Dell 2407WFPb Monitor - Bad Cap

[regino3rd@yahoo.com]

Re: Dell 2407WFPb Monitor - Bad Cap

Let's be precise in our terminology. SOT-23 is an SMD package. It usually contains a transistor, but can contain a diode, pair of diodes, zener diode, hall effect switch, or voltage reference. Heck, it could even contain a (very low power) voltage regulator!

It is important to look at the component designator. Q101 would be a transistor; D101 would be a diode; ZD101 would be a zener diode.

PlainBill

Q564 is SOT-23 transistor, not a diode as Newbie1 wrote.

[regino3rd@yahoo.com]

Re: Dell 2407WFPb Monitor - Bad Cap

I'm thinking that the main reason people order a replacement power supply is that either they gave up trying to repair or didn't want to deal with attempting to repair it. The problem I see is that you received a defective board that you had to repair. Luckily, it was an easy repair, but what if it wasn't? I supect most people would be unhappy if this seller is dealing untested/broken power supplies. Nothing worse than taking something out of the box and having it not work.

I won't give up, the only thing holding me is sourcing of parts, i have to travel long way to get into the city and buy parts. I'm pretty confident I can repair this thing. Internet access to search for part #'s another thing too.
I'm going to sit this out one time I know i can do it, just a matter of concentrating and will power.

Thanks a lot with the information guys!

[WillyBobz]

Re: Dell 2407WFPb Monitor - Bad Cap

Sorry.... apologies for my mistake, the Diode was actually D655 NOT D665 which is located next to R668 just beneath Heatsink for Q651. I can not give you the Rating as it is now back together again and being used by my son.

[zomi]

Re: Dell 2407WFPb Monitor - Bad Cap

Anyone have the values for

Zd601, d603, d604, d655

Zd601= 15v 0.5 yes
not sure about the other 3

[regino3rd@yahoo.com]

Re: Dell 2407WFPb Monitor - Bad Cap

Anyone have the values for

Zd601, d603, d604, d655

Zd601= 15v 0.5 yes
not sure about the other 3

i'm pretty sure they are signal diodes, maybe a 4148 is a nice try, i try it to my board later.

Can someone lighten us!
thanks.

[zomi]

Re: Dell 2407WFPb Monitor - Bad Cap

Scrap that removed diodes from the board and they all tested fine.

The way these diodes are situated on the baord will give false results.

Has everyone been testing the diodes on the board or remvoing and testing.

[regino3rd@yahoo.com]

Re: Dell 2407WFPb Monitor - Bad Cap

Scrap that removed diodes from the board and they all tested fine.

The way these diodes are situated on the baord will give false results.

Has everyone been testing the diodes on the board or remvoing and testing.

I do mine on the board first, then remove to verify because just like what you said it might give false result. i'm using a digital multimeter with a diode tester.

[zomi]

Re: Dell 2407WFPb Monitor - Bad Cap

Thats cool

Zd601, d603, d604, d655, all these will give a false reading on the board.
And need to be removed to be tested.

[regino3rd@yahoo.com]

Re: Dell 2407WFPb Monitor - Bad Cap

Thats cool

Zd601, d603, d604, d655, all these will give a false reading on the board.
And need to be removed to be tested.

you got proof in hand now if d603. d604. d655 were signal/switching diodes.

[AdrianM]

Re: Dell 2407WFPb Monitor - Bad Cap

I've joined this thread because my monitor was dead today :-( Looking at the schematic from the Russian site it seems there's two halves to this SMPS - first is a Power Factor Correction circuit (boost) followed by a buck taking the HV down to LV. My faulty parts appear to be restricted to the PFC section (Q654 PNP, Q651 mosfet, R670 0.22R and the cause of this mess C605 120u/450V)

I'm pretty sure the PFC front end is an optional extra to comply with power line etiquette and even appears to be enabled/disabled via opto-coupler IC652. The rectified and smoothed Line voltage across C605 is bypass around the first transformer T651, via D651 which suggests to me that simply replacing the big cap and removing the blown parts around the PFC ought to get it working again! (I't a weekend and I'm desperate for the PC monitor to work)

SO, having tested all the semis around the Buck convertor and put another Cap in place of C605 I fired it up. No drama, 320V on the HV but nada around the LV end. Pity, thought I'd get some qudos for spotting the unnecessary-ness (?) of the PFC part. Maybe the PWM chip is down after all, but nothing measures dead short and all the semis around are good. Maybe someone can shoot down my theory about the "optional" nature of the PFC at the front end?...

[AdrianM]

Re: Dell 2407WFPb Monitor - Bad Cap

Got it fixed now. New approach though. I began to wonder what voltage(s) were supposed to be produced by the SMPS so traced out the circuit around the output transformer. The feedback for the buck stage is obtained via an opto switch that comes on at about 20V (determined by a LM431 variable Zener). Just the one 20V supply rail then! A quick hookup to a bench supply confirms a happy monitor when juiced by 20V@3A.

Because the final output is a bunch of diodes fed from the switching transformer, it's OK to run an external 20V supply straight onto the board (across the row of 470u/35V caps). The diodes block current from going back into the transformer. I tweaked the voltage around and monitored the opto switch switch-on point confirming that the SMPS is set to run at 20V.

Now, I remembered that I had a spare power brick from my previous Dell monitor and sure enough it's rated 20V@3.5A. Not only that, but the DC jack fits the Loudspeaker output - so I cut the inductor (L702) that fed it power from the main board and jumpered a wire to the 20V rail (photo). I also grounded the switch contact that tells the MB the speaker isn't plugged in, even though the power plug is in. Oh, and I left the fuse off the SMPS just incase it ever gets plugged into the mains by accident!

[jetadm123]

Re: Dell 2407WFPb Monitor - Bad Cap

Got it fixed now. New approach though. I began to wonder what voltage(s) were supposed to be produced by the SMPS so traced out the circuit around the output transformer. The feedback for the buck stage is obtained via an opto switch that comes on at about 20V (determined by a LM431 variable Zener). Just the one 20V supply rail then! A quick hookup to a bench supply confirms a happy monitor when juiced by 20V@3A.

Because the final output is a bunch of diodes fed from the switching transformer, it's OK to run an external 20V supply straight onto the board (across the row of 470u/35V caps). The diodes block current from going back into the transformer. I tweaked the voltage around and monitored the opto switch switch-on point confirming that the SMPS is set to run at 20V.

Now, I remembered that I had a spare power brick from my previous Dell monitor and sure enough it's rated 20V@3.5A. Not only that, but the DC jack fits the Loudspeaker output - so I cut the inductor (L702) that fed it power from the main board and jumpered a wire to the 20V rail (photo). I also grounded the switch contact that tells the MB the speaker isn't plugged in, even though the power plug is in. Oh, and I left the fuse off the SMPS just incase it ever gets plugged into the mains by accident!

Great job on troubleshooting! Any idea what's causing the loss of the onboard 20V?

[AdrianM]

Re: Dell 2407WFPb Monitor - Bad Cap

Great job on troubleshooting! Any idea what's causing the loss of the onboard 20V?

Thanks - well, once the bypassed PFC stage was out of the way, it was all down to the NCP1200 convertor. This particular implementation deviates from the typical application circuit in the datasheet - mostly around the VCC connection which has a SCR connecting an additional winding on the output transformer. I started running out of patience staring at this part of the circuit though.

All the semis were intact - 0.6V between appropriate junctions in-circuit, Zeners biased off-board etc. But the NCP1200 VCC hunted around 3V to 5V the whole time and I don't particularly like probing around 330VDC circuits so I didn't look into it much further.

I think there may already be a few misconceptions about this board - some seem to think there's a standby stage and a main stage, possibly because of the part description in the Onsemi datasheet? I suspect that the MB just shuts off the PFC when in standby using opto IC652 and adjusts cycle skipping via opto IC605 but other than that runs the whole thing as a low power (60W) convertor all the time.

[Tyan]

Re: Dell 2407WFPb Monitor - Bad Cap

@AdrianM:
nice Job!
could you make a simple step-by-step workaround for "Dummies"? I´d rather buy an external Dell power supply (DELL PA-6 ~10$ @Amazon) than go on with t&e... I´ve already dumped 70$ in electronic parts (I have 3 NCP1200 here ).

I think, this would be the easiest/best solution for everybody experiencing this little pcb issue :-/

[AdrianM]

Re: Dell 2407WFPb Monitor - Bad Cap

Tyan, the speaker socket/power mod is a simple three-step process:

1) In the photo attached below I have marked both ends of L702, which is to be removed from the PCB, with magenta crosses. De-solder or just cut both ends of this inductor (a bit of wire that passes through a ferrite bead on the component side). This isolates the power socket from it's normal supply circuit.

2) The short blue line shows where to solder a bridging wire to permanently ground the switched contact for the socket CN703 - making it seem that the speaker isn't plugged in. A blob of solder across C718 would also do the trick.

3) Run a decent gauge stranded, insulated wire between the points marked with an orange line. To get a good solder joint on the land between C713 and F701 scrape away the lacquer first. The other end of this wire goes down onto the middle pin of socket CN703 which is already quite well exposed for soldering.

That's really all there is to it! The central pin in the speaker socket may be a little smaller than the power-brick plug (2.1mm vs 2.5mm I think) If it doesn't connect very well, it might be necessary to roll a small piece of copper or tin shim to increase the diameter of the centre pin. If that's too fiddly then you can always swap the socket for the correct sized one, or find an adaptor lead. I slipped a slender wire crimp insert over the pin which worked a treat.

[newbie1]

Re: Dell 2407WFPb Monitor - Bad Cap

Tyan, the speaker socket/power mod is a simple three-step process:

1) In the photo attached below I have marked both ends of L702, which is to be removed from the PCB, with magenta crosses. De-solder or just cut both ends of this inductor (a bit of wire that passes through a ferrite bead on the component side). This isolates the power socket from it's normal supply circuit.

2) The short blue line shows where to solder a bridging wire to permanently ground the switched contact for the socket CN703 - making it seem that the speaker isn't plugged in. A blob of solder across C718 would also do the trick.

3) Run a decent gauge stranded, insulated wire between the points marked with an orange line. To get a good solder joint on the land between C713 and F701 scrape away the lacquer first. The other end of this wire goes down onto the middle pin of socket CN703 which is already quite well exposed for soldering.

That's really all there is to it! The central pin in the speaker socket may be a little smaller than the power-brick plug (2.1mm vs 2.5mm I think) If it doesn't connect very well, it might be necessary to roll a small piece of copper or tin shim to increase the diameter of the centre pin. If that's too fiddly then you can always swap the socket for the correct sized one, or find an adaptor lead. I slipped a slender wire crimp insert over the pin which worked a treat.

Sir, you are a genius!!..

I almost dump this monitor out of my apartment window after MONTHS of troubleshooting. I am really glad i did not do it.

I have tried the mod using an old Laptop 19.5v power adaptor & it fired right up!!!!... However, because the power adaptor i am using is rated 2.35A, the lamps are not 100% lit & i am having patches of 'blacks' on the screen. Now, to searching for a reliable 20v 3A power..

A million thanks to you AdrianM..

[AdrianM]

Re: Dell 2407WFPb Monitor - Bad Cap

I almost dump this monitor out of my apartment window after MONTHS of troubleshooting. I am really glad i did not do it.

Well done for hanging in there

I think the AC power specs on this monitor are in the range 55W to 110W. Of this 20% or so will be lost in the switch mode PSU and my measured 3A @ 20V at the DC supply point sits right in there. Bear in mind of course that this depends a lot on screen brightness!

The PA6 power brick is rated at 70W on the output side, which will equate to something like 85W from the AC Line. Mine has been powering the monitor for more than a week now and runs at about 30degC above ambient (measured on the case). I'd say it's OK, but if you get the chance to buy a 100W brick, it might be better in the long term. I'm looking at 27" monitors now.

[jetadm123]

Re: Dell 2407WFPb Monitor - Bad Cap

Sir, you are a genius!!..


the lamps are not 100% lit & i am having patches of 'blacks' on the screen. Now, to searching for a reliable 20v 3A power..

A million thanks to you AdrianM..

Hopefully , those black patches are not pressure marks resulting from something pushing/pressing against the screen. Those type of marks are not repairable.

[klkmalik]

Re: Dell 2407WFPb Monitor - Bad Cap

Hey All,

I have a problem on my hands! I have a very beautiful (and also very dead) 24 inch monitor that has a bad capacitor on the power supply. However, thats not really the problem. The problem is that the capacitor that died is the big kahuna -- 450V 120uf Elite branded capacitor -- and I can't find a replacement for it anywhere.

I'm trying to find a replacement capacitor so that I can attempt to fix this monitor. I've done a lot of reading on the matter but I simply don't know enough to make a decision on which capacitor to buy and where to buy it. I need your help, this is beyond my area of expertise.

Any help is greatly appreciated!

EDIT: Subscribed to the thread...

I think you can take from old or damajed laptop adaptor inside main cap.
it will 400/120 or 400/100uf . you can fixed.

[klkmalik]

Re: Dell 2407WFPb Monitor - Bad Cap

Got it fixed now. New approach though. I began to wonder what voltage(s) were supposed to be produced by the SMPS so traced out the circuit around the output transformer. The feedback for the buck stage is obtained via an opto switch that comes on at about 20V (determined by a LM431 variable Zener). Just the one 20V supply rail then! A quick hookup to a bench supply confirms a happy monitor when juiced by 20V@3A.

Because the final output is a bunch of diodes fed from the switching transformer, it's OK to run an external 20V supply straight onto the board (across the row of 470u/35V caps). The diodes block current from going back into the transformer. I tweaked the voltage around and monitored the opto switch switch-on point confirming that the SMPS is set to run at 20V.

Now, I remembered that I had a spare power brick from my previous Dell monitor and sure enough it's rated 20V@3.5A. Not only that, but the DC jack fits the Loudspeaker output - so I cut the inductor (L702) that fed it power from the main board and jumpered a wire to the 20V rail (photo). I also grounded the switch contact that tells the MB the speaker isn't plugged in, even though the power plug is in. Oh, and I left the fuse off the SMPS just incase it ever gets plugged into the mains by accident!

why not use universal power adaptor 19v 4.2 AMP FOR EXT power supply. it is enough.