Benq Inverter board C5707 Transistors

[avsubvert]

Hello everyone,


I have a faulty HP 1940 with a Benq 48.LIG02.A01 inverter board. It has the fault where it the screen goes black after a few seconds, after putting a flash light to it I could see that the image was still there. i Opened it up and inspected the inverter board I could not see any faulty caps and the soldering looks good to me (I'm a novice) so after reading through the posts here I decided to pull the four C5707 transistors off the board and test them with my DMM, as I have read here that these transistors are a common fault with this board. Because I have little experience testing transistors I was hoping for some conformation from the experts (you)


I pulled 4 transistors off the board and kept them separated in pairs as I'm told the fail in pairs.


The first pair gave me the following results (all tested with the diode function on my DMM)


Transistor 1 (first pair)


B-E .575 v
B-C .575v
C-E Open
E-C Open


Transistor 2 (first pair)
Same as above


Transistor 3 (second pair)
B-E 0.00v (it did not read 0L )
B-C .349v
C-E .349v
E-C .360v


Transistor 4 (second pair)
B-E 0.00v (it did not read 0L )
B-C .444v
C-E .478v
E-C OPEN


it seems to me that the second pair is faulty I was wondering if anyone can confirm this? I have found a place on ebay where I can order some new C5707s. Also As a precaution I have touched up the soldering on all the transformers in case this was the cause of the fault. Anything else I should check?


Thanks heaps for all the great tips I found here I would never have tried this without the advice I have found on this site.

I have also posted this on fixya hope you dont mind me posting this in more then one place.

Mike

[Richie_asg1]

I'm normally on Fixya !

I am still trying to get to the biottom of my dell E173FPb

I bought 4 c5707 and the fuse from Ebay - replaced them anyway as the problem seemed only to occur when hot - and I couldn't test this.

Checked for any cold joints around the transformers.

Do your power transistors look hot - slight browning of the board around heatsinks ?

Anyway - replaced the 4 + fuse - and still had same problem.
Then replaced the major Elec' Caps - still same problem. I now suspect the 2 squat caps near the header to the small board. (on RHS next to inductor on photo below)

From other forums and google pics - there are another 2 power transistors sitting on their in the centre of the board - marked on this photo in blue -
http://www.ccl-la.com/images/LCD5_9.jpg

May be worth pulling these too.

[electroguy75]

2nd pair r faulty. cheers

[Bobdee]

Originally Posted by avsubvert

Hello everyone,


I have a faulty HP 1940 with a Benq 48.LIG02.A01 inverter board. It has the fault where it the screen goes black after a few seconds, after putting a flash light to it I could see that the image was still there. i Opened it up and inspected the inverter board I could not see any faulty caps and the soldering looks good to me (I'm a novice) so after reading through the posts here I decided to pull the four C5707 transistors off the board and test them with my DMM, as I have read here that these transistors are a common fault with this board. Because I have little experience testing transistors I was hoping for some conformation from the experts (you)


I pulled 4 transistors off the board and kept them separated in pairs as I'm told the fail in pairs.


The first pair gave me the following results (all tested with the diode function on my DMM)


Transistor 1 (first pair)


B-E .575 v
B-C .575v
C-E Open
E-C Open


Transistor 2 (first pair)
Same as above


Transistor 3 (second pair)
B-E 0.00v (it did not read 0L )
B-C .349v
C-E .349v
E-C .360v


Transistor 4 (second pair)
B-E 0.00v (it did not read 0L )
B-C .444v
C-E .478v
E-C OPEN


it seems to me that the second pair is faulty I was wondering if anyone can confirm this? I have found a place on ebay where I can order some new C5707s. Also As a precaution I have touched up the soldering on all the transformers in case this was the cause of the fault. Anything else I should check?


Thanks heaps for all the great tips I found here I would never have tried this without the advice I have found on this site.

I have also posted this on fixya hope you dont mind me posting this in more then one place.

Mike

You are correct as electroguy75 confirms, also check inverter fuse as it will probably be open
When in circuit you measure pairs of C5707s, when one is open they will both read open
When removed from board only one reads bad, usually,but its best to replace in pairs anyway
As you have the pair bad you may find there may be a faulty CCFL or something else,check your solder joints carefully

[Bobdee]

Originally Posted by Richie_asg1

I'm normally on Fixya !

I am still trying to get to the biottom of my dell E173FPb

I bought 4 c5707 and the fuse from Ebay - replaced them anyway as the problem seemed only to occur when hot - and I couldn't test this.

Checked for any cold joints around the transformers.

Do your power transistors look hot - slight browning of the board around heatsinks ?

Anyway - replaced the 4 + fuse - and still had same problem.
Then replaced the major Elec' Caps - still same problem. I now suspect the 2 squat caps near the header to the small board. (on RHS next to inductor on photo below)

From other forums and google pics - there are another 2 power transistors sitting on their in the centre of the board - marked on this photo in blue -
http://www.ccl-la.com/images/LCD5_9.jpg

May be worth pulling these too.

You seem to have done the ground work
As you say this happens when hot, that sugests to me that you need to check out your CCFLs
The 2 transistors you marked in blue turn the backlights off, when they go the backlights stay on when monitor is turned off

[Newbie2]

If you can't get the original 2SC5707 transistors, you can use D44H11 transistors to do the job. They're in a TO-220 package though, larger than the original transistors, but they can probably still fit.

My BenQ FP731 17" LCD monitor failed last year, after a long inspection I found that the 2SC5707 transistors had failed along with the fuse. Digikey doesn't have 2SC5707 transistors, so I ordered D44H11 transistors. My monitor still works good.

[Bobdee]

Originally Posted by Richie_asg1

Checked for any cold joints around the transformers.

Afterthought
Its best to resolder the transformers as it is the ring cracks that you find with them
As in this Link
http://www.badcaps.net/forum/showpos...&postcount=231

[Richie_asg1]

Thanks for the pointer BobDee - and Excellent photo by the way of cold solder cracking. Will resolder all transformer pins.

My symptoms are- works fine daily - for an hour or more, then display goes black - green power button stays on. turn off, turn on - will stay on for a few seconds before going black again. Leaving to cool results in good working again.
Display goes bright for 2 seconds before going black completely. Tried adjusting brightness - but has little effect. This rings a bell with something I've read - but can't remember which bit.

I'm thinking a transistor is getting too much current - then crowbaring into full conduction - resulting in a shutdown. Possibly one of the limiting resistors - but which one ?

Also I'm not too ok with working on live equipment to check working voltages. Is there a safer way of doung things besides being suspended by a rubber bungie rope?

Cheers.

[Bobdee]

Originally Posted by Richie_asg1

Thanks for the pointer BobDee - and Excellent photo by the way of cold solder cracking. Will resolder all transformer pins.

My symptoms are- works fine daily - for an hour or more, then display goes black - green power button stays on. turn off, turn on - will stay on for a few seconds before going black again. Leaving to cool results in good working again.


Cheers.

Do you get the odd flicker of backlights before they fail,
With the symptoms you mention above and the odd flicker of lights before failure sugest that its a faulty CCFL
They are worthy of a check to see if this is the problem as well as resoldering transformers

Never come across your sugestion of limiting resistors being the problem
These Benq boards are in Benqs + HPs + Dells etc I have done many ( 2 seconds to black problems ) as they are called
standard fix = replace faulty C5707s + pico fuse + resolder transformers, and check and resolder any other bad solder joints if that dont repair it then its usualy bad CCFLs

[Richie_asg1]

Originally Posted by Bobdee

Do you get the odd flicker of backlights before they fail,
With the symptoms you mention above and the odd flicker of lights before failure sugest that its a faulty CCFL
They are worthy of a check to see if this is the problem as well as resoldering transformers

Never come across your sugestion of limiting resistors being the problem
These Benq boards are in Benqs + HPs + Dells etc I have done many ( 2 seconds to black problems ) as they are called
standard fix = replace faulty C5707s + pico fuse + resolder transformers, and check and resolder any other bad solder joints if that dont repair it then its usualy bad CCFLs

No flicker - just bright for a second or two - then blackout.
I'll stick with replacing the last two 1000uf 10v caps I didn't have last time, testing those FU9024N transistors and resoldering the transformers complete.
Will also check the CCFL's when I read how to do that.

Will report back how things go. Thanks for your help again. Great forum - great people.

[avsubvert]

Originally Posted by avsubvert

Hello everyone,


I have a faulty HP 1940 with a Benq 48.LIG02.A01 inverter board. It has the fault where it the screen goes black after a few seconds, after putting a flash light to it I could see that the image was still there. i Opened it up and inspected the inverter board I could not see any faulty caps and the soldering looks good to me (I'm a novice) so after reading through the posts here I decided to pull the four C5707 transistors off the board and test them with my DMM, as I have read here that these transistors are a common fault with this board. Because I have little experience testing transistors I was hoping for some conformation from the experts (you)


I pulled 4 transistors off the board and kept them separated in pairs as I'm told the fail in pairs.


The first pair gave me the following results (all tested with the diode function on my DMM)


Transistor 1 (first pair)


B-E .575 v
B-C .575v
C-E Open
E-C Open


Transistor 2 (first pair)
Same as above


Transistor 3 (second pair)
B-E 0.00v (it did not read 0L )
B-C .349v
C-E .349v
E-C .360v


Transistor 4 (second pair)
B-E 0.00v (it did not read 0L )
B-C .444v
C-E .478v
E-C OPEN


it seems to me that the second pair is faulty I was wondering if anyone can confirm this? I have found a place on ebay where I can order some new C5707s. Also As a precaution I have touched up the soldering on all the transformers in case this was the cause of the fault. Anything else I should check?


Thanks heaps for all the great tips I found here I would never have tried this without the advice I have found on this site.

I have also posted this on fixya hope you dont mind me posting this in more then one place.

Mike

Hello, sorry I have not kept you updated with my progress. Thanks for your replies. I got called into another project so I had to put the monitor repair on hold for a while.

So after reading your replies about the transistors I order in some from farnell and installed them into the board. Touched up the solder on the transistors and checked the fuse. I replaced all 4 c5707's just to be sure. After turning on the monitor I was delighted to see that the back lights now comes on and stays on.

However when I connected a computer to the VGA input the monitor did not detect the signal. After turning it on the monitor gives the message after about 30 seconds: Monitor going to sleep and goes into standby. When I reset my computer the monitor came of standby for a second and displayed the monitor going to sleep message again... no image


is there something I could have done to damage the board as part of my repairs? How would I go about diagnosing this issue?


Thanks for all your help


Mike

[PCBONEZ]

Originally Posted by Newbie2

If you can't get the original 2SC5707 transistors

http://cgi.ebay.com/ws/eBayISAPI.dll...m=320310321434

http://cgi.ebay.com/ws/eBayISAPI.dll...m=160359156562

[Geordieboy]

If you find what model Benq your inverter board is from and go here:- http://www.elbase.ru/app/webroot/files/cd/monitor/BENQ/ you may be able to find something to get you a stage further. There's a wealth of technical knowledge for loads of monitors on the site.

[MarcoPau]

Hello guys, my BenQ stopped working (lights won't start) the other day after doing a bit of on and off lasting a few seconds, then just dark. I stripped the board and found a pair of the C5707 conducting C-E, thus I replaced them with a pair of TIP3055 together with the 3A picofuse which wasn't working any more as well.

Turned on the monitor and seemed it was working, displaying BenQ's screen and then the motherboard boot after turning on the computer. Only a few seconds and it turned off, then on again and so on till I turned everything off before going back again to the previous stage ;-)

Do you have any hint? I only have a simple multimeter to test the components.

Thank you very much for your help.

[PlainBill]

Originally Posted by MarcoPau

Hello guys, my BenQ stopped working (lights won't start) the other day after doing a bit of on and off lasting a few seconds, then just dark. I stripped the board and found a pair of the C5707 conducting C-E, thus I replaced them with a pair of TIP3055 together with the 3A picofuse which wasn't working any more as well.

Turned on the monitor and seemed it was working, displaying BenQ's screen and then the motherboard boot after turning on the computer. Only a few seconds and it turned off, then on again and so on till I turned everything off before going back again to the previous stage ;-)

Do you have any hint? I only have a simple multimeter to test the components.

Thank you very much for your help.

The common problem with this inverter design is cracked solder joints around the the transformers. Since the TIP3055 has greater current capacity than the 2SC5707, you are probably overloading the power supply. I suggest resoldering every pin of both inverter transformers, then checking the transistors for shorts. If you STILL have the problem replace the transistors with good quality 2SC5707 (C5707).

PlainBill

[Bobdee]

Originally Posted by MarcoPau

Hello guys, my BenQ stopped working (lights won't start) the other day after doing a bit of on and off lasting a few seconds, then just dark. I stripped the board and found a pair of the C5707 conducting C-E, thus I replaced them with a pair of TIP3055 together with the 3A picofuse which wasn't working any more as well.

Turned on the monitor and seemed it was working, displaying BenQ's screen and then the motherboard boot after turning on the computer. Only a few seconds and it turned off, then on again and so on till I turned everything off before going back again to the previous stage ;-)

Do you have any hint? I only have a simple multimeter to test the components.

Thank you very much for your help.

I am not sure if this is the cause of your problem but as you did not replace with C5707s (that is considered the best for the job) with new C5707s, then you may need to replace the other two so as to have four TIP3055s to balance circuit(as TIP3055s are slower) Better still four C5707s as they are easily available on Ebay
Also did you resolder the four inverter Transformers, if not do so or you may end up with new failures in transistors and fuse

[hotdig]

An interesting article on the Benq C5707 transistor problems.

http://www.electronicrepairguide.com...or-repair.html