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

[DJM_PHX]

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.

[jetadm123]

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

Check with and ohm meter out of the board, or in the board? Hot or not?

In the board I see resistance but not shorted. I should be able to check voltage on each post by making a probe tip to reach in there.

Also, I can not get to the SMPS control chip while powered up to check it as in the other thread.

The fluctuating voltage of 10-15V across the 22uf cap indicates that the power supply is trying to start up. Refer to the datasheet application circuit, page 14 regarding the following:

With power off and monitor unplugged,

1) The circuit shows a 56Kohm 1W resistor (your value will be different) connected to the output of the large cap. Find and measure the value of this resistor.

2) In the circuit, you have a 1N3613 diode (your values will be different) feeding the 10uf cap. Locate this diode and check it for shorts.

3) Measure resistance across pins 5 (gnd) and pin 7 (vcc) of the SG3842.

4) You've already checked the mosfet and replaced the small cap.

5) If the above checks good, I have some reservations about the voltage fluctuation on the 450V large cap, you might consider replacing it.

6) If the above checks good, I'm leaning towards replacing the SG3842 chip. It also goes under the part # UC3842 and is dirt cheap, so buy a couple of them.

[nirvnyc]

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

114 and 115 are 25V - 470uF
117 and 118 are 10V -1200uF

Is the yellow square transformer looking unit the SMPS?

Thank you!

[jetadm123]

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

Check with and ohm meter out of the board, or in the board? Hot or not?

In the board I see resistance but not shorted. I should be able to check voltage on each post by making a probe tip to reach in there.

Also, I can not get to the SMPS control chip while powered up to check it as in the other thread.

I meant to check for shorts with power off.

[DJM_PHX]

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

hi, can anyone tell me what the original uF and voltages are on those four caps c114 115 117 and 118 on the LM004 Ver:3 board... thanks

i had replaced the caps a long time ago with caps of higher uF and voltage ratings... and now those went bad after a few years... and i have no idea what the original specs were..

114 and 115 are 25V - 470uF
117 and 118 are 10V -1200uF

Is the yellow square transformer looking unit the SMPS?

[DJM_PHX]

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

There's a mosfet mounted on the heatsink directly below the large cap. I suggest you check it for shorts.

Check with and ohm meter out of the board, or in the board? Hot or not?

In the board I see resistance but not shorted. I should be able to check voltage on each post by making a probe tip to reach in there.

Also, I can not get to the SMPS control chip while powered up to check it as in the other thread.

[nirvnyc]

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

hi, can anyone tell me what the original uF and voltages are on those four caps c114 115 117 and 118 on the LM004 Ver:3 board... thanks

i had replaced the caps a long time ago with caps of higher uF and voltage ratings... and now those went bad after a few years... and i have no idea what the original specs were..

[retiredcaps]

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

Have you had any time to put a check list together of what I should do next?

Check for shorts on the power mosfet as jetadm123 suggested in post #123.

I have the same smps chip in my monitor. Once I replaced the small cap, I got secondary voltages. See

https://www.badcaps.net/forum/showthread.php?t=9904

for details. If the power mosfet is okay, then the SMPS controller may be bad. Replacements are available on ebay.

[DJM_PHX]

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

I and probably others will have suggestions. The next post will probably be a long one and I don't have time right now to write it all out.

Have you had any time to put a check list together of what I should do next?

[jetadm123]

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

There's a mosfet mounted on the heatsink directly below the large cap. I suggest you check it for shorts.

[DJM_PHX]

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

Yes. Screws in and tight.

[retiredcaps]

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

The large cap V is the same and fluctuating as before.

Just to confirm, are all the boards are connected together during these tests?

Okay, I have to leave now and won't be posting until later tonight.

[DJM_PHX]

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

I appreciate it. The large cap V is the same and fluctuating as before. I read that the Chengx brand is not liked. I can try to get a replacement for it. Since I see the same voltages and fluctuation, I for now will leave it in. The board is looking a bit rough at the solder points.

[retiredcaps]

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

retiredcaps, and all, have a Happy New Year.
I'm calling it a day and will pic this up next year. But, if you think of anything else, let me know.

I and probably others will have suggestions. The next post will probably be a long one and I don't have time right now to write it all out.

Any chance of getting the Rubycon? Most here will question the Chengx brand even though the little cap may not root cause here.

If it is not the small cap, please re-measure the large cap to see if the fluctuating voltage is there or not. Other possibilities is that the SMPS controller chip is bad. I'm trying to guide you through each scenario one at a time.

[DJM_PHX]

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

retiredcaps, and all, have a Happy New Year.
I'm calling it a day and will pic this up next year. But, if you think of anything else, let me know.

[DJM_PHX]

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

They were suppose to be Rubycon's but they sent Chengx.
The voltage is not stable and is 10V-15V DC.
Is the large sqr. yellow unit the SMPS?
Why can I detect voltage on the heat sink by the large cap?

[retiredcaps]

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

Are they power transistors? The top one, on my board, the outer 2 to the center pin only have 0.1 ohms res.

The power transistor is just above the big cap in post #2. The two ICs on the heat sink next to all those capacitors on the right are schottky diodes. That reading of 0.1 ohms could be right because it is measured "in circuit". What you are looking comes after the SMPS starts.

What is the voltage across the new small cap and is it stable or fluctuating like before?

[DJM_PHX]

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

Ok. In post #2 on the right circled in green is the power filter caps. behind them are two units connected to the heatsink (in Yellow). Are they power transistors? The top one, on my board, the outer 2 to the center pin only have 0.1 ohms res. The other two have on the board have much greater amounts. That one is directly connected to the large yellow transformer, or is this the SMPS?
I have not even a flicker of light from the power LED on the front panel any more.

[retiredcaps]

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

I am not even gettiing 1V out of any of the wires on the long connector as listed before, even with the new cap.

That is expected because the SMPS still hasn't started (the clicking sound).

[retiredcaps]

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

Ok, the 22uF cap is in and no change. The clicking noise is still there. Replace the 100uF large cap next I assume?

What did brand and series is the replacement cap?

Before replacing the big one, what is the voltage across the 22uF cap?