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

[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

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

[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

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

[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

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

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

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

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.

[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.

[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

I check the inverter/power board find out had cold solder at quickconnect to VGA card, resolder is working final
Thanks

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

[DJM_PHX]

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

That's OL with black lead on GND. 0.448 reversed.

[DJM_PHX]

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

When I measure between pins 5(Gnd) and 7(Vcc), I see an decreasing meg Ohm resistance. I assume the little cap is charging back thru the ground side of the cap.

Using the diode selection, OL between 5 and 7.

[DJM_PHX]

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

Nope, not it!

[retiredcaps]

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

On D103 I get 0.058 in both directions.

If D103 tests the same out of circuit, it is bad.

[DJM_PHX]

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

On D103 I get 0.058 in both directions. In the above pic, D103 is just above and to a little right of the control chip (orange and black). D101 a 0.462 and OL, D012 returns a 0.441 and OL.

[DJM_PHX]

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

The beep tells you if there is a connection and if possible, the Fluke will continue to provide the resistance measurement. I'm using an Fluke 87 DMM.

[retiredcaps]

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

If the diode is on the bottom of the board, D103, I have 99 ohms each way.

Diodes are best tested with the diode test function on your multimeter. Put red probe on one side, black on the other. Record reading. Now reverse the probes and record reading.

One reading should be between 0.3 and 0.8V and the other is "1" or 0L (depending on your multimeter).

[jetadm123]

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

Please excuse my ignorance about circuits, but when you refer to "in the circuit", are you talking about in the pdf (which figure) or another diagram? I attached pics of the top and bottom of my boards. It would help if you indicate which is which. I don't see a resistor connected to the large cap, at least what I would recognize a one.

If the diode is on the bottom of the board, D103, I have 99 ohms each way. If on the top of the board I have a D101 and D102. D101 I get nothing in both directions accross it, D102 I get a short beep with no reading but I can flip the leads and get the same result, a short beep. I am not sure what I am seeing.

The applications circuit on page 14 of the pdf file was what I was referring to on my previous post. Since we don't have a manufacturer's schematic to work from, I chose to use the one for the SG3842. Please keep in mind that the SG3842 schematic is a "suggested" design. In reality, your circuit board will differ a little from it. The key is to try and match what you're seeing on SG3842 schematic to what's on your board.

Either D101 or D102 are probably connected to the positive terminal of your 22uf cap. Since you're getting unusual values from both (I have no idea what a "beep" means on your meter), try removing them from the circuit board and check them again.