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Old 08-25-2018, 02:42 PM   #541
Valetebellum
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Default Re: Dell 2407WFPb Monitor - Bad Cap

Hello,
I got the exact same problem with various defective components on the board and want to try the external powering hsck by adrianm.
I read that instead of the supposed 19volt also somebady got it to work with a 24v power supply, which is 5v off.
So i'm hoping, that my 16v power supply with 5amps could also work, because it would only be off by 3volts.

I dont have the option off buying a 19v power supply online or in my surrounding area, so i either have to use the 16v power supply or dump the monitor into trash.


Sorry for not being able to do it the easy way, but any kind of response would be nice :-)
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Old 08-27-2018, 05:36 AM   #542
r-p
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Default Re: Dell 2407WFPb Monitor - Bad Cap

Edit: nevermind... The guy using 24V was me and all my measurements were with my then faulty powersupply (since replaced the capacitor and it is fine again), so any measurement I did back then is untrustworthy. So I cannot answer your question.

Original answer before the edit:
I'll try to find my post, but I remember having tried several voltages (adjustable powersupply) after the hack. I cannot recall if I went as low as 16V.

It worked fine for years, but then someone posted a link to dirtcheap replacement powersupplies, I ordered one and it runs absolutely perfect on the new PSU. Do note it is only used occasionally at the moment, so I cannot vouch for it also working when used 24/7.

Since you have nothing to loose (apart from the probably quite well protected 16V supply), why not simply try it?

Not related, but just an example: I hacked a Dell external DVD drive caddy which should get 20V from the laptop. It came with a USB-plug with a different plug integrated into it, I chopped it off and replaced it with a normal USB cable and put 12V on the PCB where the 20V should go. It works perfectly. But that supply only steps down the 20V (and now the 12V) to 5V so that was quite easy and failsafe.

Last edited by r-p; 08-27-2018 at 05:48 AM..
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Old 05-28-2019, 09:34 AM   #543
Octoberon
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Lightbulb Re: Dell 2407WFPb Monitor - Bad Cap

I hope this may help somebody. I had a 2405FPW that was in a bad way.

Usual failure mode: Open circuit C605. Cascade failure ensues.

It is working now. There was some parts that were definitely bad, and some parts that tested OK. There were some parts that I just did not feel comfortable proceeding without replacing them.

Note: the attached circuit diagram is for a 2407. That said, the similarities between the 2405 and 2407 PSUs are very handy. Check my comments in the PDF to see what I found.

Good luck, these PSUs generally will work again with a little love.

Jon
Attached Files
File Type: pdf PSU - Belt & Braces.pdf (202.0 KB, 35 views)
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Old 08-03-2019, 10:34 AM   #544
sean0118
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Default Re: Dell 2407WFPb Monitor - Bad Cap

Quote:
Originally Posted by Octoberon View Post
Note: the attached circuit diagram is for a 2407. That said, the similarities between the 2405 and 2407 PSUs are very handy. Check my comments in the PDF to see what I found.
Thanks, the circuit diagram is really helpful.


I think there's actually a design issue. The feedback resistors for the boost circuit are 499k+232k+232k and 8.25k. The L6561 has a reference voltage of 2.5V. This will cause it to try to regulate to +294V DC.

2.5V *971.25/8.25 = 294V

But if you rectify 230V AC you get 325V DC, which is the input to this boost converter (and 30V higher than its output set point). Shouldn't the output be set to regulate at +400V DC? Or at least 350V?

It's 2am here so I might be missing something, but I don't see how this works with a 230VAC input.

Last edited by sean0118; 08-03-2019 at 10:43 AM..
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Old 08-04-2019, 06:37 AM   #545
sean0118
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Default Re: Dell 2407WFPb Monitor - Bad Cap

Ah, never mind, I was wrong.

R666 (24.9K) gets put in parallel with R674. Which does make it regulate to 391V DC as would be expected.

So the design looks ok, the cap is just bad quality.
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Old 08-04-2019, 06:57 AM   #546
Octoberon
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Default Re: Dell 2407WFPb Monitor - Bad Cap

The Capacitor, C605, 120uF, 450V is definitely and by far the most common failure on these Benq made PSUs. It is so sad that the failure of this capacitor can be so catastrophic. Most of the failures I have seen, are where electrolyte has leaked out through a lead-hole in the rubber bung. The capacitors lead then appears to get etched away by the electrolyte.

Last edited by Octoberon; 08-04-2019 at 07:00 AM..
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Old 10-02-2019, 10:03 PM   #547
sean0118
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Default Re: Dell 2407WFPb Monitor - Bad Cap

I managed to fix mine as well.

I thought it wasn't fixed when I first tested it, because C605 only had rectified mains on it, not the ~400V DC expected if the PFC is boosting the voltage. But it turns out the PFC circuit doesn't turn on until it's connected to the other board, this is the signal "Power_off" which controls the optocoupler IC652.


I ordered all the parts from LCSC, but I'm sure Digikey/Mouser would have them too. There were some parts which aren't available anymore so I had to find a similar part to replace it with:

Designator - Original part on board - Replacement part
Q654 - 2N3906S - KEC 2N3906S-RTK/PS
IC651 - L6561D - ST L6562DTR
IC601 - NCP1200AD - ONsemi NCP1200D60R2G
D655 - LL4148 - ONsemi LL4148
D604 - LL4148 - ONsemi LL4148
Q651 - 2SK3502 - ST STF24N60M2
Q601 - 2SK3548-01SC - ONsemi FQA11N90C-F109
R670 - 0.22ohm - Futaba Elec RWS03JR220A750NH
R669 - 1K 0805 (0603?)
ZD652 - TZMC24 - TZMC24
C605 - 450V, 120uF - To save money I used a Nichicon part from a donor board for this, I think it was 100uF, but seems to work.

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