Dell 3008wfp PSU problem

[retiredcaps]

Re: Dell 3008wfp PSU problem

What do the experts think?

I think you need to re-read post #9 and follow my instructions exactly.

[leapius]

Re: Dell 3008wfp PSU problem

Pin | Good PSU | Bad PSU

P801 (to a/v board)

1. 5.15 | 5.11
2. 5.14 | 5.11
3. 5.14 | 5.11
4. 5.14 | 5.11
5. 5.14 | 5.11
6. 0 | 0
7. 0 | 0
8. 0 | 0
9. 0 | 0
10. 0 | 0
11. 18.18 | 19.1 > 19.65 > 19.18 > 19.71 > 19.21 > 18.8 > 19.18 > 18.86 (across about a minute)
12. 0 | 0

P802 (to a/v board)

1. 2.56 | 2.56
2. 3.27 | 3.26
3. 2.98 | 2.97
4. 2.97 | 2.97
5. 0 | 0
6. 0 | 0
7. 0 | 0
8. 12.82 | 12.32
9. 0 | 0
10. 5.18 | 5.14

P803 (to inverter)

1. 24.3 | 25.1
2. 24.3 | 25.1
3. 24.3 | 25.1
4. 24.3 | 25.1
5. 24.3 | 25.1
6. 0 | 0
7. 0 | 0
8. 0 | 0
9. 0 | 0
10. 0 | 0
11. 0 | 0
12. 3.26 | 3.26
13. 2.56 | 2.55
14. 0 | 0

The only voltage that fluctuates is Pin 11 on P801, all others remain constant. I was unable to verify the panel was in fact blank when testing (read above) but I believe it was due to the power led flickering once, which is the only other noticable condition.

Thanks.

[retiredcaps]

Re: Dell 3008wfp PSU problem

First, the reason for asking for measurements in the exact format is we get the raw data without filtration and interpretation. Too often, we get people posting here saying "the voltages measure good" only to find out they were bad or out of specification or they miss reporting a key voltage pin. With raw data, we can see for ourselves.

Second, a multimeter only reports the quantity of the voltage, not the quality. The fluctuating 18V could have lots of ripple which may be causing a problem. As an example, see this article

http://www.jonnyguru.com/modules.php...Story4&reid=71

and the 12V rail.

Since you have fluctuating 18V, I suggest changing out the caps on this rail to see if you get a solid 18V.

[leapius]

Re: Dell 3008wfp PSU problem

I get what you're saying. What tactic should I use to trace this 18v rail backwards and associated capacitors? I was trying to follow the trace visually but it becomes very difficult quickly. My limited knowledge about capacitors is that their purpose is to smooth voltage, so in seeing this fluctuation made me think that could be where the problem lies. My theory as to why the PSU is staying on but the screen goes blank was that it was sending fluctuating voltage to the a/v board and that it didn't like so shut down (and cascaded that to the tcon and then to the inverter).

[retiredcaps]

Re: Dell 3008wfp PSU problem

What tactic should I use to trace this 18v rail backwards and associated capacitors?

Continuity beep mode on your multimeter is your friend.

Put one probe on the 18V pin. Put the other probe on the + leg of the capacitor. If the multimeter beeps, it is on the 18V rail.

In addition, the caps on the 18V rail are likely to be 25V or 35V rated. Thus, no need to check the caps rated at 5V, 10V or 16V.

[retiredcaps]

Re: Dell 3008wfp PSU problem

Too often, we get people posting here saying "the voltages measure good" only to find out they were bad or out of specification or they miss reporting a key voltage pin. With raw data, we can see for ourselves.

We invite and want people thinking and interpreting the raw data, but we would like to confirm.

[leapius]

Re: Dell 3008wfp PSU problem

Continuity beep mode on your multimeter is your friend.

Put one probe on the 18V pin. Put the other probe on the + leg of the capacitor. If the multimeter beeps, it is on the 18V rail.

In addition, the caps on the 18V rail are likely to be 25V or 35V rated. Thus, no need to check the caps rated at 5V, 10V or 16V.

Thanks for the tip! Very useful - a cursory adventure around the board revealed 2 capacitors connected to that rail (a 35v next to it and a 25v further away in the middle). I will do some more thorough investigation tomorrow morning.

[leapius]

Re: Dell 3008wfp PSU problem

I have now replaced the two capacitors (new Rubycons) that I found on the 18v rail and the voltage still fluctuates between 18.8~19.7 seemingly randomly but after a while it settles down. It tends to go high on cold start and then slowly drop back down.

What can I try next? Re-cap the whole board?

[retiredcaps]

Re: Dell 3008wfp PSU problem

One of the 3 pin schottky diodes, probably mounted on a heat sink, is likely responsible for generating the 18V. See if it is stable 18V DC (likely the middle pin).

If it is, then you have to trace the path from that diode to the 18V connector on the board. Something in the 18V "path" could be bad. Since you replaced the capacitors, you can rule them out.

[leapius]

Re: Dell 3008wfp PSU problem

Of the 5 diodes on the heatsink:

2 are at 25.2v constant
2 are at 5.17v constant
1 varies 17.77~17.58v (and dropping. This is the notorious D22 schottky - voltage seems to go slowly in the downward direction)

That was the only one close to 18v. As you can see it's not producing the same voltage as the output pin. Is that variance of interest?

[retiredcaps]

Re: Dell 3008wfp PSU problem

1 varies 17.77~17.59v (this is the notorious D22 schottky - voltage seems to go slowly in the downward direction)

That was the only one close to 18v. As you can see it's not producing the same voltage as the output pin. Is that variance of interest?

It wouldn't hurt to change it since it has been problematic for many people. Eventually, it will fail like so many others have reported.

[leapius]

Re: Dell 3008wfp PSU problem

Do you think the root of the problem is further back down the 18v rail somewhere? Could a schottky going bad affect the voltage like that?

[retiredcaps]

Re: Dell 3008wfp PSU problem

Do have a schematic for this PSU? If not, you will have to trace out all the components on the rail .

[leapius]

Re: Dell 3008wfp PSU problem

Do have a schematic for this PSU? If not, you will have to trace out all the components on the rail .

I've not found one as of yet but could you give me an idea of how to do this myself? Also do you know what failing components are likely causes for voltage fluctations like this? Thanks.

[retiredcaps]

Re: Dell 3008wfp PSU problem

This is a schematic of a VM2235wm-3.

It gives you an idea of what to look for. See the 24V rail as a rough guide to the 18V rail. Imagine D821 as D22.

As I mentioned before, continuity, when tracing out a circuit is your friend.

[leapius]

Re: Dell 3008wfp PSU problem

I am having a lot of trouble tracing the rail back using the continuity method. Nothing seems to be connected. If the D22 has a voltage 17.59~17.77v does that mean it is on the 18v rail? Because the continuity beep is not present. I can only make it beep on 2 capacitors in fact.

Also does this flucation indicate the problem is further down the line towards the transformer? Or could the diode itself be causing a fluctuation? ~0.2v fluctuation is not as much as the ~1v difference seen at the pin itself.

Many thanks for any help.

[clanteam]

Re: Dell 3008wfp PSU problem

Hy all.monitoromról the picture goes away after 10 minutes.background lighting remains.what may be the problem?yeah the nutriment is getting warm strongly.
Excuse me my wrong English!