Dell 2408WFPb Monitor - No Power

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

The voltage across the cap varies between 321-327 v. Doesn't seem to change in time to clicking.

[selldoor]

Re: Dell 2408WFPb Monitor - No Power

Ok What voltage is there at the torpedo fuse? Measure each side of fuse to ground

Also near the fuse there is IC701 - can you test it - they dont often seem to go wrong but we have had a couple lately and as the fuse was blown perhaps it got taken out as well?

You mentioned D 602 can you retest and state the resistances ideally should be
resistance above 30ohms one way and very high/ off scale the other.- You may have to lift one leg to test.

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

Voltage at torpedo fuse cycling 5-7.5 volts on both sides. D602 out of circuit shows 28.9 Kohms and 0 ohms. Don't know how to test IC701. Identified it with markings KA431AZ as being a three terminal adjustable regulator.

[Toasty]

Re: Dell 2408WFPb Monitor - No Power

Check C601, the blue one.

T

[selldoor]

Re: Dell 2408WFPb Monitor - No Power

Can you test the diode on diode mode Oohms suggests a short

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

Apologies meant to say no reading. In diode mode shows .533v and 0v

@Toasty-I take it you mean the blue capacitor next to the transformer. Any tips on how to test these?

[Toasty]

Re: Dell 2408WFPb Monitor - No Power

Diode is fine.

Yes. Look at the underside of the board to see the component numbers.

You'll need a meter that can check the capacitance. Some DMM's have a capacitance check function. It will need to be removed from the circuit to verify the value.

Transcribe the numbers/letters line by line from the cap here so we can determine the values.

T

Additional: Did you replace both electrolytic caps on the hot side? C611 & C606?

Since the fuse popped, something is wrong beyond the PSU. Getting the PSU working is the first step towards resolving the other issue. Had someone worked on this before?

There maybe a standby voltage produced by the PSU that is passed to the circuits beyond, but is controlled by the Opto IC652 which tells the PSU "I'm okay" and allows it to switch on when needed.

Probably produced by the regulator IC603 (3-legged, TO-92 package) above the mains cap. What are its' markings & find a datasheet for it. What voltages do you get measured from HOT ground to each leg?

T

[retiredcaps]

Re: Dell 2408WFPb Monitor - No Power

Apologies meant to say no reading. In diode mode shows .533v and 0v

I assume you mean 0L (out of range).

PS. A multimeter always gives you a reading when it is turned on. There is no such thing as "no reading".

Interpreting it correctly is key.

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

@Toasty
Tested the blue capacitor 1.01 nF,
Markings
102J
2J

IC 603 is marked MCR 100-6 816
seems to be a silicon controlled rectifier, datasheet here http://www.futurlec.com/Diodes/MCR100-6.shtml
Tested all legs,
a pin cycling 6-12v
all other pins showing 0v

[jetadm123]

Re: Dell 2408WFPb Monitor - No Power

Did you ever verify that mosfet Q601 is not shorted?

[Toasty]

Re: Dell 2408WFPb Monitor - No Power

Cap is okay.

Marking it on the board as an IC is a mistake/misnomer. Should be a "D" something.

T

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

Q601 is marked K3548 6692 01
Tested across pins in circuit.
Pins 1 2 3, G D S
1-2 0L
1-3 0L
2-3 225 ohms then 0L

I think this is maybe worth removing and testing further.
Couldn't find a datasheet.

[Toasty]

Re: Dell 2408WFPb Monitor - No Power

Have you tested the 3 output rectifiers? D70? ? (can't make out the numbers)

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

Yes they are actually 2 schottky diodes and tested ok.

Also tested the cold side for shorts with the transformer removed. Again tested ok.

[Toasty]

Re: Dell 2408WFPb Monitor - No Power

You mean 3 don't you?

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

No, third position on heat sync (middle) is not populated.

[Toasty]

Re: Dell 2408WFPb Monitor - No Power

Ok.

Q601 is marked K3548 6692 01
Tested across pins in circuit.
Pins 1 2 3, G D S
1-2 0L
1-3 0L
2-3 225 ohms then 0L

I think this is maybe worth removing and testing further.
Couldn't find a datasheet.

Fuji Electric 2SK3548 - been looking for a long time for that one.

Use diode check function and test both ways. Unlikely that it's bad if the 0.22Ω resistor is good.

Have you checked that little electrolytic over at the far left of first picture?

This uses PFC, if I'm not mistaken. You should be getting 385-400v on the mains cap when on. I think perhaps the boost controller or its' circuit may be the culprit. Love to get hands on with one of these someday.

What is IC651?

T

[ForensicFixer]

Re: Dell 2408WFPb Monitor - No Power

IC651 is marked L6561, Power Factor Corrector apparently?

however c622 that's the cap on the far left is 100v 1uf
measured out of circuit o.92uf and esr 2.4 ohm

This could be the culprit?

Will try to get replacement tomorrow

[Toasty]

Re: Dell 2408WFPb Monitor - No Power

Cap values measured sound okay.

Clean the backside with alcohol and a soft brush to get rid of the flux. Visually inspect every chip transistor and resistor for burn marks with a magnifying glass/loupe. Test the transistors using the diode check function.

I'm usually pretty good at picking them out in the pics, but there's too much flux distorting the pictures.

Does R620 have a crater?

Are there any Zener's on there?

T