and still didn't work so he cut his losses and gave it to me.

First I have disconnected everything from the mainboard and power supply and just connected the power supply to the mainboard. It still just clicks over and over about 1 second apart. All the voltages (even standby) are going down to 0v and then flicking on for a brief second. (all low voltages (5s and 15s) come up for a momentary second, but drop quickly back to 0).

If I disconnect the mainboard and force the PS on (ps_on to ground), it comes on and all voltages look normal. I can even force VS_on and the Vs and Va look good.

Back to when the PS and mainboard are connected:
The mainboard light flashes when the power briefly turns on. What concerns me is that standby voltage drops to ground. This makes me think there may be a short in the mainboard causing the power supply to reset or something.
When forcing PS_on with a jumper and still connected to the mainboard, the power supply still shuts off, so it's not that the mainboard is switching PS_on.

So at this point, I think perhaps mainboard. But my knowledge of power supplies is not very good and this one is known to be trouble. Could be one of those ghost loose solder joints that shows proper voltage under no load (mainboard disconnected) but with the load of the mainboard it drops down and causes a reset? Could be still a bad PS if perhaps a cap or something else is dead and even the slightest load on it causes it to shut off. I tried putting a 600 ohm resistor across every voltage output to the mainboard (the 5's and 15's). It powered the resistor to 3/4-1watt without trouble.

Only thing I could think of to do next is to try and remove pins from the plug and see if I could measure current draw on each supply voltage to the mainboard. Perhaps see if one is shorting out. Then look for shorts on the mainboard? bad voltage regulators? or keep eye out on ebay for replacement mainboard.

Thanks so much anyone who can give me some tips on this!

[ReeceyBurger123]

Re: Samsung PN51F8500AF continually clicks

Good testing Id say change Mb in that case.

[martino]

Re: Samsung PN51F8500AF continually clicks

Got a variable power supply? I would feed a few volts into the mainboard on the 5volt in line.by itself while freezing the board with aa can of air thats up side down. Slowly increase the voltage till you see a component that thaws the fastest. Gives a good indication on either the defective part or the area at least. Prob a voltage regulator or cap on main thats shorted.

[ReeceyBurger123]

Re: Samsung PN51F8500AF continually clicks

Or if you dont have one use a Pc Psu but make sure everything is disconnected from the mainboard.

[mtnickel]

Re: Samsung PN51F8500AF continually clicks

Got a variable power supply? I would feed a few volts into the mainboard on the 5volt in line.by itself while freezing the board with aa can of air thats up side down. Slowly increase the voltage till you see a component that thaws the fastest. Gives a good indication on either the defective part or the area at least. Prob a voltage regulator or cap on main thats shorted.

Thanks for the tips guys! I did 3 years of engineering before switching to business, so it's fun to get back to my roots.

I've got an ebay search alert for a mainboard should one come up. They are NLA and supply for these high end plasmas is not plentiful.

I don't have a variable supply, but could see how big of a potentiometer I have (may have an audio based 8 ohm one that will handle a bit of current). Otherwise, I feel it'd be risky using the PC power supply directly as they can provide a substantial amount of current. I'll see what I can find locally and maybe just buy one.

Thanks again, will keep you updated.

[mtnickel]

Re: Samsung PN51F8500AF continually clicks

EDIT: irrelevant now.

Well, I powered all the 5V lines. Is there a reason not to test the 15v Lines as well?
What I did was use a phone charger type thing which was about 1amp max. Hooked up it drew about .75Amps and dropped the voltage to around 4.7v.

Attached are pictures of the components to heat up first.

The marking is: N5 over BN.

From looking through mouser, I figured it's this SMD package: SOT-89-5

Anyone know how to identify it further? Perhaps try and source the Samsung service manual? I can probe the pins, but need to figure out the layout to know if it's a correct voltage or not.

There were also 2 small transistors I believe near the bottom that heated up fairly quickly as well. Marked "31" which i think is a NPN transistor. I will have to measure those shortly to see if either is shorted.

[mtnickel]

Re: Samsung PN51F8500AF continually clicks

Sigh. 1 step forward, 2 steps back. I didn't realize in my connections I had mixed up my left and rights on one of the pins for 5v. So during my first test i was sending 5v down a line that required 15v. That didn't harm anything, but I then put test probes into the back of the plug for the 15v connections (to keep separate from the 5v in front) and then I applied 15v to the same pin which sent 15v to the entire 5v system.

So my first test was probably a fail and didn't really check the 5v system since it was perhaps trying to drive a 15v lead as well and I missed applying 5v to a different pin.

Then when I connected 15v (was lazy and used the PS board) it sparked a bit and definitely blew a very small 16 pin chip...one I will likely not repair.

It definitely shows up fast as a dead short/heat area on the freeze test. and Board won't light up the LED anymore (in the attempt to startup).

So looks like i'm on the lookout for a new mainboard. I'll tell myself the other fault I may have found was a bad one anyways :P.

[mtnickel]

Re: Samsung PN51F8500AF continually clicks

Update: got a replacement mainboard.

It responds to the power button, but only clicks on then off. Just a fraction of a second.

But now when I force PS_On with a jumper, the board will actually power up and flash green led nice and steady (1 second apart). This also brings VS_on from the mainboard, yet I can't get any images to come up, no menu, no input change, no volume, etc. I do see the screen come on black now.

ALSO! if I force PS_on with the service jumper (pins 3/4 connected) I get all the test screens looking perfect.

I'm baffled.

I wondered if a defect in the power supply was causing a really high drain for it to turn on PS_on (a bad mosfet or something overloading the Mainboard Ps_on line). But I measured, and it only takes 2mA to turn on.

My thought is perhaps that the mainboard works yet its' getting a reset signal (DRV or DS? not sure how these work) or some other fault to turn off. Some sort of power on test that's not completing. I feel it's a step forward as now jumpering PS_on at least doesn't keep reseting/shorting to ground. I notice there are temperature sensor pins coming from the sustain boards, maybe one of those is bad?

Anyone have any ideas from here? There is always the possibility of this mainboard being bad as well. Could purchase another provided they have a decent return policy.

[martino]

Re: Samsung PN51F8500AF continually clicks

if I force PS_on with the service jumper (pins 3/4 connected) I get all the test screens looking perfect.

You received a faulty mainboard. All other boards work fine.

[mtnickel]

Re: Samsung PN51F8500AF continually clicks

You received a faulty mainboard. All other boards work fine.

Keep in mind that jumper is on the same mainboard (it's not separate mainboard and control board like some). It's an all-in-one. Would it not be possible that it's a signal reporting back to the mainboard that won't allow it to start it's turn on procedure?

As I said the mainboard does have a steady flashing green light when PS_on is forced on. Makes me think it's running correctly, but an input to the board is causing it to turn off. Guess it's worth another try. Replacement boards are like $150USD which is adding up fast.

[mtnickel]

Re: Samsung PN51F8500AF continually clicks

Could it have anything to do with the power supply if it has a weak standby voltage?
I measured again and it requires 20mA (not 2) from ps_on to ground to turn it on. Normal still.

I measured the standby voltage and when I hit the power button, it doesn't stay a steady 5v, it drops to 3 very briefly during the click where it's trying to power on. (By briefly, I mean like an instantaneous drop coinciding with the click). Maybe the mainboard isn't getting enough juice from the standby line to go through proper power on. But strange the IR and power button and power led are all fine.

These things are known to have bad power supplies and I'm just wondering if there's anything else I can check on it.

[dskall]

Re: Samsung PN51F8500AF continually clicks

You now have different symptoms. You can now plug your mainboard in without clicking. You can get a test pattern. Means rest of tv and power supply are good. I doubt if you would get test pattern with bad standby

[mtnickel]

Re: Samsung PN51F8500AF continually clicks

You now have different symptoms. You can now plug your mainboard in without clicking. You can get a test pattern. Means rest of tv and power supply are good. I doubt if you would get test pattern with bad standby

I can plug the mainboard in without clicking...but only with a manual jumper keeping the PS on (PS_on jumpered to ground). Without the jumper the TV will only click on/off right away (when using the power button or remote). But the test images are encouraging regarding knowing the panel works, and likely the sustain and buffer boards are ok (but small chance of a false fault being sent? bad temp sensor?).

Wondering now if it's maybe the PS still. I did some testing on the Standby Line:

1) the Standby line can hold voltage fine into a 40 ohm load (about .6 watts), but fails with a 5 ohm load (5 watts). I could rig up some more resistors and see how much I can get it to hold.

2) With the VS and VA cable removed from the PS, when I try to turn the unit on, it continually clicks (like before). The voltage on Standby drops from 5 to like 4.8V. Nothing bad.

3) With that VS and VA cable connected, when I try to turn the unit on, it clicks just once, but the Standby voltage drops down to 2-3V. It comes back up to 5V after maybe 1-2 seconds.

4) With the mainboard jumpered on (PS_on), My computer detects the display but still no output, no menu. Tried all 4 HDMI ports to get sound and also hit the source key many times but no sound.

Found a seller online who will send me another mainboard and return it if I pay for to/from shipping. Might be worth the $20 for peace of mind to see if it responds exactly the same way as the last one.