Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn on.

[pswayze]

I've got a 50" Samsung plasma that has sound but no picture. The internal model number shows S50HW-YB01. I've been going through a service manual I found online. There are no fuses blown and all voltages on the power supply board are in spec. Now I'm at the step in the service manual where it has me test for blown FETs on the Y-MAIN board. The manual says a shorted FET will read 0 Volts across the terminals (duh), and anywhere from 0.3-0.9 Volts if it is working properly. I'm assuming that is the saturation voltage. The readings I get are about 90V across all the FETs which is telling me they aren't shorted but they also aren't on. The service manual doesn't actually say what to do if they are off but not blown. Any ideas on what to look at now?

[tom66]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

You need to first test the Vs testpoint to see if the PSU is giving you a good ~200V.

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

You need to first test the Vs testpoint to see if the PSU is giving you a good ~200V.

Vs is 198V.

[tom66]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

OK, that is good.
Next step can you please post some pictures of the Y-main and X-main boards, and one of the overall layout of the set?

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

Vs is 198V.

I found a much better manual in your sig. I have verified that all voltages are correct on the SMPS output.

[tom66]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

S50HW-YB01 by the way is just the panel model, the model of the screen in the TV. We would also need the TV model e.g. PN50C100.

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

S50HW-YB01 by the way is just the panel model, the model of the screen in the TV. We would also need the TV model e.g. PN50C100.

The model of the TV is HPS5073.

The board pictured that is mounted vertically is something of interest. I'm not sure what it does but I see a transformer on it. It is also on the same board with the FETs that are off. There is a component on the board that makes a faint snapping/clicking/popping sound about once every second.

[tom66]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

OK, here is a quick way of verifying some parts of X-main and Y-main functionality.

You should see a voltage of around 60 to 120V across these blue capacitors; test one cap in each group

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

OK, here is a quick way of verifying some parts of X-main and Y-main functionality.

You should see a voltage of around 60 to 120V across these blue capacitors; test one cap in each group

Each set of three caps is between 98V and 100V

[tom66]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

It would appear the plasma panel is being correctly sustained. In a dark room, does the screen glow at all?

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

It would appear the plasma panel is being correctly sustained. In a dark room, does the screen glow at all?

I will try to find out but it will be a bit difficult because of the room it is currently in is very bright and moving it is problematic. I do not believe it is glowing. All the FETs are still open/off. Should I be concerned about the ticking noise?

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

It would appear the plasma panel is being correctly sustained. In a dark room, does the screen glow at all?

I've been patiently waiting for sunset. The screen does not glow at all even with a source playing.

[tom66]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

Those FETs you describe I believe they are known as the PASS fets. If one device is shorted, it can make the entire set look bad. Turn the set off and check any one for resistance between the three pins. A low reading probably indicates a defective fet.

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

Those FETs you describe I believe they are known as the PASS fets. If one device is shorted, it can make the entire set look bad. Turn the set off and check any one for resistance between the three pins. A low reading probably indicates a defective fet.

All the FETs read 10K Ohm between the legs, and 5M Ohm between each leg and the base. It's like something is keeping the FETs from turning on?

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

Those FETs you describe I believe they are known as the PASS fets. If one device is shorted, it can make the entire set look bad. Turn the set off and check any one for resistance between the three pins. A low reading probably indicates a defective fet.

I removed the transformer from the vertical board. For the life of me, I couldn't pinpoint what component the clicking was coming from. After removing the transformer, the clicking stopped but obviously it could have been any component in that circuit. There is still 98V across the FETs with that transformer removed.

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

I removed the transformer from the vertical board. For the life of me, I couldn't pinpoint what component the clicking was coming from. After removing the transformer, the clicking stopped but obviously it could have been any component in that circuit. There is still 98V across the FETs with that transformer removed.

Tom, any ideas?

[pswayze]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

Those FETs you describe I believe they are known as the PASS fets. If one device is shorted, it can make the entire set look bad. Turn the set off and check any one for resistance between the three pins. A low reading probably indicates a defective fet.

Sorry to keep bugging you. I'm about to give up on it. Any last suggestions?

[Alfa12]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

Hey all,

I am repairing the same type of plasma and it works now.
But the vs voltage cannot be set lower than 209V and the panel asks 198V.
Pot is at the end, anybody any idea what is wrong?

[plbrand]

Re: Samsung Plasma S50HW-YB01 has sound but no picture. PSU OK but FETs don't turn o

I've got this same TV with the same problem - no picture! I hear the same light clicking sound from somewhere on the vertical board shown above.

I note that the temperatures of several of what I think are FET's on this board seem quite high, as show in the attached photos.

Any idea where to look next?

Thank you,

PLBrand