Dell 2405FPW blown transformer?

[ucdzombie]

Hi everyone. I've searched the forum thoroughly about this problem but couldn't find anything similar to my Dell's symptom.

I've repaired this monitor before by replacing two 5v slow blown fuse (F701 and F702) and modified the bad MOSFET on the logic board (under the direction of an electrical engineering professor). I've enjoyed my repair for two years and now it's dead. When power is on there's no response at all, no front LED light, no image.

I'm not an electrical engineer so I can only do a couple of test on the power board. I've attached a picture for you to review. Basically all the fuses have been tested and they're all normal (zero resistance, normal continuity with the multimeter buzz sound). However, when power on, the big fuse next to the power input has no current reading at all, so are the rest of the components. I've not tested the transformer since I don't know how to test the legs. But I'm suspecting that it's a blown transformer?
The only two places that have 120v reading are the power connector and the small resistor behind it. Any help would be much appreciated! Thank you all.

[budm]

Re: Dell 2405FPW blown transformer?

What DCV do you get between the two legs of that large black 450VDC main filter cap?

[Per Hansson]

Re: Dell 2405FPW blown transformer?

Just to nitpick when you say "current" you actually mean voltage.
It's a very important distinction to make

https://en.wikipedia.org/wiki/Voltage
https://en.wikipedia.org/wiki/Electric_current

[ucdzombie]

Re: Dell 2405FPW blown transformer?

What DCV do you get between the two legs of that large black 450VDC main filter cap?

Hello budm. Thanks for helping me again! The reading of the 450v capacitor is 160 volt. I've attached a picture in case I'm wrong. Initially I test it under the scale of 500v on the multimeter. But since the reading is low, I switched the scale to 250v.

Also I tried testing f702 fuse again. The voltage reading is about 7v. Let me do it again tonight to confirm on this.

As far as the big fuse next to the power connector, I tested it again. The needle on the multimeter barely moved on the lowest scale.

[ucdzombie]

Re: Dell 2405FPW blown transformer?

Just to nitpick when you say "current" you actually mean voltage.
It's a very important distinction to make

https://en.wikipedia.org/wiki/Voltage
https://en.wikipedia.org/wiki/Electric_current

Yes! Thank you for correcting me. Forgive me for my ignorance.:p

[ucdzombie]

Re: Dell 2405FPW blown transformer?

OK just some updates. I just ran an ohm test on the 7 bigger capacitors (including the 450v), all of them are dead (although there's no signs of physical damage). The needle on my multimeter went to the right, stayed, and never returned to infinity. So maybe the transformer is OK?

[ucdzombie]

Re: Dell 2405FPW blown transformer?

Anyone?

[budm]

Re: Dell 2405FPW blown transformer?

OK, since you are getting the 160VDC on the main cap, then no need to check the main fuse, etc.
Right now we have Voltage to run the switching power supply but it is not active so we need to find out why next.
I need to find the pictures of my power supply board so I can can give you the test points on the board to test next.
BTW, you should get the Digital Multi Meter.

[ucdzombie]

Re: Dell 2405FPW blown transformer?

OK, since you are getting the 160VDC on the main cap, then no need to check the main fuse, etc.
Right now we have Voltage to run the switching power supply but it is not active so we need to find out why next.
I need to find the pictures of my power supply board so I can can give you the test points on the board to test next.
BTW, you should get the Digital Multi Meter.

Thank you. Would it help if I take a picture of of the bottom of my board?

[jetadm123]

Re: Dell 2405FPW blown transformer?

There are different variations of this power supply. The 160 volts you are seeing should be boosted to approx. 390Vdc if the supply is working properly. I suggest you check the following:

1) with power off, measure the resistance between pins of mosfet Q651 below. If it's the power supply I think it is, then Q651 (2SK3502) is mounted on the heatsink behind the yellow transformer.

gate-->source

source-->drain

drain-->source

You will see very low resistance if there is a short.

2) is step 1 checks ok, then you will need to check the dc voltage of pin 8 (VCC) of IC651 (L6561) located underneath the board. It's the black 8-legged chip. Use the neg lead of the 450V cap for ground. You will probably have to solder a small wire to pin 8 and run it clear of the board to take your measurement. Be careful as you are working in a HIGH VOLTAGE area.

[budm]

Re: Dell 2405FPW blown transformer?

There are different variations of this power supply. The 160 volts you are seeing should be boosted to approx. 390Vdc if the supply is working properly. I suggest you check the following:

1) with power off, measure the resistance between pins of mosfet Q651 below. If it's the power supply I think it is, then Q651 (2SK3502) is mounted on the heatsink behind the yellow transformer.

gate-->source

source-->drain

drain-->source

You will see very low resistance if there is a short.

2) is step 1 checks ok, then you will need to check the dc voltage of pin 8 (VCC) of IC651 (L6561) located underneath the board. It's the black 8-legged chip. Use the neg lead of the 450V cap for ground. You will probably have to solder a small wire to pin 8 and run it clear of the board to take your measurement. Be careful as you are working in a HIGH VOLTAGE area.

I do nor remember if the PFC is running all the time or not or only when it gets the PS-ON signal from the logic board, but at this point he indicate that there is no any DC from the connector at all so the it means there will there no VCC from the AUX winding of the power transformer to run the PFC circuit if I remember the circuit setup on this board. I cannot find my spare board yet.

[ucdzombie]

Re: Dell 2405FPW blown transformer?

There are different variations of this power supply. The 160 volts you are seeing should be boosted to approx. 390Vdc if the supply is working properly. I suggest you check the following:

1) with power off, measure the resistance between pins of mosfet Q651 below. If it's the power supply I think it is, then Q651 (2SK3502) is mounted on the heatsink behind the yellow transformer.

gate-->source

source-->drain

drain-->source

You will see very low resistance if there is a short.

2) is step 1 checks ok, then you will need to check the dc voltage of pin 8 (VCC) of IC651 (L6561) located underneath the board. It's the black 8-legged chip. Use the neg lead of the 450V cap for ground. You will probably have to solder a small wire to pin 8 and run it clear of the board to take your measure ent. Be careful as you are working in a HIGH VOLTAGE area.

Thank you. Ok I check the leads. They all have uniformal reading, which is 80k. I've attached a picture in case I'm reading it wrong. I was also curious about Q601 (which looks the same as Q651). It also has the same reading. Is this considered high or low resistance?

[ucdzombie]

Re: Dell 2405FPW blown transformer?

I do nor remember if the PFC is running all the time or not or only when it gets the PS-ON signal from the logic board, but at this point he indicate that there is no any DC from the connector at all so the it means there will there no VCC from the AUX winding of the power transformer to run the PFC circuit if I remember the circuit setup on this board. I cannot find my spare board yet.

Ok another update. I just checked the logic board. It has voltage reading. The mod I did 2 years ago (the little voltage converter in the middle of the picture) has a reading of 17 volt. So is this a bit low since it's supposed to be 19 volt?

[jetadm123]

Re: Dell 2405FPW blown transformer?

I do nor remember if the PFC is running all the time or not or only when it gets the PS-ON signal from the logic board, but at this point he indicate that there is no any DC from the connector at all so the it means there will there no VCC from the AUX winding of the power transformer to run the PFC circuit if I remember the circuit setup on this board. I cannot find my spare board yet.

budm-I see your point. The problem now seems to be the mod that the OP did to the mainboard. Seems like the mod is putting out 17v, which is substituting for the normal 19v that the power supply would be putting out. Not really sure how to approach this type of mod.

[budm]

Re: Dell 2405FPW blown transformer?

Wait a minute, are you using the external power supply with the mod you made?
If not then you cannot have 0V at all the pin of the power supply connector.
I know some people use that buck converter module and run it from the power supply board, not from the external power supply.

[ucdzombie]

Re: Dell 2405FPW blown transformer?

Wait a minute, are you using the external power supply with the mod you made?
If not then you cannot have 0V at all the pin of the power supply connector.
I know some people use that buck converter module and run it from the power supply board, not from the external power supply.

No. I'm sorry for the confusion. That mod is just to convert 19v to 5v since the original MOSFET was bad. As you know, I'm not an electrical engineer so I apologize again for my ignorance. So I've tried to test my board carefully again and I came up with more new readings hopefully to narrow down the problems. I've attached a picture to explain. Basically the mod (buck converter) is working properly by itself (converting 17v to 4v). However its source is only 17v, that means the power board is not working. And this is confirmed by testing the F701 fuse on the power board, which shows only 17v on both ends. In order words, the power board is not outputing 19v. Last time I fixed this monitor, the F701 fuse had 19v on both ends, so did the buck converter source.

[budm]

Re: Dell 2405FPW blown transformer?

So you DO have output Voltage from the power supply, not 0V like you have indicated in the post.
If that is the case then you should have boosted Voltage close to 400VDC on the cap, all this time I thought you have no output Voltage from the power supply at all, but it is not the case but the Voltage is not 19'~20V like it should be, at this point you need to get the PFC boosted Voltage on the cap which will involve checking to see if the PFC Controller IC has DCV on the VCC pin for it to run.

[ucdzombie]

Re: Dell 2405FPW blown transformer?

There are different variations of this power supply. The 160 volts you are seeing should be boosted to approx. 390Vdc if the supply is working properly. I suggest you check the following:

1) with power off, measure the resistance between pins of mosfet Q651 below. If it's the power supply I think it is, then Q651 (2SK3502) is mounted on the heatsink behind the yellow transformer.

gate-->source

source-->drain

drain-->source

You will see very low resistance if there is a short.

2) is step 1 checks ok, then you will need to check the dc voltage of pin 8 (VCC) of IC651 (L6561) located underneath the board. It's the black 8-legged chip. Use the neg lead of the 450V cap for ground. You will probably have to solder a small wire to pin 8 and run it clear of the board to take your measurement. Be careful as you are working in a HIGH VOLTAGE area.

Ok, so I will solder a wire between the negative leg of 450v cap to the pin 8 of the vcc? But which pin is pin 8? And how do I check the voltage? Do I measure pin 8 in relation to the chassis ground? What scale should I be using?

[ucdzombie]

Re: Dell 2405FPW blown transformer?

Ok, so I will solder a wire between the negative leg of 450v cap to the pin 8 of the vcc? But which pin is pin 8? And how do I check the voltage? Do I measure pin 8 in relation to the chassis ground? What scale should I be using?

OK never mind. I think I understand what you meant. Solder the wire to pin 8 so I can test it easily without shorting other parts. Use the negative leg of the 450v cap as ground. Check the voltage of pin 8 in relation to the 450v neg leg. Am I correct? But which one is pin 8?

[ucdzombie]

Re: Dell 2405FPW blown transformer?

Ok I just did some research and run the test (see picture, assuming I've identified pin 8 correctly), the voltage is very very small. On the scale of 0.1v, the multimeters needle barely moved about 3 clicks. Also I've noticed the surface of IC651 is somewhat melted. The part number is so faded and unrecognizable.