Dell 1901fp

[PCBONEZ]

Re: Dell 1901fp

I replaced the four larger caps on the board and none of them were bad. Someone earlier had mentioned the smaller caps by the big cap. but they don't seem to be bad either.

If you didn't check ESR and uF then you don't actually know if the old caps were bad or not.
Those can be out of spec independently of each other and you can't tell by looking.

Make sure you push the lead tips firmly into the solder or test point when you measure.
- Especially on solder and component leads.
There is usually a coating [conformal coating] you have to get through to get a good reading.
That might be why your readings keep changing.
.

[retiredcaps]

Re: Dell 1901fp

.... something seems really strange.

Try this. Measure the blue/purple wire for about 2 minutes. See if it stable 5 V DC or fluctuating. Pay attention the entire 2 minutes and don't let the probes slip otherwise you may short out something.

Also, maybe try a fresh battery in your multimeter and make sure the test leads are firmly inserted into the jacks.

[littlski717]

Re: Dell 1901fp

I don't mean any disrespect by this, I know you guys are trying to help, but I am not a total newb when it comes to electronics. I am actually a EET major in my junior year, but I haven't messed with this type of board. And yes, I know that makes me nowhere near a professional, as a matter of fact, i'm far off from most of you i'm sure. Please forgive me if I ask questions, but this is all actually very interesting to me, so I ask questions to learn for the future.

And yes, I know how to use a DMM to make a measurement. lol

[littlski717]

Re: Dell 1901fp

Try this. Measure the blue/purple wire for about 2 minutes. See if it stable 5 V DC or fluctuating. Pay attention the entire 2 minutes and don't let the probes slip otherwise you may short out something.

Alright, I did this. I showed a constant 5.16V on the output connector on both blue and purple, while is was disconnected from the logic board.

[retiredcaps]

Re: Dell 1901fp

I showed a constant 5.16V on the output connector on both blue and purple, while is was disconnected from the logic board.

Can you retest with the logic board connected and see if the 5.16 V DC holds steady for 2 minutes. If it doesn't, then something is pulling down the voltage.

[littlski717]

Re: Dell 1901fp

Can you retest with the logic board connected and see if the 5.16 V DC holds steady for 2 minutes. If it doesn't, then something is pulling down the voltage.

Yeah, when I did this previously, I used the pins on the bottom of the board with the board connected and it never got close to 5V. That is when I was getting 2.86V

[littlski717]

Re: Dell 1901fp

***I don't know what the deal is, but when I plug it in after about 30 seconds, I start to hear a high pitch whining noise.***

[retiredcaps]

Re: Dell 1901fp

Yeah, when I did this previously, I used the pins on the bottom of the board with the board connected and it never got close to 5V. That is when I was getting 2.86V

Okay. So with a load, the power board is not holding steady at 5 V DC. That hissing/whine noise could be a capacitor slowly dying.

Here are some suggestions.

1) Replace all the remaining electrolytic caps on the power board excluding the largest one which rarely, but not never, fails. This includes the discussion on the startup cap. Capxon makes terrible quality caps.

I see you used Rubycon so you can continue to use that.

2) If you have 2 identical monitors like you said in post #1, take the power board from the other monitor and put it into this one and repeat the same tests looking for steady 5 V DC with the logic board connected.

If you have steady 5 V DC, then re-measure all 5 pins on that IC and compare to the datasheet. If the voltages are stable and match the datasheet IC, then you are closer to getting to the power LED turning green/blue.

[littlski717]

Re: Dell 1901fp

I have actually replaced all of the bigger caps (excluding the giant one) on both boards. I am currently working on both boards simultaneously, but I am only talking to you guys about one, because I can survive if I just get one running. Now in regards to those startup caps, is there any way possible someone could willingly point out on my pics in this thread which caps those would be? That would be awesome if you could.

BTW, do I have to use the same logic board to test both PS boards? Or can I work with each PS board and logic board together as they would be in the monitor?

[retiredcaps]

Re: Dell 1901fp

Now in regards to those startup caps, is there any way possible someone could willingly point out on my pics in this thread which caps those would be? That would be awesome if you could.

Members here recommend replacing ALL the electrolytic caps. That means all on the power board. As PCBONEZ mentioned earlier, unless you have an ESR meter, you can't tell if a cap is bad by looking at it.

BTW, do I have to use the same logic board to test both PS boards?

I would use the same logic board. You don't want to change too many variables at one time.

[littlski717]

Re: Dell 1901fp

Ok cool. I will look into pricing to replace all of the caps, and see if it's worth just buying a replacement board or replacing the caps.

I would also guess that these caps are the most common problems in this case, correct? what else could there be that may cause the same problem? -- just from all of your experiences.

[PCBONEZ]

Re: Dell 1901fp

Caps are the most common problem in screens and usually the cause behind other problems.
Few [if any] screens use good caps as original.

Make sure to use caps with low enough ESR.
The smaller caps are probably best sourced at digikey or mouser if you are in the US or CA but they don't carry Rubycon.
Need to fail safe to at least Panasonic FM or FR to ensure they are good enough if you're not going to spec the old ones for the original ESR.
I'm not a 'screen guy' but I am a 'caps guy'. [And a 'mobo guy'.]
EVERYBODY gets to mess with power supplies now and then no matter what they are into.
.

[retiredcaps]

Re: Dell 1901fp

I will look into pricing to replace all of the caps, and see if it's worth just buying a replacement board or replacing the caps.

A replacement board is likely to come with Capxon caps (same as the original).

[littlski717]

Re: Dell 1901fp

Sorry for keeping this thread rolling, I'm sure some of you trying to help are frustrated. I've been waiting for parts and replaced them to see if that helped.. all caps are replaced, and all my voltages are unchanged.

I'm kind of looking for a theory opinion here. Looking back at all that has come out of this so far, we have figured out that my output connector (when not connected to the logic board) has the correct output voltages. Once connected, the logic board from both screens is drawing a load (one that it doesn't like), which to me seems to be putting the power board into some sort of protect mode. Since at this point I am ruling out the caps, does anyone think it could be something on the logic board that's not acting as it should??

This is all killing me mostly because I want to figure out this problem. I never give up on a project!!

**I did for Sh*ts and giggles, use a fluke to test the old caps, and they checked out fine, but replacing them surprisingly did nothing.

[PCBONEZ]

Re: Dell 1901fp

**I did for Sh*ts and giggles, use a fluke to test the old caps, and they checked out fine, but replacing them surprisingly did nothing.

As pointed out before, you can't completely check caps without an ESR meter [or an O'scope].
Everything can look perfect by other tests but the ESR is too high.
Too High an ESR => Too much Ripple in circuit => IC chips won't work right.
.
.
Check for shorts in the rails.
.

[Toasty]

Re: Dell 1901fp

Late to the party as usual....

Did the hissing/whining sound go away now that all caps are replaced?

The power LED indicator is still not working? Not orange or green?

What cap brands and series did you use for replacements?

What are the voltages supposed to be -or- what does the board marking indicate on the PSU output connector to the graphics board? One -should- say "BL On" or similar. Please list in order and color.

If the 5v line(s) are being pulled down when connected to the graphics board, my thinking is (1) the PSU output is weak and can't sustain the load or (2) there is a failed regulator on the logic board that is pulling excess current.

Because this is/was an -identical failure- of both units, I really suspect (as PCBONEZ indicated) there is something still wrong with the PSU. Now, that being said, when you are testing this, are all connectors and lamps plugged in? You can get standby (5v) okay but they won't run unless the lamps are hooked (Protect circuit).

I also noticed there are no pics of the trace/foil side of the board(s). Sometimes there are clues there that go unnoticed when we only see the component side. There are also components on the trace side of the PSU that may provide some answers.

I have 2 1905FP's that use a very similar PSU.

Toast

PS: Would you update your profile with your location, please? At least the country...

.

[ronreb]

Re: Dell 1901fp

I am new to this forum, I currently have a problem with one of these Dell monitors. My problem may be bad CCFL or inverter supply. I haven't looked into much yet.
From what you describe as whining is a power supply trying to run at its designed frequency. You have some kind of short on the power supply load or a defective component in a power supply.
Check all power supply voltges & see if they are correct. This may give you aclue to where the defect may be.