Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

[scokljat]

Hello,

Well, this one is for my mother in law ;-) So, you know the level of importance...

It's a year 2003. model 32" LCD TV Samsung LW32A23W. Quite solid and quality TV as I can see... But, TV came to me year and a half ago. Initial symptom was "sometimes it won't turn on". Really, TV sometimes turn on OK, works OK, you can switch it on/off with remote or soft buttons. But sometimes it fails to turn on with soft switches or remote. Relay clicks, LED is orange (not green) and it's stuck. Once when you are stuck in that point you must turn it off with hard power switch or eject AC plug, and than retry. After few retries TV would switch on OK again.

Now, I left it aside to repair it when catch some free time, and year and a half has passed ;-) Yesterday I finally decided to troubleshoot it and repair. Now after year and a half in storage it won't power on at all. Problem deteriorated, when TV is plugged to AC and hard power switch is turned ON, relay clicks, LED is orange and that's all. No response to soft switches or remote.

First I was sure it's caps because problem deteriorated over time, but I checked PSU caps with ESR meter, all caps are OK. I didn't checked caps on inverters, t-con and digital board. I have no ideas what to check next.

Please, could you give me some advice, where to start, what to check...

Service manual is attached.

Thank you!

Best regards,

Suad

[CapLeaker]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Wow that's an old TV! Not worth frigging around with much, unless you do it for "experience".
Try heating up the main board with a hair drier nice and warm. See if the set will respond then.

STBY voltage goes to the main board from the PSU (and that is there, because the LED is on), the main board gives the PSU the PSon voltage, which turns the rest of the power supply on. I think the main board isn't giving PSon. You can try and "force on" the power supply maybe, but I got to look at that schematic first.

[ben7]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

As CapLeaker said, check to see if the PSU is getting the signal to turn on.
If it is, then check to make sure none of the PSU outputs are shorted.

Also, check for cracked solder joints.

You sure the caps are OK ESR wise?

Also, have you shone a flashlight up close on the screen? If the backlights aren't running (and the rest of the TV still works), you should still be able to see a faint picture (you have to look closely though, with a bright flashlight).

[scokljat]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

As CapLeaker said, check to see if the PSU is getting the signal to turn on.
If it is, then check to make sure none of the PSU outputs are shorted.

Also, check for cracked solder joints.

You sure the caps are OK ESR wise?

Also, have you shone a flashlight up close on the screen? If the backlights aren't running (and the rest of the TV still works), you should still be able to see a faint picture (you have to look closely though, with a bright flashlight).

Main PSU has only two inputs and three outputs. Inputs are 220V main input and 25V ps-on voltage. Also there is 6V standby voltage on standby board. So, main PSU gets 220V and 25V ps-on but gives no 12V, 12V audio or 5V output. I think problem is on main PSU (big pcb in center of tv). All caps on main PSU check OK with ESR meter.

[ben7]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Main PSU has only two inputs and three outputs. Inputs are 220V main input and 25V ps-on voltage. Also there is 6V standby voltage on standby board. So, main PSU gets 220V and 25V ps-on but gives no 12V, 12V audio or 5V output. I think problem is on main PSU (big pcb in center of tv). All caps on main PSU check OK with ESR meter.

Check to make sure there is no short on the output of the PSU. If there is, that would prevent it from turning on.

[scokljat]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Check to make sure there is no short on the output of the PSU. If there is, that would prevent it from turning on.

That is exactly what I gonna try first thing tomorrow. Stay tuned. Thanks!

Btw, maybe some short on inverter power supply section (120V) on the main PSU?

[ben7]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Btw, maybe some short on inverter power supply section (120V) on the main PSU?

That could be possible, but I don't know what the schematic looks like so I can't say for certain. I don't think it is likely though. A short on the primary side would very likely make the fuse(s) blow.

I would think that the TV would still power on but the backlights wouldn't work, if the inverter power supply wasn't working. Then again, it might monitor that and not turn on the other output voltages if the backlight PSU cirucitry won't turn on. Again, can't say for sure since I haven't seem the schematic. But it is still totally possible to fix it without a schematic ... might be a bit more difficult, but it is a good problem solving exercise

[budm]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Disconnect the cables to the Inverter board and see if the 24V will stay on.
So you do have PS-ON signal for sure? If you do, then the DCV on the two main filter caps in the primary side should be >360VDC (measure right at the two legs of the caps), this PFC Voltage booster must be working for the main 24V/12V supplies to function.

[scokljat]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

That could be possible, but I don't know what the schematic looks like so I can't say for certain. I don't think it is likely though. A short on the primary side would very likely make the fuse(s) blow.

I would think that the TV would still power on but the backlights wouldn't work, if the inverter power supply wasn't working. Then again, it might monitor that and not turn on the other output voltages if the backlight PSU cirucitry won't turn on. Again, can't say for sure since I haven't seem the schematic. But it is still totally possible to fix it without a schematic ... might be a bit more difficult, but it is a good problem solving exercise

I attached schematics along with pictures in the original post. Take a look...

[ben7]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

I attached schematics along with pictures in the original post. Take a look...

Whoops, LOL, sorry, I didn't see that at first xD
I just usually look at the pictures, that distracted me :P

[scokljat]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Here is the update:

I did lot of testing on the main power supply board. 220V input, standby power, PS-ON voltage is OK, all fuses are OK.

12V sound power supply is OK! (12.5V), but 12V and 5V are dead.

I found when I connect DVM to STR-W6853 MOSFET input (pin 4). TV powers ON! Sometimes it turns ON imidiately when I connect DVM to pin 4, and sometimes I have to touch pin 4 several times, but from that point TV works OK. Look at the schematics picture. Hi res schematics you can download on my initial post, page 34. Once TV is switched ON, I get normal 5V (4.98V) but 12V looks little high (13.3V). Is it normal?

Does it mean STR-W6853 MOSFET is my primary suspect?

Just in case I replaced all caps on 5/12V secondary and I found one small 100uF cap that reads 9uF on my ESR micro. It's a solid state capacitor (see picture), I was puzzled why it's bad. Is it some special sort of capacitor so my ESR micro gives bad reading? I replaced it with standard low ESR capacitor - no change, TV still won't turn on with all secondary filter caps replaced. Funny thing is, I cannot find this 100uF solid cap on schematics. It's not there!

I will check all the caps on 5/12V primary as well.

So, what do you think now? What to check next? Should I change places of STR-W6853 MOSFET from audio power supply so I can monitor changes?

I'm quite puzzled with this one.

When I switch TV ON (with touching pin 4 with DVM), TV works OK. But, when I turn it off with remote and than back ON, it won't switch ON unless I touch pin 4 of STR-W6853 MOSFET again. I checked for cold/cracked solder joints - all is OK.

Waiting for your response...

Thanks!!

Cheers!

S

[budm]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Pin 4 of the STR-W6853 is the VCC pin and it must have 18.2V (see page 14) for it to start up.
So did you check the start-up/running cap for this IC?

[scokljat]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Pin 4 of the STR-W6853 is the VCC pin and it must have 18.2V (see page 14) for it to start up.
So did you check the start-up/running cap for this IC?

Is it the one I marked on schematics? (see picture) Please could you mark what to check/replace. When I measure pin4 and GND (GND directly, not thru RM808 resistor) with DVM I get 100V.

Thank you!

Cheers!

S

[budm]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

Measure between the legs of CM801, also between the two legs of CM802.
Also check that resistor RM808 to see if it is OK.
Do not use the chassis for your meter.

[scokljat]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on! - SOLVED!

Measure between the legs of CM801, also between the two legs of CM802.
Also check that resistor RM808 to see if it is OK.
Do not use the chassis for your meter.

SOLVED!!!

It was STR-W6853 MOSFET after all! I checked, RM808 - OK, checked and replaced all small caps, and finally I swapped STR-W6853 with other one from audio 12V exclusive. After swap I got 12V and 5V but lost 12V audio voltage. I bought new STR-W6853 for 5$ in a local shop. Problem solved.

Few final questions:

1. how do I measure startup voltage on pin 4 of STR-W6853 MOSFET? Where to put DVM probes?

2. now I have nice 5V but 12V is 13.3V. Is that OK? There is a 5V trimmer and it affects 5V and 12V simultaneously. I trimmed it to get perfect 5.00V but 12V is 13.3V. Is it OK?

3. I used standard small caps (10u/50V, 33uF/50V, 22V/50V...), not low ESR. Should I leave it or return old ones? I have low ESR ones but all are 35V and not 50V.

4. any suggestions what more to replace just for maintained?


Thanks for all your help!

Cheers!

Suad

[budm]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

"1. how do I measure startup voltage on pin 4 of STR-W6853 MOSFET? Where to put DVM probes?" on the two legs of the startup/running cap CM801 10uF 50V.

"2. now I have nice 5V but 12V is 13.3V. Is that OK? There is a 5V trimmer and it affects 5V and 12V simultaneously. I trimmed it to get perfect 5.00V but 12V is 13.3V. Is it OK?" As you can see, the regulation feeddback circuit only monitoring the 5V output not the 12V output so the 12V is not regulated, the circuit the 12V is feeding probably does not require tightly regulated 12V.

You can recap the whole power supply board but this one has lots of caps.

[scokljat]

Re: Help needed - Samsung LW32A23W 32" LCD TV won't turn on!

"1. how do I measure startup voltage on pin 4 of STR-W6853 MOSFET? Where to put DVM probes?" on the two legs of the startup/running cap CM801 10uF 50V.

"2. now I have nice 5V but 12V is 13.3V. Is that OK? There is a 5V trimmer and it affects 5V and 12V simultaneously. I trimmed it to get perfect 5.00V but 12V is 13.3V. Is it OK?" As you can see, the regulation feeddback circuit only monitoring the 5V output not the 12V output so the 12V is not regulated, the circuit the 12V is feeding probably does not require tightly regulated 12V.

You can recap the whole power supply board but this one has lots of caps.

Thanks!

Cheers,

Suad