Fix for Benq FP737s 17" (Q7T3)

[jayoung]

Re: Fix for Benq FP737s 17" (Q7T3)

LCD monitor manufacturers use various power supplies and inverter boards for monitors with the same model number. You will need to open up the monitor and then you will be able to read the values of the needed capacitors. Many members here recommend replacing all of the electrolytic caps at the same time (except for the large main filter cap). There are several threads on here about ordering capacitors and recommended brands. If you make a list of the capacitors that are on your board, and list what replacement caps you think you should order, someone here can verify if you are ordering the right ones. Pictures also help people help you, use the manage attachments below the text area when posting through advanced messages.

[balu]

Re: Fix for Benq FP737s 17" (Q7T3)

Hai Everybody,

I joined in this forum yesterday. It is a very useful and helpful one. I could get some essential information regarding my damaged Benq LCD Monitor. Could somebody give the values of Caps @ C501 and C509 in Benq Q7C5 Power Supply Inverter Board? Thanks....

[Smiffy]

Re: Fix for Benq FP737s 17" (Q7T3)

Thanks RetiredCaps

Smiffy

[retiredcaps]

Re: Fix for Benq FP737s 17" (Q7T3)

My FP737S has 2SJ598s in Q743 and Q 751.Is it ok to replace them with FU9024Ns?

PlainBill answered this ...

https://www.badcaps.net/forum/showpo...7&postcount=20

[Smiffy]

Re: Fix for Benq FP737s 17" (Q7T3)

My FP737S has 2SJ598s in Q743 and Q 751.Is it ok to replace them with FU9024Ns?
All transformers resoldered.all 2SC5707s replaced and bulging caps done(nearly all of the 470uf 35v ones)
Smiffy

[jetadm123]

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!!

Did you also remember to resolder all the transformer pins?

[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

[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!!

[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?

[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.

[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.

[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

[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)

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

[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.

[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 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?

[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

[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

[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.