Samsung LN40B630N1FXZA Power Board Troubleshooting

[ksideris]

Hello folks. Found a Samsung LN40B630N1FXZA. Brought it home, trying to fix.

Symptoms:

Started it up, got the startup melody, screen comes up, then 15 seconds after that, turned off, no response to buttons. Unplugging and re-plugging it, no response unless I leave the TV unplugged for a long time (more than a few hours), where I get the same routine, startup melody 10-15 seconds, screen off.

Opened it up. Has a BN44-00264A power board. Unplugging the main board and attempting to power it up, get the backlight to come up for a second or so, then screen goes out. From the little I know, that seems to indicate a bad power board?

Trying to troubleshoot a little more, learn, and hopefully avoid the cost of a full new board.

No bulging or leaking caps that I can tell. Took board out, turned around, nothing totally evident. Pictures attached.

Have searched the forum, but unfortunately it seems the stuff that I can understand have not pointed me somewhere. Looked at the schematic but cannot figure it out too much.

Multimeter shows big caps jumping for a sec to 400V, or a little less at startup, then dropping to 166V. Fuses look good, measure fine on continuity.

Cannot get any readings at the CNM802 plug, not sure if I am doing something wrong to not get anything there.

CM806 shows 166V, CM805 shows 0V. All the caps on the right side of the board (closer to CNM802 plug) that I can measure appear to show 0V.

My soldering, desoldering skills are not that great, so I am trying to avoid taking unnecessary components off to test them.

Any pointers as to next steps to troubleshoot would be greatly appreciated.

[Ltank]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

Power Supply PFC circuit is shutting down. What about your 5vdc standby?

[ksideris]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

LTank, thank you for your response.

Unfortunately I do not get any readings at all at the CNM802 plug.

Where else on the board can I measure for it?

[tom66]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

You must have some readings at that plug or the TV would not be working at all, you may need to press the probe in fairly hard to reach the metal sheath around the connector pin.

[ksideris]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

tom66 thanks, will try again tonight to see if I get anything.

I tried it without the cable plugged in at all and was not getting any readings straight from the pins though. Does it go into some sort of disabled mode?

I saw some other threads where some readings were requested on caps that can only be read from the bottom of the board.

I did not want to just assume that I can take those readings with the board not plugged in to the bulb connectors and just the power plug connected through, but I can see no other physical way of accessing those pins, short of soldering wires to all of them, which I doubt is what is being asked.

[budm]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

What do you use for Ground ref for your meter when you are measuring the Voltage in the Primary Hot side of the circuit? The DC GND in the primary will be the negative leg of the main filter cap. You are getting power otherwise you will not have 160VDC on the main filter cap.
SCH provided by TOM66.
BTW, if you search the site with 'BN44-00264A' you will see lots of threads.

[Shinju]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

Also check for bad cold solder joints on the Aluminum heatsink and fets. Also your issue might be a bad transformer. I have had my fair share of fun with these!

I have yet to see any bad caps on these boards, mostly bad inverter transformers/Fets.

[ksideris]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

Thank you for the responses folks.

When measuring the pins I think I may have tried with one of the other pins on CNM802 labeled as ground for reference. I did that because I thought that was the cold side?

It dawned on me this morning to search by the power board model rather than the TV model. Did find a couple more threads where the issue at least appears to be similar in that the power board alone will not light up the backlight and was planning to go through the steps suggested in those tonight as well.

I tried to test the FETs (QI820, QI821, QP801S, QP802S) in circuit yesterday. Not sure if I was screwing up the procedure or you cannot get reliable results in circuit. Is it possible that all 4 of these have gone at once?

[tom66]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

I tried to test the FETs (QI820, QI821, QP801S, QP802S) in circuit yesterday. Not sure if I was screwing up the procedure or you cannot get reliable results in circuit. Is it possible that all 4 of these have gone at once?

What did you measure?
You can't use a transistor tester in circuit. But I've never had a problem checking if a transistor was dead short in circuit. A good transistor usually tests open, most of the time, the failure mode is almost always a short circuit condition drain-source and often drain-gate/source-gate too.

[ksideris]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

Ok folks, a bit delayed because I managed to blow a fuse by applying power while the board was not connected to the bulbs and had to run to radioshack to buy replacements.

So I basically get 0V anywhere on the cold side of the board within a second of putting in the power cord.

Let me go through the events for tonight.

Plug in TV, startup melody (given it was unplugged for quite a few hours). CP815, CP816 measured at 390V+. Unfortunately did not have a chance to measure anything else with screen on. Screen turns off promptly 5 seconds after it comes on. CP815, CP816 fall to 170V.

Once this happens, subsequent connections of the power cable will not bring TV up unless I leave TV for more than at least an hour. If I disconnect the main board, I may see the backlight flash on for less than a sec (depends on how long I have left the TV off the cord, longer off, more likely to happen).

At one point I think I was able to catch standby blip to 4V or so before draining down again.

Measurements on the hot side with ground ref being the negative side of CP815:

CP815: 170V
CP816: 170V
CM806: 170V
CM805: Both sides show 170V (?)
CM808: ranges between 9 and 13V.

Then I started measuring resistances on the between drain and source on my FETS (QI820, QI821, QP801S, QP802S).

All of them seemed to be infinite and then I started second guessing myself (at this point the post takes a darker turn) and thinking 'should I be plugged in for this'? So the answer is of course NO....

But the result is that I got some nice sparks, and I think I killed RP806. I am getting a resistance of 1.2 Mohms on it now, not sure what the spec is. Photo is attached.

So I guess I am feeling pretty discouraged right now... Did I kill off my board or is there still hope for it with some fine soldering skills (that I do not possess)?

I know I can buy a new power board and most likely get a TV for $70, which will probably save me a few nights, but still, damaging something more than what it was is a disappointment to my dreams of reviving my electrical skills.

[Neelix]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

Photo is blurry and so it difficult to see if the pads got damaged, but RP806 is definitely killed. According to diagram, it's resistance value should be 10Kohms. Measuring resistances in a circuit with live voltages can also damage your meter. What or where did you measure when RP806 got killed and you saw the sparks?

[tom66]

Re: Samsung LN40B630N1FXZA Power Board Troubleshooting

RP820 and the QP801/QP802 could be bad.