Dell 2408wfp - No Power - High voltage reading on Filter Cap

[indecided]

Hi guys,

I've got a Dell 2408wfp which for a couple of days did power up but was in a constant black screen (power light started out amber, the last day it started up in green, and today it doesn't power up at all).

Disassembled it and did some quick preliminary testings -

All diodes are not shorted out, no caps "appear" to be blown based on preliminary physical inspection

Something quite unsettling though is that the large filter cap reads at 720V!.

That would put it at ~240v x 3 instead of the 230v x 1.414 that is to be expected, so it looks to be WAY overvoltage (based on 450v on the very helpful schematic drawn up by bicycle6228).

Also, all of the heatsinks show live potential (based upon a quick test pen/voltage tester, did not check actual voltage reading) - is that finding normal?

With that diagnosis, should I be looking at the transformer or based on that voltage across the capacitors, should I just consider replacing the power boards? i'm wondering if the entire board isn't already fried.

FYI, I've got fair experience in basic diagnostics and do have about a decade of hobbyist electronics repair behind me, but when it comes to SMPS I usually just replace the offending board, but for the 2408wfp the power section looks to be pretty hard to come by.

[victor_cj]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

Unplug the connections from main board and inverter module and check the smps with external "on" signal command. At the larges filter cap must be 385-400V DC in operating mode and 295-315V DC in stand-by mode if you have 230V AC.

[indecided]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

Hi Victor,

Thanks for the info. Do you mind elaborating on the trigger for the external ON - is that going to be a pin shorted to ground? If so, which pin on which IC?

Thanks!

[diif]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

When you measured the voltage on the big cap, did you measure across the cap ? (negative lead on the negative side and positive on the positive)

Whilst i have the slightly bigger 2709 which is a slightly different but similar design, i can confirm the heatsinks are live.

[victor_cj]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

In this case external ON is 2,7-5V applied to green or gray wire from the main board, i do not have connecting diagram now.

[indecided]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

diif, yep, the voltage is measured across the cap. Looking at the voltage made me want to buy a pair of more sturdy DMM leads....eep.

[indecided]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

victor, OK. i'll measure the voltage potential in "standby mode" first across the cap and report back with my findings, i'll probably hook up the external on tomorrow.

[indecided]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

victor, OK. i'll measure the voltage potential in "standby mode" first across the cap and report back with my findings, i'll probably hook up the external on tomorrow.

Victor, with all connectors removed from inverter and mainboard, the filter cap sill registers at 722-723 volts DC... this would be presumably in "standby" mode.

[SteveNielsen]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

Are you sure the meter is set on DC volts and not AC volts?

[indecided]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

Are you sure the meter is set on DC volts and not AC volts?

Oh dear. This is really embarassing. Steve, you pretty much hit the spot. I was measuring the cap on AC. I'm beginning to think i'm dyslexic.

So yes, the cap reads 332V, which works out to be about 234V. So, back to square one.

What should I be checking next?

[victor_cj]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

Check for cracked soldering at IC601 or bad caps.

[SteveNielsen]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

Oh dear. This is really embarassing. Steve, you pretty much hit the spot. I was measuring the cap on AC. I'm beginning to think i'm dyslexic.

So yes, the cap reads 332V, which works out to be about 234V. So, back to square one.

That's a fairly common mistake so don't feel too embarrassed about it.

[indecided]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

So, it's been a couple of weeks.. the 2408 has been sitting belly down on the desk for a while because of lack of time to look at it. I just did some blanket resoldering of the most crucial elements on the power board and repowered it back up.. no luck.

However, I cycled the power a couple of times, and volia - the monitor sprung back to life!

I can't quite explain it, but over the last year or so i've had the same issue - the monitor wouldn't power up, and cycling the power a few times would turn it on.

I'm inclined to say something else was wrong/different here as I did the power cycle a few dozen times before deciding to open up the monitor.

Now, before I put the monitor back in its case, does anybody have any ideas as to why it behaved the way it did, and if that is something I can fix for good so as to avoid having to threaten the monitor that it would be replaced before it starts working again?

[viziks]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

I would suggest you replace the startup capacitor 47uf 50V or similar on the PWM IC VCC. If I am not mistaken the small capacitor just behind the Big 450V mains Filter.

[alas3376]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

use ESR Meter to control the startup ca^pacitors

[indecided]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

viziks,

I've replaced the 2 47uF 50v caps on the board, the rest of the replacement caps are on their way.

Unfortunately, still the same thing. The monitor powers on "now and then", and i've discovered that when it doesn't, i can switch off the monitor, long press the power button, then switch on and long press the power button again, 3/5 times that works to power up the monitor.

If i short press the power, it doesn't power up. The power LED just "blinks" at start, that's all.

[Agent24]

Re: Dell 2408wfp - No Power - High voltage reading on Filter Cap

Did you check all the voltage rails and any secondary rails\regulators on the logic board?

You should probably check each one in both working and non-working states and see if anything is obviously different.