Fix for Benq FP737s 17" (Q7T3)

[bizu]

Re: Fix for Benq FP737s 17" (Q7T3)

Thanks to Davmax for initiating this thread.
I have a Benq FP731 and the problem was just the same as most others have found.
After replacement 4x2SC5707 2xFU9024 and the fuse 3A all was well again. I resolder joints on the inverter transformers.

[epiman46]

Re: Fix for Benq FP737s 17" (Q7T3)

First, a big thankyou to all on this thread. I had two BENQ 737s that died, more or less simultaneously. I replaced the SC5707s, and the 3A fuse, and they both worked. Yay! For not quite a week. Then they both died.

One will run for about 5 minutes, fade (get dim) and then croak. The image is visible under inspection with a strong light. Giving it a 10-minute rest will give me about 5 minutes of usable function. The other, just flashed, then died. Again, the image is visible on strong illumination.

So replacing a few transistors has not solved the problem. I am not interested in going that route again, unless I have a solution that addresses the problem of why they failed in the first place. I have a few questions:
- Which is the better candidate for repair?
- has anyone else had any success with Davmax's current-limiting protection circuit?
- should I be recapping, on general principles? I don't see any bulges, or discharge. All the solder joints looked good.

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Did you resolder all the inverter transformer connections? It sounds as thoughthe 5707s are gone again. There are claims that the resoldering will enable a permanent fix. However as you have seen I have implemented the current limit system on three monitors with no further failures. This method catches all heavy overcurrents FAST (10usec).

[ozcar]

Re: Fix for Benq FP737s 17" (Q7T3)

I would just like to thank all of you who contributed to this thread. When my FP737s died last week ago I was about ready to give up on it and get a new monitor. Then I found this thread, and one 2SC5707 and one fuse later I am back in business. I resoldered the transformers too - not that I could see any problem there.

With a bit of luck maybe it will keep going for another few years. If it dies again I might consider the suggested circuit changes.

[msubzwari]

Re: Fix for Benq FP737s 17" (Q7T3)

Greetings from Karachi, Pakistan. This thread is a treasure trove for Benq affectees.

I have a 5 years old Benq 17 incher. Model is FP767-12. It gave up on me couple of days ago. While searching on the net for repair/service centre, I stumbled into these forums & boy am I glad that I did. The board/circuit is the same as being discussed/shown in photographs posted in this thread.

To cut the long story short, the 3A fuse & one of the 2SC5707 were busted in my board. I could not find 2SC5707 in the market but one shop had 2SC5706. It has slightly lower Collector & Base Current max ratings compared to 2SC5707 but rest of the specs were a close match. So decided to go for it instead of going the TIP3055 way.

I replaced one pair but the lamps did not work. Thinking the C5706 are busted, I took them out & tested. They were Ok.

Checked Hfe value of 5706 and it was around 320. Checked the working 5707 and it has Hfe of 380. I bought some extra C5706 and two of those gave me Hfe reading of 400 & 395. So removed the ones I soldered earlier and installed the higher Hfe rating C5706 pair. That brought the life back to the display.

Packed it up, connected to PC, turned it on.....nothing. Opened it up again, checked all of my soldered components which seemed fine. Double checked any problems in the caps, nothing obvious observed.

Decided to resolder anyway & while I was at it, resolderd all transformer legs. Powered it up and it was alive again. Packed it up again & it has been working well since then.

I just turned it off after being used for about an hour..so far its holding.

My sincere thanks to all the contributors to this thread for sharing their valuable insight into this very interesting problem.

[enterthehatrix]

Re: Fix for Benq FP737s 17" (Q7T3)

Greetings from the UK.

Our work office is loaded full of these benq monitors (all Q7T3). Needless to say they'll all eventually fall to this problem as we've had 2 die already out of 14 identical models.

Anywho, I was wondering if anyone could clarify exactly what I should replace in order to get it working (identifier code on board, and part name / code) and any basic and simple preventative measures I could take to stop it happening in the future, if not stop it happening quickly again in the future.

The reason I ask is that the information I've read here conflicts with some information on another site. However, I trust the judgement from this forum more since there are several of you vs the writer of the article I read.

[enterthehatrix]

Re: Fix for Benq FP737s 17" (Q7T3)

I replaced the 3a fuse, and it instantly blew up along with the SC5707's.

Smoke was pouring out of it and it stank the house out.

my guess is that I've blown a bit more than the SC5707, but there are burn marks only around the SC5707s.

I've ordered replacements they should turn up in a few days. Anything else need replacing besides SC5707 and the fuse?

[dvorav]

Re: Fix for Benq FP737s 17" (Q7T3)

Hi there,
I have two questions: (possibly for Davmax, but of course, everyone's answers are welcomed)

1) Have you replaced all/some of Elite Caps? Or did you simply leaved as it is?

2) Add to the overcurrent protection - have you replaced the C770 by polyester 1uF one? I did so, but then protective circuit didn't work, monitor not as well. (Probably I have done some mistakes, but I'm not sure.) After I removed main part of the circuit, monitor went on. (It's not a pleasant look on the cut and bridged-again tracks, but anyway...)

Thanks for suggestions.

[dvorav]

Re: Fix for Benq FP737s 17" (Q7T3)

TO: enterthehatrix

Sometimes the IRFU9024N are needed to be replaced. Try to short-circuit-test them.

And possible all caps, but I have questioned that in previous reply.

[Jacknewboy]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello Badcaps Forums.

Thank you for the birthday wishes. They have prompted me to report on my activities with my Benq FP737s-D 17” LCD monitor.

The monitor was about two months out of the warranty period when the backlight failed. This was diagnosed because computer Control Panel – Display program still recognised the monitor's existence and, with a torch at a suitable angle and a high contrast picture, the image was identifiable. Without a service manual, I cautiously dismantled the display to look for any obvious problems. The most obvious was that it would not be easy/possible to run the monitor in a dismantled state to make measurements. This would be even more difficult because it would be necessary to connect the monitor to a computer for it to be activated (come out of standby mode).

At that time I did a search of the internet looking for a source of a service manual or at least a circuit and stumbled onto Badcaps Forums. With the information that the most likely cause of the backlight failure was the two FETs (2SC5707) of the high voltage inverter, I inspect a couple of catalogues and made a couple of phone calls and found the FETs in stock at The Go Company in Brisbane for a total cost of $3.30 each. (Incidentally, they had had to replace the same components in a Dell monitor on there service counter). At that price, I bought two to install and two more in case another problem had caused the initial failure.

At that time I had not registered with Badcaps and so had not been able to download the Forums' diagrams and photos. After a static check of the components around the area of the inverter, I installed the FETs. I had checked the main fuse but did not recognise the existence of the mini fuse. Having registered and downloaded the Forum attachments, I found that the simplest way to replace the fuse was to fit an M205 PCB fuse mount – with slightly extended leads – placed strategically between other components. The unit was assembled and worked correctly and I did not think it worth dismantling again to take a photo of the fuse location.

The monitor has now been working many hours almost every day, for between one and two months. It is used as the second monitor on a note book computer which rarely moves from the desk top. As a precaution, I have developed the practice of not pushing the power button on the monitor until Windows is in full operation, and turning the power off after the computer has removed the video signal and the display has gone to standby. At this time I do not intend to include other suggested modifications.

Thank you for the information contained in the discussions on your Forum. It has certainly saved me considerable time investigating the monitor failure.

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Hi Jacknewboy.


Good to see you were successful. A few items for future reference.
1. This is a good source for all the components you need: http://cgi.ebay.com.au/ws/eBayISAPI....m=320301193969 I see that this auction keeps renewing.
2. The 2SC5707 is a bipolar transistor. The black ones in the link above and the MOSFETs are the blue ones.

Hope you resoldered all the transformer connections on the board.

[epiman46]

Re: Fix for Benq FP737s 17" (Q7T3)

The latest news: One of my FP737s seems to have recovered after having its transformer leads resoldered. This is the machine that would run for about 5 minutes, then fade out. It has now been running for a day or so without any problems. I am very hopeful about the other that may need a some 5705s replaced. I'll do the transformer leads for that one at the same time.

[enterthehatrix]

Re: Fix for Benq FP737s 17" (Q7T3)

I've got the one to work (oddly) and the other doesn't.

The odd working one has backlight lit up even when the power is off, which means I have to leave it on to turn the backlight off.

The other doesn't light up at all.

I'm not willing to spend any more money on these than I already have, so I might make a projector out of the one that doesn't turn on.

Having said that, the smaller PCB with the VGA connection connects to the power PCB. Could you get the pin out for that for me?

[Wragie]

Re: Fix for Benq FP737s 17" (Q7T3)

Just to add another data point and something else to look for.

I have a Q7T3 manufactured in June 2004. (Price was free to good home)
On mine it exhibited sort of what everyone has described as the inverter section having problems. On mine I always had a display but it would white out or grey out. And I noticed that a tap on the side of the monitor would sometimes clear this up or make it better.

I took it apart when this got annoying enough and didn't see anything obvious. I soldered the transformers as they had stress cracks around the pins. Anyone know when Benq went to low lead or lead free solder? This looks like lead free done using lead temps etc. Anyway this fixed it for about a week and it started acting up again. My wife said she caught a wiff of hot/burnt electronics. So I took it apart to see what popped. NADA, no smells on parts.So this time I went through and soldered a few more things including of the caps. Plug it back in and I have a solid green power light but no image.

This time I took it apart and checked just what I had soldered. One capacitor C710 hadn't looked like it was soldered right so I had cleaned off a bit of solder and soldered it again. So I took it out and checked it. It was well within specs. However looking at the board I see that the pad looked like it was lifting but the trace looked ok. Thought I had cooked it or it was part of the poor quality a few people had mentioned. Put the cap back in, soldered it, no continuity. So I solder on a jumper put everything back together and the monitor is now working again flawlessly.

The trace and pad must have just had enough connection to work. Judging by the tapping having an effect on it I would say a very slight contact connection. When I soldered I must have moved it or removed what was making the connection. This one is hard to spot and I only saw it (about the third time) as I was using a magnifier. The pad and trace look that normal.

Dave

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Yes Wragie you have described the poor quality of the PCB of the lamp inverter circuit. Certainly great care is required when reworking this board.

[enterthehatrix]

Re: Fix for Benq FP737s 17" (Q7T3)

Another one bit the dust today at work, so I'll have to give this a shot using parts from the failed one. The parts still work, my guess is the fault it elsewhere.

[enterthehatrix]

Re: Fix for Benq FP737s 17" (Q7T3)

Oh, and I can't find a supplier for IRFU9024N in the UK. Any suggestions?

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Try these.

http://cgi.ebay.com.au/LOT-OF-TWO-2-...3286.m63.l1177

http://cgi.ebay.com.au/KIT-4-PCS-2SC...3286.m63.l1177

[Wragie]

Re: Fix for Benq FP737s 17" (Q7T3)

Yes Wragie you have described the poor quality of the PCB of the lamp inverter circuit. Certainly great care is required when reworking this board.

Just thought I'd reply here as well so if anyone else runs across this.

When I had pulled the cap to check it, thinking it was toast I noticed that just a part of the pad was lifted. What was strange was at this point there was no crack separation or lifting of the pad from the track or the track itself. ie it was just a small portion of the pad. At this point I thought perhaps I had been a bit too hot on the iron (thinking of previous bad board comments). I was looking at this under good magnification so I should have seen anything else. What I did see was that the edge of the pad was actually resting on top of the track. And as I mentioned to Dave off list, my board has a fair amount of hand written marks on it such as using a grease pencil.

What I am wondering is that if Benq was reworking boards and then shipping them out? The overlapped edges show that there was something added or fixed (but not too well). So far since my discovery the monitor is working perfectly. And it is running quite a bit cooler than before which is quite interesting.

Cheers

Dave

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

Please note that the unmarked capacitor in the diagram is a 1uF polycarbonate. This cap can be seen in the top of board view at top right posted on 15th October. It can replace C770 in so doing cut the PCB track effectively to the right of this capacitor as shown on the circuit diagram, not where to cut is XXX.

I read somebody had problems with the mod. make sure you use the latest diagram posted 14th Oct and note the above.