Dell Monitor blinking LED

[Pentium4]

This thing on rare occasion will turn on for a split second, most of the time just have a blinking LED. Fuse F601 and PF751 tested good. 164V across the input cap. Diodes D751 and D761 aren't shorted. I think the problem is either with the caps or the voltage regulator on the main board. That regulator U10 (which reads G061 950T65) acts very strange:

Pin 1: Bounces between 0.985V - 1.300V
Pin 2: 0.00V
Pin 3: Bounces between 0.980V - 1.060V

Could that be one of the caps next to the regulator? All the caps in this thing are trash (It is a Benq afterall...) The power board has Lelon/Elite and the main board has Lelon/Jamicon/Elite

[UniT3d]

Re: Dell Monitor blinking LED

Hmmm .....

This has so many revisions and yet they have the same common issue.

If my memory isn't that fuzzy :

- Check/replace Q739, Q740, Q759, Q760, Q743, Q751
- Resolder T751, T752, T753, T754
- Replace all secondary caps

[budm]

Re: Dell Monitor blinking LED

Check those 4 inveter tansistors 2SC 5706/5707 for low resistance reding between E and C, and yes, the solder joints
Which Dell is this?
http://s807.photobucket.com/albums/yy352/budm/DELL/

[Pentium4]

Re: Dell Monitor blinking LED

It's a Dell E172FPb. All the joints look good on both boards, could they still be bad? When checking the transistors which legs are E and C? Well some are labeled some aren't

[Pentium4]

Re: Dell Monitor blinking LED

#1 it gets < 1 Ω, and #2 gets 7.7 Ω

[budm]

Re: Dell Monitor blinking LED

These are the transistors that usually shorted out.

[Pentium4]

Re: Dell Monitor blinking LED

All of those between C and E (going both ways) either tested ~7.3Ω or ~287Ω. So I replaced just the secondary caps and now the voltage regulator on the main board is still drastically fluctuating but now the voltages are lower (the low and high) Can that indicate anything?

[Lumberjack777]

Re: Dell Monitor blinking LED

Yeah. retiredcaps knows this board very well. It's the "BenQ board". It's almost always one of the inverter transistors circled in red. Or 1 of the 2 circled in blue. And also check the picofuse circled in red.

[Pentium4]

Re: Dell Monitor blinking LED

Thanks for the reply. I checked the picofuse and it's good. I gave the readings for the transistors on post #7, does that indicate if any are bad?

[retiredcaps]

Re: Dell Monitor blinking LED

All of those between C and E (going both ways) either tested ~7.3Ω or ~287Ω.

Shorted. They are bad.

[Pentium4]

Re: Dell Monitor blinking LED

Shorted. They are bad.

All of them? Do I need exact replacements? Oh and by the way I tested them still soldered to the board. Did I need to desolder them before I tested them?

[retiredcaps]

Re: Dell Monitor blinking LED

All of them? Do I need exact replacements? Oh and by the way I tested them still soldered to the board. Did I need to desolder them before I tested them?

Desolder all of them and verify out of circuit. We generally recommend they be replaced in pairs.

The c5707s are sold as "kits" on ebay because these Benq have known problems.

http://www.ebay.com/sch/i.html?_trks...313&_nkw=c5707

Resolder all your inverter transformers regardless how they look.

[Pentium4]

Re: Dell Monitor blinking LED

Desolder all of them and verify out of circuit. We generally recommend they be replaced in pairs.

The c5707s are sold as "kits" on ebay because these Benq have known problems.

http://www.ebay.com/sch/i.html?_trks...313&_nkw=c5707

Resolder all your inverter transformers regardless how they look.

Thank you very much for the link! Does it not matter which ones I get on that search list? Okay I will resolder the inverter transformers and I will also replace all of the smaller caps on the primary. For future reference is it the high resistance that indicated those transistors were shorted?

[retiredcaps]

Re: Dell Monitor blinking LED

Does it not matter which ones I get on that search list?

I just buy from sort by lowest price + shipping. A while back I order about 5 "kits" so I have enough to repair anymore Benqs. I picked up 2 Benq's a couple of weeks ago, but I have been busy with other things. I'm sure it will be the "usual" fix.

For future reference is it the high resistance that indicated those transistors were shorted?

From my 2 seconds guide (post #19)

2) Shorted Transistors/MOSFETs

On some LCD brands (example: Benq) the transistors (C5707) are sometimes shorted. Transistors are usually marked with a "Q" designation.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for shorted transistors "in circuit" (with power off and unplugged)

a) put black probe on pin 1 and red probe on pin 2 - read/record ohm
b) put black probe on pin 1 and red probe on pin 3 - read/record ohm
c) put black probe on pin 2 and red probe on pin 3 - read/record ohm

If any reading is less than 30 ohms you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify.

Note: a shorted transistor would likely cause a very brief flash of the backlight or no backlight.

Mosfets

If you have a manual range multimeter, set it to 200 ohms (two hundred). This is for 3 pin ICs only. You can test a mosfet (Q, IC designation) "in circuit" by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

[retiredcaps]

Re: Dell Monitor blinking LED

I forgot to mention that thecapking.com also sells the c5707 and FU9024. He is based in USA.

http://thecapking.com/sanyoc5707.html

http://thecapking.com/sanyoc5707.html

$2 First Class USPS Shipping on All Orders in the US!

[Lumberjack777]

Re: Dell Monitor blinking LED

Resolder all your inverter transformers regardless how they look.

Thanks RC. That's vitally important for the repair to last. And I might add that it doesn't mean just touching the joints with a solder iron and remelting the solder. You want to get your wick up there and remove most of the solder a side at a time. And resolder using good quality leaded solder. Then it should last quite awhile.

[Pentium4]

Re: Dell Monitor blinking LED

Awesome! Thank you very much retiredcaps. I ordered 12 of them so I will report back when I get them and test it out! Yeah I always redo the joints myself. Plus it looks better

[retiredcaps]

Re: Dell Monitor blinking LED

Link to

http://www.thecapking.com/irfu9024npbf.html

if the FU9024 are shorted.

Did you test these as well?

[selldoor]

Re: Dell Monitor blinking LED

I have skimmed through and cant see it - you should also resolder the joints on the capacitors between the 5707s

[Pentium4]

Re: Dell Monitor blinking LED

I replaced those transistors, replaced all the caps, and re-soldered all the joints of the 4 inverter transformers. Now it will turn on but I have to use a flashlight to see a picture. Could one of the inverters be bad?