Fix for Benq FP737s 17" (Q7T3)

[agentsmithitaly]

Re: Fix for Benq FP737s 17" (Q7T3)

Krankshaft thank you for the reply!

I'm sorry if I haven't posted yet but the guys i bought the 2SC5705 shipped me the SMD version, so I had to wait they restock through hole version and sent me again.
Well I replaced all the 5705's to be sure and the 3A fuse. The monitor worked for four days, I was so excited, then it faulted again.

This time the CCFLs stay on only for half a second then they turn off. In this half second you can see the BenQ logo fading.
Then the power led is still green but you can see nothing on the LCD, even with strong light pointed at it. Before repairing attempt I could see the picture with this solution.

It seems that 5705 failed again (but no blown fuse) or they are ok and something else is faulty.
What can I do? I could replace the small 945 and 733 transistor, but getting new mosfets or DQ10DPF could be too much expensive and not worthing the waste of time...

[agentsmithitaly]

Re: Fix for Benq FP737s 17" (Q7T3)

I forgot to say there is no bulging capacitors, and solder joints seem fine at my inexperienced eyes. How can I test the capacitors?

[Newbie2]

Re: Fix for Benq FP737s 17" (Q7T3)

I forgot to say there is no bulging capacitors, and solder joints seem fine at my inexperienced eyes. How can I test the capacitors?

The quick blink seems to suggest a blown fuse as I did see that happen when I ran a BenQ monitor with good transistors but with a bad 3A fuse.

A "slow blow" fuse is recommended for replacement of the small 3A fuse, if you are not using the "bel MS3" 3A fuse (the original fuse) you can order them off Digikey (Digikey part number: 507-1034-ND).

Check solder joints and replace the fuse and transistors and see what happens.

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

It is not a good idea to have a 3A slow blow fuse "not recommended if it is to protect the transistors". A fast fuse may save the transistors.

agentsmithitaly

I suspect blown transistors again.
1. Make sure that all the inverter transformer connections are resoldered
2. Use a fast 3A fuse
3. Check the MOSFET in each circuit Q751 & Q743 (FU9024N) they should not read a short circuit.

http://www.lcdrepair.us/fu9024n.html

See my experiences earlier in the thread. These transistors are not well protected by the circuit design. I modified the circuit and the monitor is very well protected.

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Further note on fuses.


My FP737s is working long term on a 3.15A quick blow fuse part 226-0787 from RS Electronic Components. Wickmann part 3951315000

[agentsmithitaly]

Re: Fix for Benq FP737s 17" (Q7T3)

Thank you guys for the suggestions. I'll check them out and let you know.

If I understood the schematics correctly (btw i'm just a hobbyist), the 3A fuse provide current to power the inverter section, so blown fuse = no light, not even for half a second. am I right?


Please notice that I never rebooted the pc in these four days before failing, just stand-by (yellow led) since i read the Davmax investigation about the BLT_ON issue, unfortunately the workaround is too complicated for my electronic skills to implement

[Newbie2]

Re: Fix for Benq FP737s 17" (Q7T3)

Thank you guys for the suggestions. I'll check them out and let you know.

If I understood the schematics correctly (btw i'm just a hobbyist), the 3A fuse provide current to power the inverter section, so blown fuse = no light, not even for half a second. am I right?


Please notice that I never rebooted the pc in these four days before failing, just stand-by (yellow led) since i read the Davmax investigation about the BLT_ON issue, unfortunately the workaround is too complicated for my electronic skills to implement

If the 3A fuse is blown but the transistors are fine then the monitor will turn on with a very quick blink on the LCD panel. I don't understand schematics either.

Davmax's modification does not need to be put in, my two FP731 monitors are working fine without it. My electrical enginner buddy thinks that davmax's modification actually makes the design worse than its original configuration, and I heard that he said he has good reason for that claim.

[robd]

Re: Fix for Benq FP737s 17" (Q7T3)

I got one with the same problems, resoldered all trans and transformers and now i get a slow blink on the green LED any ideas is there a refurb board out there for this thing/

RD

[robd]

Re: Fix for Benq FP737s 17" (Q7T3)

Discard the top message and this is whats it still doing

I got one power led lights up for one second then off and wont come back on

RD

[agentsmithitaly]

Re: Fix for Benq FP737s 17" (Q7T3)

If the 3A fuse is blown but the transistors are fine then the monitor will turn on with a very quick blink on the LCD panel. I don't understand schematics either.

X Davmax:
1. Make sure that all the inverter transformer connections are resoldered
these connections seem fine to naked eye. I don't have a professional solder neither excellent soldering skills, i suppose i could worse the problem
2. Use a fast 3A fuse
Ok, I'll try to find it in a repair shop
3. Check the MOSFET in each circuit Q751 & Q743 (FU9024N) they should not read a short circuit.
I have ST D10PF06, I suppose they are equivalent. Could you provide a link to how to test with diode mode? I have to desolder them to test, right?


Thanks to everybody for all the suggestons


Hi Newbie,
I checked the 3A fuse and it isn't blown; I looked also the schematics and if the fuse was blown, there won't be current to the inverter, and no light.

However I tried to connect one lamp at a time and found that only one inverter it's working; the other one probably have a problem and triggers TL1451 protection mode.
But could be just another faulty transistor? the bad news is to get replacement parts from farnell or RS components would cost a lot (considering the price of new lcd monitor) and you aren't sure to fix it...

[bassbindevil]

Re: Fix for Benq FP737s 17" (Q7T3)

I suspect the failures are caused or at least aggravated by heat. On my FP731s, there aren't enough openings at the bottom of the metal shroud to allow cool air to enter.

I'm going to grab the Unibit and cut some bigger holes on the lower left and right of the shroud, next to the openings in the plastic outer cover, and along the bottom. That should improve air flow.

Only one of the four transistors was bad, but I've ordered 8 anyway, in case I can score another one of these.

[HOODLAM]

Re: Fix for Benq FP737s 17" (Q7T3)

Hi Guys.
Just a followup after sevral months, my HP1955 is still on the go. I bought a spare set of 9024's which I will keep spare for just incase something happenes again.

One thing i found strange was the original fuse was a 3amp and the supply shop suggested i repalce with a 2amp cause it had to do with the voltage supply(220V), i installed the fuse about 7 months ago and still running!

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Hi Guys.
Just a followup after sevral months, my HP1955 is still on the go. I bought a spare set of 9024's which I will keep spare for just incase something happenes again.

One thing i found strange was the original fuse was a 3amp and the supply shop suggested i repalce with a 2amp cause it had to do with the voltage supply(220V), i installed the fuse about 7 months ago and still running!

The shop guy gave you bad advice. The amp rating has nothing to do with the supply voltage of 220V. The fuse circuit operates on 15 volts DC irrespective of the supply. Lots of misinformation gets out from shop/counter guys.

[Cas123]

Re: Fix for Benq FP737s 17" (Q7T3)

Hi,
Just joined this forum. I have a Q7T3 that I inherited. Appears to have blown the 2 2SC5707 transistors as per this thread. Does anyone have any spare transistors they'd like to sell? Also the rear case mounting screws are missing. If anyone has a Q7t3 monitor they have given up on I would appreciate the screws.

Thanks, Ron...

[-LeonM-]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello,

This is my first post on this forum, my name is Léon, and I am an electronics student from Holland.

I got a FP731, which has the same problem as the rest of these LCD's: the 2SC5707 transistors were blown, the LED was constantly flashing (screen kept rebooting). After opening the case, it quickly became clear that someone else already had a try at fixing it, the fuse was shorted (!) and there was some very messy soldering done on some of the transistors.

After replacing all four 2S5707's (I got them from Ebay), the screen worked for about 1 second, it displayed the Benq logo, and then white stripes appeared on the screen and it died. Once again one of the 5707's was blown.

So, after some investigation (I had not found these topics on this forum yet) I found out that:

1. It does not matter if the CCFL's are connected or not, the transistors wil blow anyway.
2. The transformers on my PCB are NOT shorted (desoldered every one of them and tested them with a signal generator and oscilloscope)
3. Just one of the two inverters keeps destroying transistors, the other one works just fine.
4. The two inverters seem to work parallel, because after removing the broken transistors form the inverter that kept failing, the screen actually worked!! As far as I could judge it had a normal brightness and the whole screen was lit (not just the upper/lower side).

After about 5 seconds, the backlight is turned off, probably because the single inverter gets overloaded, the image is still visible, the screen does not go on stand-by mode, it just turns the backlight off.


So, offcouse the question is: what could cause these transistors to die immediatly after the power is applied? I've checked all other transistors on the board, they all seem to be fine, the diodes are all good and there are no visible broken/shorted traces.

Does anyone have a suggestion where to look next?

[-LeonM-]

Re: Fix for Benq FP737s 17" (Q7T3)

update:

Today I continued testen the board, to be 100% sure that the FET's were not faulty I swapped the FET's between the inverters, after powering up the inverter that always worked before still did. So the FET's, transformers and all others transistors besides the 5707's were actually fine.

After desolering, checking and resoldering almost every component from the faulty inverter I still had no idea what the problem could be. I decided to give the monitor just one more chance to live before I would bring it to the scrapyard... I inserted the last pair of 5707's I had, powered it up and.... IT WORKS!

I really don't have any idea what the problem was, it's now standing here on my desk next to my LG L203WT and it's working fine for over 2 hours now. Maybe I will build DavMax's circuit in someday, if it decides to die again

For now, it's time to repair the next TFT: A Philips 170C...

[bassbindevil]

Re: Fix for Benq FP737s 17" (Q7T3)

An update... I got the transistors and a fuse from a guy on eBay, stuck four new ones in along with the fuse, crossed my fingers, and plugged it in. It worked. Before buttoning it back up I checked one of the power supply caps, and it measured OK.

I've been using the monitor for a while now, and it's still working. I'm starting to think these LCD monitors just might catch on... it's a lot smaller than my old 17" CRT, and even though it's not running at the native resolution, it is perfectly legible.

Since getting this monitor, I also picked up a Viewsonic VX922... that one had bulging caps, which measured bad. I'm waiting for a DVI cable to put it into service, but it worked fine when I powered it up after changing the bad caps. I cut more ventilation holes in the shroud of that one too.

[gaztoo]

Re: Fix for Benq FP737s 17" (Q7T3)

G'Day Folks.

Thanks to Davmax for initiating this thread and the fine work that he has done on the problem. I got one of these FP373's to repair and the problem was just the same as most others have found. One s/c 2SC5707 and o/c FP751 3A fuse.
After sourcing the replacement parts all was well again. I did however, find many "dry" solder joints on the Inverter transformers and on other parts. I wonder if these could precipitate the problem.

I am from Australia and if it will help, I found the following local sources for the 2SC5707s. They are rather special and no substitutes should be used.

1 Wagner Electronics (Sydney) $1.75ea
2 Farnell Electonics $2.12ea (currently out of stock)
3 The Go Company (Brisbane) $7.95 ea (go figure)

[caddyroyalty]

Re: Fix for Benq FP737s 17" (Q7T3)

I just wanted to thank everybody here. I found a Benq FP731 in the recycle bin of a company and changed the 4 transistors and fuse and that took care of it. I'm using it now! I took pics and put them on Flickr, http://flickr.com/photos/caddyroyalt...7606746620272/

[electron]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello,

I would like some help to fix my LCD. I am not entirely sure, but it looks like the Q7T3, silver and black bezel. I dont have the bezel anymore. So I have a couple of questions and requests.

1. If my screen is 17 inch, silver and black, does that narrow it down to a Q7T3, my screen looked like this

Picture of my screen

2. I see some different numbers and serial numbers, on the screen itself, internal bezel etc, but couldnt find any reference to them online. Can anyone help? Or no need to, since if my screen looks like the one above, it is a Q7T3?

3. Most importantly and most appreciated, can anyone help me diagnoss, and then fix my screen?

I can take pictures of the board as well.

Whats broken: The screens backlights do not turn on. I really dont think the lamps are broken. I checked the LCD is still working, just no lights.

Green power light turns to amber after a few seconds.

voltmeter is getting a 1. volt reading from the power outputs to the lamps.

Thanks so much, if you would really like to help please email me!