Dell 2405FPW power supply fault.

[(*steve*)]

This came to me in a "partially repaired" state.

Stop reading here

Skip to the end... The stuff in the middle is what I've found out. But I think I want to bypass the original power supply, so my REAL question relates to the voltage and currents for the rails generated by the power supply.

It seems to have had the classic 2405FPW fault of the 400V capacitor giving out.

I've read this, and I can spot a number of similarities.

Anyway, I've attached some images. It seems to me that Q651 has been replaced at some point. And clearly C605 is not the original.

In any case, comparing to the checklist:

a) F601: Resistance of fuse high!

Yep, it's blown.

b) C605: Capacitance reading not correct

Replaced (but reads correct capacitance)

c) Q651: Resistance only a few ohms between one pair of pins

Yes, I'm seeing a short across drain and source (0.1ohms). Also seeing 117 ohms to the gate which seems rather low...

d) IC651: visibly burnt

Best image is 3685, and yes it looks burnt.

e) R669: visibly burnt/ resistance no longer 1k

No visible issues, but resistance 1.226k

f) ZD601: resistance of about 22R across it, both ways round?

On resistance rage I see open one way and just over 1k the other way. On diode test I see 0.616V and 1.016V. SO still acting a little like a diode, but I'd have to remove it to be sure.

g) IC601: Capacitance measured across C611 appears more than 47uF even though C611 is OK.

Can't measure across C611, but IC601 looks discoloured on one side. I think it's fried.

Start reading here

Frankly, the effort in repairing this looks more than the monitor is worth to me. But the idea of applying power externally seems attractive.

The instructions for the 2407WFPb are here.

What I'm concerned about is the other voltage supplied by the 2405 power supply board that is not supplied by the 2407 board. Does anyone know what it is?

What are the rails?

What currents are they capable of supplying?

Thanks

p.s. Sorry about the quality of the photos below. I shot them by hand in failing light and they're not as sharp as I would have liked.

[(*steve*)]

Re: Dell 2405FPW power supply fault.

In another thread I've discovered that the output voltages are likely to be around 21.7V and 6.6V and that each is fused at 5A (which at least gives an upper limit on the current)

Black: 0
Brown: 0
Red: 21.7V
Orange: 21.7V
Yellow: 21.7V

[...]

Green: 6.6V

Thanks Denz & Seldor

[selldoor]

Re: Dell 2405FPW power supply fault.

I and probably others might be able to help more if your post was structured better.
I think I stopped reading when you told me to and when I read the bottom part it didnt really say what the problem was but that you were going for a power bypass so I moved on. I also assumed you knew what you were doing until I read you had not realised the fuses were fuses
Most of us work on - Do the lights work?. Is there any sort of picture? what is the output from the Power supply board. test secondary voltages then primary.
Voltage regulators on Main board. Fuses - probably could be first but some are hard to reach. I often ask for a description of all that happens from plug in to stable fault - lights lamps sounds clicks pops wheezes.
I cant help with a power bypass or understanding circuits but happy to share what little I have picked up regards testing.

[(*steve*)]

Re: Dell 2405FPW power supply fault.

A TLDR problem I see.

The power supply is currently toast as per my initial post and the cost to get the various components is higher than I'm prepared to pay (especially considering they may just let the smoke out as soon as they're replaced).

I'm currently after the current that I need to be able to supply from these rails.

To answer you *precisely*, since the main fuse is blown, I'm not going to have any lights or picture, clearly no output from the power supply, and no input voltage either (after the blown mains fuse).

[selldoor]

Re: Dell 2405FPW power supply fault.

Yes - when we dont need it.
I still dont fully understand the question -the bypass just uses a laptop type power supply -20v 4.5A 90w seems the one to go for and I dont think the 2405 will need more power than the 2407.
Have you been able to locate the points to connect the by-pass ( not that I can help as I cant often follow circuits)

[oko]

Re: Dell 2405FPW power supply fault.

Hi guys, help determine which there were? received in this form! thanks for the help! may have a schematic for this power supply??

[bicycle6228]

Re: Dell 2405FPW power supply fault.

C702, C711, C712, C713, C714, C715: All 6 of these capacitors are 470uf, 35V, 105°C.
D701, D702, D703: YG865C15
D704: YG802C10
These capacitors fail frequently, and also C605 (120uf, 450v, 105°C)
I have never seen any of those Schottky Barrier Diodes fail.

[bicycle6228]

Re: Dell 2405FPW power supply fault.

Forgot the schematic. Attached, but it's for the very similar 2407WFPb. There are some differences, such as R674, which is 8.25K in a 2407, but 11.3K in a 2405.

[budm]

Re: Dell 2405FPW power supply fault.

Can you tell me what DC voltage you get at that main 450vdc filter cap?

[bicycle6228]

Re: Dell 2405FPW power supply fault.

300VDC, measured across C605's terminals, with the monitor actively in display, not in standby.

[budm]

Re: Dell 2405FPW power supply fault.

Thanks, it is lower than I thought for the typical PFC Voltage booster.

[malkit]

Re: Dell 2405FPW power supply fault.

do you have the remaining circuit diagram that shows all components starting with 7xxx
This will help me use the external block (20V supply)
Thanks