Fix for Benq FP737s 17" (Q7T3)

[jtan]

Re: Fix for Benq FP737s 17" (Q7T3)

i'm trying to find where the fuse is located in the board....

[Bobdee]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello Jtan
On the second link in my post you will find this page

https://www.badcaps.net/forum/showpo...58&postcount=3

If this is your board, you will see a black cylindrical fuse marked BEL T2A, on the top, with the board No F601 next to the power connector No CN601,

The other fuse looks similar to a resistor only its black also, and is found by the the right side of large transformer,just above the blue capacitor, it has Bel 4A on it, and board mark of PF703,
bob

[dorofteius]

Re: Fix for Benq FP737s 17" (Q7T3)

I have a problem.
Ifound 2 c5707 blown up, the fuse and the two mosfets were ok.
So I replaced them, resolder the board, and now it comes-up for 2 seconds then turns of for 2 seconds and comes again for 2 sec, and finaly the lite goes off for good.
The led stays green.
Any ideea?

[Bobdee]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello and welcome dorofteius
As no tech guys have come in, I may be able to help, as I have done a fair few of these Benq boards, as you have covered the C5707 + fuse + resoldering, they are the norm fixes in my repairs of these boards
Did you check resistance across the PF fuse ??? there should be little or none.
Other than that you will have to suspect bad inverter transformer, or bad CCFLs tube, or a possible dried out cap on inverter side, here is a good Link to a dell Monitor that uses a Benq board like yours that gives a very good process of elimination to your problem that you can use to your advantage

https://www.badcaps.net/forum/showpo...&postcount=296

should do it for you, hope so, good luck

bob

[dorofteius]

Re: Fix for Benq FP737s 17" (Q7T3)

10x for the reply. In my case, I just don't know what seems to be the problem. I have canceld the surveilance of the IC , and now it seems to be working. None of the c5707 is heating up... All the four lamps are lighting.
The monitor is working for almowst 30 hours now whitout a stop.

[Bobdee]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello dorofteius,
Good to hear that you are up and running again, as for your last post,

( Re: Fix for Benq FP737s 17" (Q7T3)
I just don't know what seems to be the problem. I have canceld the surveilance of the IC )

That seems to be one for DAVMAX or one of the other techies,hope they come in for you, I will follow your problem with intrest, best of luck
bob

[dorofteius]

Re: Fix for Benq FP737s 17" (Q7T3)

Well my benq is still up and running since a few day's. Thanx to wonderful forum for wonderful informations.
B R.

[anyaddress]

Re: Fix for Benq FP737s 17" (Q7T3)

another solution for this monitor to work fine for people who work for them for 2 or 3 second:

my monitor was not completely dead ,the front led was on(green) but no screen,after testing the board i found the following:
1-the fuse is open,shorted c5707 transistor,burned capacitor(.22micro,250v),shorted fqu11p06
i replaced capacitor,fuse,c5707(replaced both) with original parts number but i didn't find fqu11p06(also re soldered all the inverters and that's so important),after a little search i conclude that i can use irf9z34 as a replacement,now the screen is on for 3 second then off(as happened with manu users),first i think that the c5707 i replaced was not good,after testing the switching time for the inverters i noticed that the one is starting faster than from the other and this causes the power supply to inverters to be down,the solution as so simply,i changed the other non defective fqu11p06 with another irf9z34 and by this way the switching time for both inverters is the same 100% and the monitor is working just fine (the two irf9z34 transistors temp. was not to high after working for 10hours but for extra protection i put a small aluminum cooler for each transistor

i wish that help

[Bigboy101011]

Re: Fix for Benq FP737s 17" (Q7T3)

Would it be possible to just replace the 2SC5707 with a more robust part?

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

The 2SC5707 is a robust transistor. It has excellent performance and ratings, it is ideal for the job. Just make sure new ones are used, do not install any used ones. Any transistor will fail if heavily stressed as happens under fault conditions. Unless the supreme effort is made to provide a fast protection circuit as I have earlier in this thread. This protects the transistors for all fault conditions. The original design does not have good fault protection.

[shelby5041]

Re: Fix for Benq FP737s 17" (Q7T3)

hi i also have a benq monitor T705, ie q73t

same issue no backlight ...after troubleshotting found 1 bad cap...2 bad transistors and pico fuse..reaplced all of them and the trasnistors keep blowing as well as the pico.......can you maybe assist me??

and also is there a way to test caps in board??

thanks

[Bobdee]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello Shelby,welcome,
You dont say whether the the monitor worked for a bit or just went down again as soon as you powered up with new components,
Did you resolder the 4 inverter transformers,this is a MUST when you have transistors go down on these boards,check all solder joints on the board to be on the safe side, look for ring cracks, they not easy to see, use 8x magnification glass,at the least.
You may have one of the transformers burnt out,
You can check capacitors on board with ESR meter if you can put your hands on one,ESR will give you a good indication as to caps condition.
Can you post pictures of top and bottom of the board, this will help members to help you
bob

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Bobdee,

You are doing a great job helping out others in trouble. In general there are enough posted references to fix the FP737s in this thread. Basically this monitor has poor fault protection. The 3 amp fuse is not sufficient protection.

It is difficult to determine which of several failure modes causes the C5707s to blow. That is why I decided to create a fast protection circuit that would cover all overload current conditions. This circuit then enabled me to actually find out when fualts occurred because it is a fault latch ie if a fault current occurrs the monitor power or the video signal must be terminated to reset the latched condition.

My Monitor and others fixed this way have not missed a beat since the protection was installed.

Yes resoldering transformers is one proven method of fixing failure, but only that source of failure.

Reading some posts I am not sure if it was recommended to replace the 3A fuse with a 4A. I hope not, this will make the already poor protection worse.

I am not actively involved in the forum these days and therefore pay a visit rarely.

Keep up the good work.

[sch72]

Re: Fix for Benq FP737s 17" (Q7T3)

Bobdee,

You are doing a great job helping out others in trouble. In general there are enough posted references to fix the FP737s in this thread. Basically this monitor has poor fault protection. The 3 amp fuse is not sufficient protection.

It is difficult to determine which of several failure modes causes the C5707s to blow. That is why I decided to create a fast protection circuit that would cover all overload current conditions. This circuit then enabled me to actually find out when fualts occurred because it is a fault latch ie if a fault current occurrs the monitor power or the video signal must be terminated to reset the latched condition.

My Monitor and others fixed this way have not missed a beat since the protection was installed.

Yes resoldering transformers is one proven method of fixing failure, but only that source of failure.

Reading some posts I am not sure if it was recommended to replace the 3A fuse with a 4A. I hope not, this will make the already poor protection worse.

I am not actively involved in the forum these days and therefore pay a visit rarely.

Keep up the good work.

I am new to this forum and found it searching for information on the dead BenQ Q7T3 that came in to a local charity that my wife and I volunteer for a couple of times per week. I retired after forty years as an electronics tech. so get a lot of computers and high tech stuff to either refurbish for them or haul to the recycling center. We live just outside of Edmonton, Alberta, Canada.

The monitor was bound for recycling so I decided to see if it could be repaired for use as a test monitor with a smaller footprint than the CRT monitor curently on my work bench.
A web search for Benq Q7T3 repair brought up this forum.

Previously in my career I designed an built switchmode power supplies up to two kilowatts so was quite impressed with all of davmax's work on this monitor power supply.

Ok, the trouble with this monitor was a shorted Q759 (2SC5707) and blown fuse so I bought six Sanyo replacements from Rich Tech in Richmond, B.C.. Around $1.00 Cdn per transistor but shipping by express post adds about $12.00 to each order.

First attempt (Lazy!): Replaced Q759 and replaced the fuse with an in-line fuse holder and 5a fuse, the smallest in my junk box.

The monitor flashed the BenQ logo for a few seconds, then went blank. Tried re starting a couple of times then got the unmistakeable odour of something overheating. Q759 and the fuse blown again and 5 amps may have smoked one of the inverter transformers.

Second attempt (Lets get serious)

Perused davmax's notes and decided to get serious even though the quality of the circuit board in this thing doesn't inspire confidence. Replaced Q759 and Q760 and resoldered all the suspect areas. Found FET Q751 also shorted so removed that.

Hooked the inverter outputs to the CCFL's and ran voltage from an old computer power supply to the cathhode of D751 first then to D751 and D761. One inverter drew 1.15 amps at 12 volts in, half the display lit up. Two drew a total of 1.6 amps and lit the whole display so at least the inverter transformers are ok. It looks as if this thing might be worth ordering more parts for and installing davmax's current limiter.

Third attempt (Pipe dream or maybe..)

In the tradition started by the kid who took the cuckoo clock apart to see what the little bird ate I decided to disassemble the display to check the feasibilty of replacing the ccfl's with white L.E.D.'s . Space there is quite limited, also there are a number of plastic sheets that may not have been reassembled in the correct sequence so if anyone has data on that I would appreciate it.

More web searching:

FWIW the led thing has been done at least once on another brand of monitor.
Apple are currently making a big to do about the Macbook not containing any Mercury so they most likely use LED backlighting and Samsung are also using it now.

Low cost case lights also have worked for some people and the case light come with their own inverters.

One of the more amusing repairs was the one that consisted of removing the innards fron an old CRT monitor, installing the flat panel display in the front with the display back off, and using a light bulb inside the monitor case as a backlight. Sort of redneck but it workd for someone.

For now its time to order some more parts and that current limiter will definitely go into this thing.

[dariusdwtt]

Re: Fix for Benq FP737s 17" (Q7T3)

bump

[dariusdwtt]

Re: Fix for Benq FP737s 17" (Q7T3)

Hey, just popping in to say thanks to all of you for your help

I am sending this from my "new" benq fp737s.

A buddy's sister had this monitor and it bombed out, so he went to have a look and concluded it was hardware related, so she gave it to him and he gave it to me saying "it's yours you're just gonna have to fix it...".

Found you guys and voila!

Mine was a simple inverter transformer resolder, but for the sake of longevity I'll be recapping (they look pretty bad) and maybe replacing transistors. If I have the time I'll add Davmax's protection circuit, coz in my area we do get the odd power surge (assuming that it will help for that?)

What a score! The fuse wasn't even blown! lol

Easier than fixing my toaster...

XD Thanx!

[FIXITNOW]

Re: Fix for Benq FP737s 17" (Q7T3)

drive tip mod found on web
comments ?

[FIXITNOW]

Re: Fix for Benq FP737s 17" (Q7T3)

psu and inverter circuits

[Toasty]

Re: Fix for Benq FP737s 17" (Q7T3)

@FIXITNOW -

Those are davmax's who started this topic. See the first 2 pages of this thread.

[cliffpage]

Re: Fix for Benq FP737s 17" (Q7T3)

i have one of these monitors (benq q7t3)with the usual problem. I have not tried repairing monitors before, but i have recapped loads of dell motherboards (gx270) so i know what i am doing with a soldering iron.
Please answer this
- exactly how do i test the transistors to see if faulty?

Do the transistors have to be removed from the circuit board to test them?

Are all transistors tested the same way or are different methods / readings needed for different types.

I note the fuse is 125v 3amp. is the voltage actually important on a fuse. I thought from my physics lessons at school (over 30 yrs ago) that all that mattered on a fuse is the current (amp). would it make any difference if i used a 250v 3a fuse?

i look forward to your advice please