Dell 2407wfpb

[knoxrebel]

Dell 24 inch that appears to be previously "repaired". It shows no signs of power (The main fuse is good but i get no power lights, sounds or video and the backlight does not turn on.) The main cap is different, not sure that they replaced it with the correct value. Does anyone know what the correct value is for the main cap? I purchased and build the blue esr meter, all the caps appear to be good, so im not sure where to start

[PlainBill]

Re: Dell 2407wfpb

Dell 24 inch that appears to be previously "repaired". It shows no signs of power (The main fuse is good but i get no power lights, sounds or video and the backlight does not turn on.) The main cap is different, not sure that they replaced it with the correct value. Does anyone know what the correct value is for the main cap? I purchased and build the blue esr meter, all the caps appear to be good, so im not sure where to start

That doesn't even qualify as a "repair". Someone should be horsewhipped, coated with honey, and staked out on an anthill.

Start by checking the voltage across that large cap. It should be 1.4 x the line voltage - about 165 VDC in the US and Canada. If it is correct, a picture of the bottom of the power supply is the next thing needed.

Whoops!!! This power supply seems to have a PFC front end; the voltage across the large cap may be as high as 400 volts. And it is entirely possible the cap is inadequate.

PlainBill

[jsog]

Re: Dell 2407wfpb

So they replaced the big cap with an axial one, poorly. Nichicon VX series is general-purpose, not low ESR. At least they used one rated for 450V.

Hard to see, but this Dell thread leads to this picture. Looks like 120uF, 450V.

[knoxrebel]

Re: Dell 2407wfpb

I looked at another thread on here and saw that someone had a problem with the main cap and was looking for a replacement for 120uf /450v. So it looks like whoever attempted to repair this monitor used the wrong value. I measure 167 across the cap.

JSOG, i agree its a 120uf.
Would a wrong main cap cause the unit to not show any signs of power?

[knoxrebel]

Re: Dell 2407wfpb

Also, can you not edit your posts after someone has posted after you???

Here is a picture of the bottom of the power board.

[jetadm123]

Re: Dell 2407wfpb

I looked at another thread on here and saw that someone had a problem with the main cap and was looking for a replacement for 120uf /450v. So it looks like whoever attempted to repair this monitor used the wrong value. I measure 167 across the cap.

JSOG, i agree its a 120uf.
Would a wrong main cap cause the unit to not show any signs of power?

The 450V cap works at a lower frequency than the small caps on the secondary side of the board, which allows it to run cooler. Therefore, in theory, you can use a general purpose cap for the filtering of the DC voltage off the bridge rectifier. You can go a little higher in capacitance without any problems.

Looking over your photos, it looks like R603? circled in yellow may be burned. Can you please verify?

[PlainBill]

Re: Dell 2407wfpb

The 450V cap works at a lower frequency than the small caps on the secondary side of the board, which allows it to run cooler. Therefore, in theory, you can use a general purpose cap for the filtering of the DC voltage off the bridge rectifier. You can go a little higher in capacitance without any problems.

Looking over your photos, it looks like R603? circled in yellow may be burned. Can you please verify?

The problem here is this SMPS has a PFC front end. That means the cap is taking pulses at about 100Khz. I haven't researched that, but IF I recall what wizard has said, general purpose caps won't last there.

EDIT: I just looked at a power supply that definitely has a PFC front end, the caps are Rubycon MXG series.

What are the part numbers of IC651 and IC691?

PlainBill

[pedro]

Re: Dell 2407wfpb

Also check PF701 for continuity (top of pic #1, legend partly obscured by the red wire of the butchery).

[knoxrebel]

Re: Dell 2407wfpb

P701 seems to be okay, it tests closed. Im not sure if i missed an ic, but the two on the bottom are IC651(Ill have to buy a better magnifying glass, its really scratched) and IC601 ( 200a6 pzil). Also the resistor that is discolored is not open.

[PlainBill]

Re: Dell 2407wfpb

Shoot!!! IC691 is the SMPS controller.

PlainBill

[knoxrebel]

Re: Dell 2407wfpb

where is IC691?

[jsog]

Re: Dell 2407wfpb

Lower right corner on the bottom of the power board, opposite side from the AC connector.

[knoxrebel]

Re: Dell 2407wfpb

Sorry about the bad pictures, i think that IC691 is really ic601

Looks like it might be L6511d or L6561d

[jetadm123]

Re: Dell 2407wfpb

Looks like IC651 is a L6561D power factor corrector chip and IC601 is a NCP1200 SMPS controller chip.

Out of curiosity, the resistor I mentioned in post #6, does it's measured value come close to the color bands on the resistor itself? You may have to lift one end of the resistor off the board in order to measure an accurate value. And is it labeled R603? Also, is one end of this resistor connected to any of the legs of IC601? If yes, which one?

[PlainBill]

Re: Dell 2407wfpb

After you answer jetasdm123's questions, I have a couple of points to check.

Pin 4 of IC601 is ground; use that for the following measurements: What are the voltages on pin 6 and on pin 8 of IC601?

PlainBill

[newbie1]

Re: Dell 2407wfpb

OP, i would like just to share as i have a similar power board, which is still waiting for my lazy bum to get going, and to get the CORRECT replacement parts. .

The main filter caps of my board is Elite PW(M) 120uf 450V. I believe this is the original capacitor & value as i had opened up 2 of these monitors & in them the same cap is the one failing.

Just another observation. I noticed that R670 in your repaired power board is of different color coded as mine. Mine (originally was there & burnt) is Red-Red-Silver-Gold (0.22ohms +/-5%). Yours (i guess it was replaced) is Red-Red-GOLD-Gold (2.2ohms +/-5%). However, i am not expert enough to tell whether the difference will cause it to malfunction as what your monitor is behaving.

Let me know if u need any resistance measurement as i have not yet assembled my monitor yet.

[knoxrebel]

Re: Dell 2407wfpb

Thanks for the heads up on resistor r670, i can tell by the way that the leads were cut and soldered that it was replaced. Not sure if this would cause a problem though.
R603 is the one that is discolored, it was removed and tests good.
IC601 pin 6 I get 0v and pin8 i get 56v

[PlainBill]

Re: Dell 2407wfpb

Thanks for the heads up on resistor r670, i can tell by the way that the leads were cut and soldered that it was replaced after the fact. Not sure if this would cause a problem though.
R603 is the one that is discolored, it was removed and tests good.
IC601 pin 6 I get 0v and pin8 i get 56v

A .22 ohm resistor would typically be used to sense current through the power FET. Changing that to 2.2 ohm is going to cause problems.

The voltage at pin 8 is unusually low, but is within specs. The voltage at pin 6 indicates there is a problem with the NCP1200 or the cap tied to pin 6.

PlainBill

[knoxrebel]

Re: Dell 2407wfpb

That resistor that was replaced is dead. I have just replaced it with a spare 3w that i had.

[knoxrebel]

Re: Dell 2407wfpb

Well, as soon as i replaced the resistor .22 with a 3w .22 ohm resistor and plugged back in the power the resistor popped and let the smoke out.


...Yet again I am brought to believe that electronics run on smoke.