Fix for Benq FP737s 17" (Q7T3)

[mecklan]

Re: Fix for Benq FP737s 17" (Q7T3)

It's teh same problem as found on any number of these monitors. Starting at the beginning of the thread may have given you this information. In your case,check the 2SC5707's. If any are shorted, replace the lot; likewise for the FU9024. Because you are getting the flash, the fuse is good. Also, resolder the transformers; that was the original cause.

NOW it may be that the transistors are fine. Then check the output voltage of the power supply; if good, see if the monitor comes on when you push the power button.

Oh, pictures help. See the FAQ on inserting pictures.

PlainBill

I have replaced the transistors in pairs, the mosfet and fuse. If I turn on the Montor now I get a white screen. I turn on the PC, I have a very normal Windows screen. I turn off the monitor again, the screen is white for a sec.
Do they have a solution for white screen?

Mecklan

[PlainBill]

Re: Fix for Benq FP737s 17" (Q7T3)

I have replaced the transistors in pairs, the mosfet and fuse. If I turn on the Montor now I get a white screen. I turn on the PC, I have a very normal Windows screen. I turn off the monitor again, the screen is white for a sec.
Do they have a solution for white screen?

Mecklan

The problem is you aren't giving explicit information. I'm tired of playing guessing games with incomplete information. Walk through this scenario.

Unplug the monitor's power cord. Hook the VGA cable up to a computer that has been turned off. Plug in the monitor, leave the computer off.

Push the monitor power button until the power LED turns off. Screen white or dark?

Push the power button on the monitor until it turns on. Screen white or dark?

Wait until the power LED on the monitor goes orange. Screen white or dark?

Turn on the computer. Does the monitor come out of standby and display the usual POST information and eventually the desktop?

Shut down the computer. Does the monitor power LED go orange? Is the monitor screen white or dark?

PlainBill

[mecklan]

Re: Fix for Benq FP737s 17" (Q7T3)

The problem is you aren't giving explicit information. I'm tired of playing guessing games with incomplete information. Walk through this scenario.

Unplug the monitor's power cord. Hook the VGA cable up to a computer that has been turned off. Plug in the monitor, leave the computer off.

Push the monitor power button until the power LED turns off. Screen white or dark?

white

Push the power button on the monitor until it turns on. Screen white or dark?

white

Wait until the power LED on the monitor goes orange. Screen white or dark?

white

Turn on the computer. Does the monitor come out of standby and display the usual POST information and eventually the desktop?

yes

Shut down the computer. Does the monitor power LED go orange? Is the monitor screen white or dark?

PlainBill

yes orange, white


mecklan

[PlainBill]

Re: Fix for Benq FP737s 17" (Q7T3)

white

white
white

yes
yes orange, white

mecklan

Excellent!!! there are two possibilities.

1. Either one (or both) of the FU9024Ns are shorted. In this case, the screen may be unusually bright, and changing the brightness from the menu would have no effect.
2. The Backlight On line from the logic card to the power supply / inverter (PS/I) is not functioning, either because of a solder short, or a bad transistor. With the PS/I lying green side up, the AC power connector to the right, the cable to the logic card should be on the bottom near the left side. The two thin traces going to the left are the brightness and On/Off lines.

The easy way to troubleshoot this is to check the FU9024Ns first with power off, then if they are not shorted, trace the On/Off signal line using a DMM and watching it change level when the power button is pressed.

PlainBill

[kturnerga]

Re: Fix for Benq FP737s 17" (Q7T3)

Question: I am about to resolder (touch up) the transformers, re-transistorize and re-MOSFET, and replace the fuse on my Q7T3's board. I'm going to skirt around doing the mod for improper circuit design. Am I courting disaster if I don't mod?

[PlainBill]

Re: Fix for Benq FP737s 17" (Q7T3)

Question: I am about to resolder (touch up) the transformers, re-transistorize and re-MOSFET, and replace the fuse on my Q7T3's board. I'm going to skirt around doing the mod for improper circuit design. Am I courting disaster if I don't mod?

I wouldn't think so. The basic design was adequate. The real cause of the transistor failures was bad solder joints on the transformers. I'm not sure if the improved design would be robust enough to survive that kind of problem. Assuming it is, it would still be necessary to open up the monitor and resolder the transformers. Use of better solder when you perform the initial repair should prevent a repeat of that.

Now if you were talking about building 10,000 new monitors, I would certainly recommend going with the better design.

PlainBill

[alexanna]

Re: Fix for Benq FP737s 17" (Q7T3)

I wouldn't think so. The basic design was adequate. The real cause of the transistor failures was bad solder joints on the transformers. I'm not sure if the improved design would be robust enough to survive that kind of problem. Assuming it is, it would still be necessary to open up the monitor and resolder the transformers. Use of better solder when you perform the initial repair should prevent a repeat of that.

Now if you were talking about building 10,000 new monitors, I would certainly recommend going with the better design.

PlainBill

I concur With PlainBill ,
On these older benq boards some times after resoldering coils and transistor replacement I have had a flashing power led.Resoldering the 4 pin octocoupler has cured the problem.
Al.

[PlainBill]

Re: Fix for Benq FP737s 17" (Q7T3)

I concur With PlainBill ,
On these older benq boards some times after resoldering coils and transistor replacement I have had a flashing power led.Resoldering the 4 pin octocoupler has cured the problem.
Al.

Ah!! Good to know.

PlainBill

[chdala]

Re: Fix for Benq FP737s 17" (Q7T3)

First, thanks to all the previous contributors. The information in this thread lead me to the cause far faster than I would have if I had started from scratch and probably stopped me from making more silicon smoke.

I agree with PlainBill. Open circuits caused by dry joints in the transformers kills the 2SC5707s. They are probably killed because their maximum power dissipation is exceeded. A current limiter will probably not prevent this from occurring.

Explanation....(from an EE of 30 years)

Dry joints (open circuits) around the transformer primary windings cause the oscillation to stop. This causes both Q759 and Q760 to be held 'on' by by R768 and R769 respectively. 2SC5707s are high gain transistors so the collector current will be high, at least 2 amps for a short period of time.

Holding Q759 and Q760 'on' for more than a few milliseconds will saturate the collector windings and cause the entire power supply voltage to be present between emitter and collector (Vce). This 20V or more Vce at 2 amps is a power dissipation of 40 watts in a transistor that is rated to 15 watts (on a perfect heatsink) so they die, usually by going short.

The power dissipation is much lower when the oscillator is running because Vce (on) is almost zero when the transistor is on and the collector current is zero when it is off.

Once the 2SC5707s have died the TL1415 IC detects that the output voltage is low and increases the duty cycle to the FU9024N which causes it to overheat as well. There is then a race, either the FU9024 goes short and blows the fuse or the fuse blows saving the FU9024.

It is not possible to easily limit the current through the 2SC5707s to values that will allow them to operate normally and also prevents them from blowing if the oscillation stops. It may be possible to modify the base drive so that they are not held on permanently if the oscillator stops, but it is far easier to re-solder the transformers.

Chdala

[wenter]

Re: Fix for Benq FP737s 17" (Q7T3)

I have exactly same monitor with exactly same problem. I re-soldered all transformators and replaced all 4 c5727 and 2 fu9024n transistors. The fuse was ok and there was only one transistor shorted - Q751 (c5727). Since I have purchased a set of transistors - I replaced them all. After I finished all work and connected LCD panel - nothing had changed. I can see the picture when I point flashlight on LCD, but no backlight.

What should I check next?

[retiredcaps]

Re: Fix for Benq FP737s 17" (Q7T3)

I re-soldered all transformators and replaced all 4 c5727 and 2 fu9024n transistors. The fuse was ok and there was only one transistor shorted - Q751 (c5727). Since I have purchased a set of transistors - I replaced them all.

I hope you mean c5707.

Did you check all the "new" replacement components with your analog multimeter before installing them to make sure they were good?

If not, you can test them all now in circuit. Also retest the picofuse. I believe it is pf751.

[wenter]

Re: Fix for Benq FP737s 17" (Q7T3)

I hope you mean c5707.

Did you check all the "new" replacement components with your analog multimeter before installing them to make sure they were good?

If not, you can test them all now in circuit. Also retest the picofuse. I believe it is pf751.

I did not check the "new" parts before installing, but I checked them in circuit. All looks fine.

I will replace the picofuse as I received replacement in the set.

[wenter]

Re: Fix for Benq FP737s 17" (Q7T3)

I did not check the "new" parts before installing, but I checked them in circuit. All looks fine.

I will replace the picofuse as I received replacement in the set.

It works!!!!!!!!

I was not careful enough to check the fuse it looked fine, but was not working.

Lesson learned. Check everything - even if it looks good, does not mean it is good

[retiredcaps]

Re: Fix for Benq FP737s 17" (Q7T3)

I was not careful enough to check the fuse it looked fine, but was not working.

Even glass see through fuses can look good, but are actually bad because you can't see the fine breaks.

Make sure you resolder all the transformer pins otherwise you may be doing the exact same repair in as little as 24 hours.

edit: The explanation by chdala in post #269 tells you why.

[wenter]

Re: Fix for Benq FP737s 17" (Q7T3)

Even glass see through fuses can look good, but are actually bad because you can't see the fine breaks.

Make sure you resolder all the transformer pins otherwise you may be doing the exact same repair in as little as 24 hours.

edit: The explanation by chdala in post #269 tells you why.

I resoldered transformators as the very first fix. Than I went onto checking transistors.

Thanks for reminder

[Antaris]

Re: Fix for Benq FP737s 17" (Q7T3)

I’d like to say BIG THANKS to everybody who has shared the knowledge and experience in this great forum! The backlight of my FP737s has been restored by replacing 4x2SC5707 (Q739, Q740, Q759, Q760), 2xFU9024N (Q751, Q743), C710 (220mkFx25V), FUSE 3A, plus resoldering of all transformers legs. Monitor is working about 6 hours after the repair without failures.
Andrey.

[aheliason]

Re: Fix for Benq FP737s 17" (Q7T3)

Season's Greetings! I am joining this thread at the suggestion of senior member "retiredcaps," who replied to my first inquiry. My repair efforts on a Benq Q7T3 are continuing at this time because today was a "Smoke day!" Here is a recap: The monitor first failed with totally dark screen but normal power lights. The green power led would go on and change to yellow if the computer was turned off or the external monitor de-selected. First look revealed no burned components nor bulged electrolytic caps. After joining this forum I found that the schematic of Diamond View DU-172 posted by Starfury1 (#2 in this thread) matches up very well with my card. The part numbers match well enough to follow, for example T-761 on the schematic is T-751 on my PCB.
As soon as I saw the micro-fuse, PF751 (3A-SB), I checked it and found it blown. I replaced it and the monitor ran properly again for two days then blew the fuse again. At that time, member "retiredcaps" suggested resoldering the transformers as well. I also replaced the 10 volt electrolytics with 35 volt ones. This time I checked components more carefully and found two of the four inverter transistors measured shorted in-circuit. After removing them, only one was actually bad. I ordered a few and they arrived yesterday. I replaced the bad one and its compliment and tried the monitor again today. The first sign that something was still wrong was that I had white screen (raster) even when the power light was off, however the display looked good with the computer on. Unfortunately, there was soon a puff of acrid smoke!! This time, on inspection, I found that high voltage capacitor C-756 (766 on the schematic) had burned up. A dc resistance check of the associated transformer shows it having the same values as the others, so I intend to find a replacement cap and try again. It is in the same inverter that failed the first time so it may, indeed, be the culprit.
If any of you have any ideas or suggestions as to something else I may have missed, by all means, please let me know. By the way, I'll offer any left-over components at cost plus 1st class postage when done, to try to help some other member save a bit of shipping cost.

[retiredcaps]

Re: Fix for Benq FP737s 17" (Q7T3)

If any of you have any ideas or suggestions as to something else I may have missed, by all means, please let me know.

The usual checklist for Benq style boards are:

1) check picofuse to see if it is open
2) check c5707 transistors for shorts
3) check fu9024 fets for shorts
4) resolder all transformer pins

What brand and series did you use for replacement caps?

[rbrt86]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello everyone,

I really need your help. I've got a Benq FP737 with the same sympthoms as post #263: white backlight when monitor is off

The blacklight used to be broken but then I:
1) replaced the picofuse
2) replaced the 4 c5707 transistors
3) replaced the fu9024 fets

Now the monitor works but smells like smoke and the backlight is white when off

I noticed that one of the fu9024 fets gets really hot
Is it a short circuit?

Please help, your advice is very much appreciated!!

[PlainBill]

Re: Fix for Benq FP737s 17" (Q7T3)

Hello everyone,

I really need your help. I've got a Benq FP737 with the same sympthoms as post #263: white backlight when monitor is off

The blacklight used to be broken but then I:
1) replaced the picofuse
2) replaced the 4 c5707 transistors
3) replaced the fu9024 fets

Now the monitor works but smells like smoke and the backlight is white when off

I noticed that one of the fu9024 fets gets really hot
Is it a short circuit?

Please help, your advice is very much appreciated!!

The FET may be shorted, inserted backwards, or there may be a solder short.

I keep harping on this. We don't have ESP, we don't have a crystal ball. We depend on you to provide the information. Your description is quite good, but a couple of pictures would help us identify the problem.

PlainBill