Need help DCLCD 19" LCD Monitor - I already replaced caps

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

This thread is a long one,One member had a blinkling PLED and found a poor solder joint.It might be advised to check the area where he had problems.
Al

If it would make you feel better I can start another one. I have gone through and reflowed all of the suggested points and then some. If you have some valid input, please add it.

[alexanna]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

If it would make you feel better I can start another one. I have gone through and reflowed all of the suggested points and then some. If you have some valid input, please add it.

I may have an idea, I just finished working on a Dell e172Fpb with a blinking PLED, Two of the inverter transistors were shorted causing an overload in the power supply causing it to shut down and restart over and over. I think this monitor uses a similar style inverter. There have been problems with the transistors on this monitor; I wonder if that might what’s happing with yours?
The reason I said this has been a long thread is because I might have missed something you have already tried.
Al.

[jetadm123]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Okay, I need to verify the following:

1) Other than the ticking sound, the monitor is completely dead with no power light blinking or quick flash on the screen when you first try powering up the monitor.

2) In order to start up correctly, some monitors require a load, or that all boards are connected together before the power supply will start.

3) The transistors, Q6,7,11 and 12 that Al had mentioned are circled in red. You can check them for shorts if you wish.

So, it looks so far that the components around the SG3842 seem to check out good.

Perform the 3 steps above and unless there's a dead short somewhere preventing the supply from starting, then you might consider replacing the the large 450V cap and SG3842 controller chip.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Okay, I need to verify the following:

1) Other than the ticking sound, the monitor is completely dead with no power light blinking or quick flash on the screen when you first try powering up the monitor.

Yes. When I first started looking at it I had a picture for a second or two and a power LED, but now there is nothing but the clidking.

2) In order to start up correctly, some monitors require a load, or that all boards are connected together before the power supply will start.

Yes, I have always reconnected all of the wires and screwed the PCB back into the frame.

3) The transistors, Q6,7,11 and 12 that Al had mentioned are circled in red. You can check them for shorts if you wish.

I am off work today but will double check them tomorrow.

So, it looks so far that the components around the SG3842 seem to check out good.

Perform the 3 steps above and unless there's a dead short somewhere preventing the supply from starting, then you might consider replacing the the large 450V cap and SG3842 controller chip.

I have already removed the large cap and I am aquiring a replacement.

[jetadm123]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Also, back in post #130 I asked that you check the resistance of a 1W resistors with one end connected to the 450V cap. I think it might be (you'll have to verify) the one I circled that's wrapped in the heatshrink tubing. While you're at go ahead and check the ceramic cap below it. I think it's connected in parallel with your 22uf cap.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Also, back in post #130 I asked that you check the resistance of a 1W resistors with one end connected to the 450V cap. I think it might be (you'll have to verify) the one I circled that's wrapped in the heatshrink tubing. While you're at go ahead and check the ceramic cap below it. I think it's connected in parallel with your 22uf cap.

The resistor is ~4ohm and the cap next to it is ~1.87uf in circuit. The diodes right there are good as well.

[jetadm123]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

The resistor is ~4ohm and the cap next to it is ~1.87uf in circuit. The diodes right there are good as well.

1.87uf seems a little high for a ceramic cap. Compare your readings to the resistor's color code and value stamped on the ceramic cap. You may have to remove these devices for testing.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I'm sure that there are other devices interacting with this measurement. Across R106 I get the same measurement.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Reinstalled the board. Question, how hot should the heat sink get on the filter part of the board? It smells like plastic is burning and to hot to touch. The caps are very hot as well.

I now have a blinking LED.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Not only is the LED flashing, the screen is flashing white as well right in time with the clicking.

[alexanna]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Not only is the LED flashing, the screen is flashing white as well right in time with the clicking.

The longer you work on something problems tend to get worse, or cause multiple problems or show up quicker and sometimes different symptoms.
Well; I am still thinking a problem with the Q transistors, Or possibly one of voltage regulators on the logic board shorting to ground most likely the 5 volt one. Overloading the power supply.
Al

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I pulled the filter sink and the schottky diodes. the first is a schottky MOSPEC SRF10100C which checks ok. The second I can not identify. It is a: 1 OH60C C433. If it is a schottky diode also, I believe it is bad. This is the unit that smells hot and seemed to heat up hot first on the sink.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

This is getting to be a nightmare!!!!
ZD102 diode is now burned out.

My bad. I jumped a solder connection.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

In #49, wenter has a pic of his schottky diodes. His and mine are the same board and monitor so I would expect the diodes should be the same. He has a SRF1060C on the right where my unknown is and according to the data sheet, both of these schottky diodes should be the same. Mine are not. My Fluke shows a flashing OL (1-, 2+) and a leakage in reverse. The other has Mohms and a large Kohm respectively.

[alexanna]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

In #49, wenter has a pic of his schottky diodes. His and mine are the same board and monitor so I would expect the diodes should be the same. He has a SRF1060C on the right where my unknown is and according to the data sheet, both of these schottky diodes should be the same. Mine are not. My Fluke shows a flashing OL (1-, 2+) and a leakage in reverse. The other has Mohms and a large Kohm respectively.

I have a P/S-Inverter board # LM004 Rev 3.
If That will help you with any part identification, I will do my best to help you
Al.

[ben7]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I pulled the filter sink and the schottky diodes. the first is a schottky MOSPEC SRF10100C which checks ok. The second I can not identify. It is a: 1 OH60C C433. If it is a schottky diode also, I believe it is bad. This is the unit that smells hot and seemed to heat up hot first on the sink.

That may be the problem.

BUT it is a 2SC433 transistor

Hope this helps

-Ben

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

That may be the problem.

BUT it is a 2SC433 transistor

Hope this helps

-Ben

Thanks Ben, but wenter's and my board is the same and the pic in #49 page 3 shows two schottky diodes. According to the spec sheet I should have 250pf for the juction capacitance. The MOSPEC has ~238pf for each direction respecitively, but the other only has ~8pf. I believe this is the bed unit. I just need to hunt them down now and get them ordered.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I have a P/S-Inverter board # LM004 Rev 3.
If That will help you with any part identification, I will do my best to help you
Al.

Can you confirm that the units on the filter heat sink should both be schottky diodes? Like the ones on page 3 #49.

[alexanna]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Can you confirm that the units on the filter heat sink should both be schottky diodes? Like the ones on page 3 #49.

The 2 that I have pictured are both Schotty Barrier Diodes.
The one on the left. # FCH10U10
The one on the right # FCQ10U06
Al.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

The 2 that I have pictured are both Schotty Barrier Diodes.
The one on the left. # FCH10U10
The one on the right # FCQ10U06
Al.

Ok then, at least one of these is bad on my board. I am going to replace both.