BenQ LED G2420HDBL Monitor Repair.

BenQ LED G2420HDBL Monitor Repair.


The BenQ monitor in my workshop that i use for testing VGA and DVI outputs stopped working. Initial symptoms were, turn the monitor on and after 5 seconds it would turn off again even though displaying an image briefly. Repeating power up gave the same problem until eventually it wouldn’t power up at all.
So i decided to take it apart and fix it and do a blog on the repair at the same time.

Disclaimer !

All information contained on these pages are here for my reference only, i will not be held responsible for any damage or injury done to you or your equipment ! Some parts of electronic equipment can hold high voltages which can shock or burn, some electronic components are very susceptible to static discharge. Aging IC’s and pcb tracks can be easily damaged if due care is not used when working on them…
You have been warned !

The service manual for this model can be found online here in downloadable pdf format, and the relevent schematics required are on pages 60-68

After removing the monitor from the wall i removed its bracket.

There are only 2 scews (Arrowed) holding the case together, the rest is just clipped together. So i undid these first..

On the bottom edge of the case there are 2 small rectangular slots (Arrowed), to allow a pry tool to be inserted to unclip the casing.

I used a plastic pry tool from an old Iphone tool kit and inserted it into one of the slots..

Using my thumb to steady myself i slowly pulled the pry tool towards me, to unclip the casing. The
clips which cant be seen are part of the plastic casing.

I then ran the pry tool up both sides, until the casing was only attached at the top, i could then get my fingers under the top plastic case and carefully unclip it from the top by hand.

With the top case removed, its just a matter of lifting the screen and its chassis out of the back case.

So using the plastic pry tool again, i levered the screen upwards, until i could get my fingers underneath it…

Lifting the bottom plastic case free, and put it to one side.

I then laid the screen face down on the work bench, checking first that the bench was clean ! This then gives clear access to the chassis.

I started by unclipping the control button ribbon cable. Using the pry tool, i flipped up the black locking catch on the top of the connector…

…and then just lifted the ribbon cable free.

The chassis is only held onto the back of the screen by self adhesive foil, so i carefully peel this back…

…just enough on all edges until it is clear of the chassis, but still attached to the screen, as i am going to reuse this, when i refit the chassis.

I then disconnect the screen loom, by gently pulling it away from the connector.

The chassis can now be tilted up to gain access to the final ribbon cable connector, which is hidden just out of sight on the parts side of the pcb (Arrowed)

This is the same style as the previous ribbon connecter just larger, so using the pry tool again i flipped up the locking tab…

… and lifted the the ribbon cable clear, you can just see the black locking tab (Arrowed)

The chassis could now be lifted clear from the screen, and the screen put somewhere safe..

With just the chassis on the bench i turn my attention to the power supply pcb, its held on with 4 screws (Arrowed) so i undo and remove these, one of them has a ribbed washer as its an earth screw, so i made note of its position, so it can be refitted in the same place.

… i then disconnect connector CN7001 by pulling gently on it…

… and then connector CN7002.

I then flip the chassis over and prise off the IEC connector locking plate..

… and remove it.

Turning the chassis back over i can now remove the power supply pcb.

A visual inspection of the pcb shows a burnt out diode at D601 (Arrowed)

… and IC601 has blown a leg off.

I meter the on board fuse and it meters open line.

Starting with the burnt diode D601 first, i dug out some UF4007 diodes…

… and cut one free from the strip, the UF4007 is a ultra fast rectifying diode. These have to be fitted the correct way round, so they have a line on one end, to indicate orientation.

i unsoldered the old one and removed any traces of old solder from the pads using solder wick (or braid)

i then soldered in the replacement and cleaned off any flux residue with flux cleaner on a Q-tip.

Next i turned my attention to the IC with the blown leg IC601 , its a TOP258PN an intelligent power switch which incorporates a power MOSFET, PWM control, oscillator, thermal shutdown circuit, with over and under voltage protection all in one small DIL package.

Using solder wick i unsoldered all the legs of the old IC and removed the chip…

Then soldered in the new replacement.

For those 2 components to be damaged the large 68uf 450v power filter capacitor at C605, can’t of been doing its job properly, so i decided to change this capacitor and whilst i was at it replace all the radial electrolytic capacitors on the power supply pcb. Mass produced Taiwanese monitors have a tendancy to be fitted with cheap sub standard capacitors, so it pays to replace all electrolytics whilst the pcb is out.

So from my stock i dug out the following radial electrolytic capacitors:

68uf 450v 105°
470uf 35v 105°
330uf 50v 105°
680uf 25v 105°
47uf 63v 105°
10uf 63v 105°

Electrolytic Capacitors have 3 key tolerances:

A  The temperature rating, usually 85° or 105°
B  The farad rating (capacitance value)
C  The operating voltage

The trick here is to match the farad rating and the temperature rating, with the voltage rating you can either match or better still go for a higher voltage rating, still bearing in mind the physical size of the capacitor, it still needs to fit in its place on the pcb.
As these are being used in the power supply section, my replacements are still 105° and my capacitance values are the same, but i’ve gone with slightly higher voltage on all the capacitors except the main 68uf filter cap which is still rated at 450v.
Another thing to be wary of, is make of capacitor, there are a lot of cheap capacitors around for sale and also alot of FAKE capacitors. Try to buy well known japanese makes from reputable companies. I use Panasonic, Nichicon, Rubycon, Sanyo/Suncon etc...

Soldering whether for removal or installing of components can be an issue, depending on the kind of pcb track the solder pad is in or is connected to. Some are fine small pads, but some have larger surface area, and these require more heat. If your soldering iron starts to stick to the pad your soldering then its not hot enough and is struggling to keep up with the heat dissipation. I use a Metcal intelligent soldering station, with twin irons. On one i have a 0.75mm conical tip installed for fine work and on the other iron a 3mm chisel tip for when i need that extra heat.


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Kernowtronic and are Copyright © 2016
They may not be copied or used without prior permission !