Another Samsung plasma issue

[YamahaFreak]

This time I have a PN60F5500 (AF) that came to me with the following symptoms: TV goes through the motions of powering up, but there are no relay clicks or other noise, no power to the panel, and (as far as I can tell) no sound from the speakers. When you press the power button, the front standby LED flashes slowly several times, like it normally would while the set is powering up, then goes out, as it would once the panel comes up. The standby LED illuminates when the remote is pointed at the TV and buttons are pressed, so the TV is 'awake' enough to see the remote. You can power the set off and the power/standby LED comes back on, as it's supposed to when the TV is off. Inside the set, the green LED on the mainboard flashes at a rate of once every 4 or 5 seconds while the power is on. Various research has told me that the problem most likely lies somewhere on the Y-main (Y sustain) board, Samsung part number LJ41-10331A. I have replaced the small 7-pin IC after being told this could be the issue...this did not fix it. (I installed a socket, just in case.) Some threads online state that replacing three ceramic capacitors on the Y board inside the 64-inch version of this TV fixes the problems, but that size TV uses a different Y board than my 60-inch one, and I was unable to find any similar caps on my Y board. Everything inside my TV looks clean and tidy, no bulging caps or burned/smoked parts, blown fuses, etc. I'd love to repair, rather than replace, the Y board (if it's the culprit), since that is of course the most expensive board in the TV. I can of course answer questions and provide detailed photos, if they will help. I invested in a stand for this set already, so really hoping I can fix it!

[JohnCT]

The first step is to check the VS and VA voltages from the power supply to the sustains. Samsungs usually have marked test points on the power supply. If one or both are missing, do a resistance check to ground on those two lines - they should be essentially open. If you get a low ohm (1-2 ohm) short to ground, disconnect the Y main. If the short goes away, you've got shorted mosfets or igbts on the sustain, and possibly bad buffers as well. If the short remains, do the same with the X main.

There's more, but start there.

John

[YamahaFreak]

Thanks John, I'll take voltage measurements and get back to you with what I find. I haven't looked at this particular set, but I assume the correct VS and VA voltages are printed on the back of the panel (they usually are).

[YamahaFreak]

UPDATE

I have replaced the Y-sustain board, and I have one change to report: the panel now flicks on for a fraction of a second upon powering on the TV. No change with repeated power cycles. That Y board cost me quite a bit, so hopefully whatever else is still wrong can be fixed cheaply. I'm sure the panel itself is fine, and the power supply should be okay...mainboard is still flashing the once every four seconds code.

[JohnCT]

What are you VS and VA voltages, and if they appear, do they stay on or do they decay after a few seconds?

John

[YamahaFreak]

Originally Posted by JohnCT

What are you VS and VA voltages, and if they appear, do they stay on or do they decay after a few seconds?

John

John, thanks for reminding me. Can you refresh my memory as to how to properly take these voltages? I assume there are VS and VA test points, and the negative probe goes somewhere on chassis ground, such as the TV frame. I want to make sure I don't fry the TV, my meter, or myself!

[JohnCT]

Originally Posted by YamahaFreak

John, thanks for reminding me. Can you refresh my memory as to how to properly take these voltages? I assume there are VS and VA test points, and the negative probe goes somewhere on chassis ground, such as the TV frame. I want to make sure I don't fry the TV, my meter, or myself!

You won't damage your meter no matter what you do. The highest voltages on a plasma are below 400V under any condition.

On the power supply, there are test points marked for the VS and VA, which are developed on that board. There are also other voltages that are generated on the sustain boards, but that's farther down the road.

If there are no test points, then use the large harness connectors. The VS will usually have two to four pins all paralleled for current capability and the VA one or two, but the boards are marked for these voltages right at the connector.

For either voltage, you use cold ground as reference, which would be any of the mounting screws (other than upper and lower buffer board mounting screws which might be above ground) or even the entire back side of the plasma display itself.

Other voltages on the sustain boards can be referenced to cold ground or depending on maker, there might be two test points printed on the board to check those voltages.

But for now, our first attack is to see if the power supply is working, so the VS and VA voltages are the first step.

John

[YamahaFreak]

Originally Posted by JohnCT

You won't damage your meter no matter what you do. The highest voltages on a plasma are below 400V under any condition.

On the power supply, there are test points marked for the VS and VA, which are developed on that board. There are also other voltages that are generated on the sustain boards, but that's farther down the road.

If there are no test points, then use the large harness connectors. The VS will usually have two to four pins all paralleled for current capability and the VA one or two, but the boards are marked for these voltages right at the connector.

For either voltage, you use cold ground as reference, which would be any of the mounting screws (other than upper and lower buffer board mounting screws which might be above ground) or even the entire back side of the plasma display itself.

Other voltages on the sustain boards can be referenced to cold ground or depending on maker, there might be two test points printed on the board to check those voltages.

But for now, our first attack is to see if the power supply is working, so the VS and VA voltages are the first step.

John

Hi again John,

I found the measurement process straightforward. Here is what I found.

VS voltage marked on the panel is 208 volts. I measured a high of 463 volts at startup, then it steadily dropped until stabilizing at 23 volts.

VA voltage marked on the panel is 57 volts. I measured a high of 126 volts at startup, dropping to zero after several seconds.

I repeated the measurements a few times...always the same. This, to me, suggests there is a power supply issue.

[JohnCT]

Originally Posted by YamahaFreak

Hi again John,

I found the measurement process straightforward. Here is what I found.

VS voltage marked on the panel is 208 volts. I measured a high of 463 volts at startup, then it steadily dropped until stabilizing at 23 volts.

VA voltage marked on the panel is 57 volts. I measured a high of 126 volts at startup, dropping to zero after several seconds.

I repeated the measurements a few times...always the same. This, to me, suggests there is a power supply issue.

Holy crap, I've never seen one do that. Are you sure about your meter and using the power supply ground screws for ground? These have a hot and cold side so if you're using the heatsinks as ground, be aware that there are one or more on the hot side.

Even when I've seen them overshoot, it's never been more than a few volts as the power supply is very good about watching for overvoltage and snuffing them out quickly.

Also, most VA and VS supplies are separate from each other, so it seems extraordinarily odd that both would not only overshoot, but by so much.

Please don't take this the wrong way, but see if you can verify your meter.

I'll see if I have a power supply for that in the shop. Is there a BN44 number on it?

John

[YamahaFreak]

Originally Posted by JohnCT

Holy crap, I've never seen one do that. Are you sure about your meter and using the power supply ground screws for ground? These have a hot and cold side so if you're using the heatsinks as ground, be aware that there are one or more on the hot side.

Even when I've seen them overshoot, it's never been more than a few volts as the power supply is very good about watching for overvoltage and snuffing them out quickly.

Also, most VA and VS supplies are separate from each other, so it seems extraordinarily odd that both would not only overshoot, but by so much.

Please don't take this the wrong way, but see if you can verify your meter.

I'll see if I have a power supply for that in the shop. Is there a BN44 number on it?

John

John, would a low battery in the meter affect its readings? It hasn't ever given me grossly wrong readings before. I am using the wall mounting screw holes for ground. I assume I'm supposed to be using the AC setting, not DC. I will get the part number off the board tomorrow when I open the TV up again. Thanks for your guidance so far.

[JohnCT]

Originally Posted by YamahaFreak

John, would a low battery in the meter affect its readings? It hasn't ever given me grossly wrong readings before. I am using the wall mounting screw holes for ground. I assume I'm supposed to be using the AC setting, not DC. I will get the part number off the board tomorrow when I open the TV up again. Thanks for your guidance so far.

The wall mounting holes are fine for ground when reading any secondary supply voltage from the power board, but you are measuring DC, not AC.

Even so, those supplies are well bypassed and I'm surprised your meter is giving you those readings on the AC scale. What kind of meter is it?

If your meter has a min/max feature, preset the range for higher voltage and record the max voltage for the VS and VA.

If it doesn't have the min/max, just read the highest value you see before it shuts down.

John

[YamahaFreak]

Originally Posted by JohnCT

The wall mounting holes are fine for ground when reading any secondary supply voltage from the power board, but you are measuring DC, not AC.

Even so, those supplies are well bypassed and I'm surprised your meter is giving you those readings on the AC scale. What kind of meter is it?

If your meter has a min/max feature, preset the range for higher voltage and record the max voltage for the VS and VA.

If it doesn't have the min/max, just read the highest value you see before it shuts down.

John

John,

Right, will switch to DC readings. The meter has no min/max function, as it's just a cheapie from Harbor Freight. It's all I have. I will open the set again tonight and get updated readings for you.

[YamahaFreak]

I have new measurements, now that my meter is properly set to DC voltage. On startup, VS voltage briefly peaks at 211 VDC, then drops, at first sharply and then slower and slower until finally settling at 11 VDC after about a minute. VA voltage peaks at 57 VDC at startup, then quickly falls off to zero.

See the attached image for the power supply board part numbers - there are several BN44 numbers.

[JohnCT]

Originally Posted by YamahaFreak

I have new measurements, now that my meter is properly set to DC voltage. On startup, VS voltage briefly peaks at 211 VDC, then drops, at first sharply and then slower and slower until finally settling at 11 VDC after about a minute. VA voltage peaks at 57 VDC at startup, then quickly falls off to zero.

See the attached image for the power supply board part numbers - there are several BN44 numbers.

Those sound about right. Check the panel, there's a sticker that will give you those voltages.

If the 212 is high (I think it's OK), then the power supply will shut itself down. There's generally a VS adjustment on the power supply. You can try moving it a few degrees counter clockwise and see if the voltage matches the label. If it's high, I've found a small value electrolytic up in that area that goes high ESR.

If the voltages are correct, I think you may have a problem at a buffer (cue scary music) that's shutting down the sustain, which tells the control board, which shuts down the power supply.

Before you do, go over the entire surface of the screen with a bright flashlight and look for a teensy tiny bullet type star crack anywhere on the screen. If it has one, it won't be on the outer surface but on the face of the pixels (internal glass). If you find one, or a burned pixel, stop there. The panel is bad and the game is over. If you don't see any cracks or burned pixels, then let's check the scan buffers.

I have a shortcut I came up with to quickly test the scan buffers, but it requires a DMM with a diode scale. I've always used a Fluke for this test but yours should work fine if it has a diode scale.

Flip open any one of the ribbon connectors that connect to the display, and put the negative probe of your meter on *any* of the connections at the connector. The pin spacing is tiny and your meter probe will probably cover at least two, but that's not a problem as we're going to do a batch test of all the output lines of the scan buffer chips on the buffer boards.

So with the meter on diode, and the negative lead on any pin under the ribbon connector, put your positive lead on any of the edge connectors between the scan buffers and the sustain. Move the probe until you get a diode reading (0.6V give or take a tenth). Once you found the right spot, slide the positive probe across all the pins for each of the ribbon connectors. Do this slowly and keep an eye on the meter. If the indicated voltage on the meter drops to zero, there's a shorted scan line on the output side of the buffer. A single scan line will shut the whole shooting match (East Coast expression) down. My experience is that when I find one bad scan line, there's close to a dozen more. All of them coming from the same buffer IC.

If you find that as you move to another connector the meter voltage goes high, move your negative lead until you once again are reading a junction in circuit (0.6v).

If all lines test clear, then you may have a bad Y main (sustain), X main, control board, or main.

John

[YamahaFreak]

I will try and perform your suggested tests tomorrow evening. In the meantime, would it be safe to assume the panel itself is okay if it does illuminate for a brief moment upon each startup cycle?