Fix for Benq FP737s 17" (Q7T3)

[Krankshaft]

Re: Benq 17" Q7T3 fix

I have read elsewhere on the Internet that another theory about the transistor blowing is that the joints on the HV transformers are cold which causes the transistor to come to an early demise.

Other sites are simply saying remove the old solder on the transformers and resolder the joints and you're all set.

Does this sound possible or is it just crap?

I do agree that at least on my board the solder joints are all crap. Yesterday I gently bent back a transistor (like I've done many times before) to read its number and before I could even get a little pressure on it snap the solder joints just cracked.

So I don't dispute the fact that the solder joints are below average.

I attached a picture of my inverter board to this post its manufactured by Benq 48.L1A02.A00. I'm just curious if it looks familiar.

Also if you look at the picture the board is a little darker around the HV transformers I hope thats normal and doesn't mean that the HV transformer got cooked.

FYI I took this pic before I started recapping I will be getting rid of all those Elite and Lelon Crapacitors very soon.

[davmax]

Re: Benq 17" Q7T3 fix

Krankshaft. The PCB photo shows a number of similarities like the same inverter transformers.

Re Soldering. I can only go from my experience in that the two monitors I have processes have not had soldering issues and the transistors have failed. But the PCB quality is poor in that the copper track too readily lifts off the board when a soldering iron is applied. There will be plenty of speculation about the transistor failure but few, if any, have witnessed the excessive transistor current problem that I uncovered due to the implementaion of the fast overcurrent protection circuit. It would be rare to have bad soldering in the tightly controlled one pass flow soldering process used on these boards.

Do not be concerned about the colour change of the board, it may indicate a hot spot, but leave that for the moment. Changing out those caps will be useful but there is no link to the transistor failure (caused by either excess voltage or current). The transistor failure seems to occur for no apparent reason, this is supported by what I have discovered, an occasional excessive current in the transistors caused by a limited set of conditions exploiting a weakness in the softstart circuitry.

A fast overcurrent protection is the complete answer.

The Benq number you quoted does not bring any results in a search. Are there any other numbers that could help? An HP number willl not , they do not provide service info.

[Krankshaft]

Re: Benq 17" Q7T3 fix

Well I took another look at the board and I snapped three more pics of what could be part numbers. It looks like the number I gave you was probably a revision number.

I believe E157925 in the first pic is the part number.

Thanks for the help.

[davmax]

Re: Benq 17" Q7T3 fix

I made a mistake reading the family specs for the HEF series. This chip does work at 15Volts. My problem was that the 15volt supply in the monitor can rise above 18V and fuse the chip. I had to use a zener for protection the next time round. Davmax

Thanks for your inputs Gonzo. I really appreciate your many inputs and comments.
Yes I have considered resistor and transistor at the soft start point. However there are two soft start circuits, one for each inverter. It is much better to block the BKL_ON signal at one point for one second. This is also much more precise, operating in nS thus ensuring no possible weak point. Also it is very simple to adjust the time independently of the soft start.

I bench tested the delay circuit at 5 volts it worked fine. I put it into the monitor (15V supply) and the 4047 chip fused. Investigation found that I had been supplied with a HEF4047 that works up to 6 volts when I expected to get a CD4047 up to 18 volts. The supplier is stocking the wrong version. So will try again. Another trap!!!!

The 4047 chip drives a transistor that clamps down the BKL_ON signal. There is no changing components on the Benq board. I use a very small vero board with four leads. +15V, ground, input taken from a board transistor output and output that clamps the signal. As the board transistor turns OFF the input to the same transistor is clamped to ground for 1 sec, this achieves the required result.

[davmax]

Re: Benq 17" Q7T3 fix

posted in error

[davmax]

Re: Benq 17" Q7T3 fix

Krankshaft. Those photos were helpful. The one on the left shows 94V, this is a Benq model number. Unfortunately I have searched for service manuals that give circuit details. So far it would seem that I am lucky in that of all the models the FP737s (Q7T3) that I own does have such a manual.

Therefore the simplest option for you, if a transistor is blown, is to replace the pair of transistors and the fuse. The fault only occurrs rarely, hopefully it will be some time before it happens again. It would seem that the computer post is the most hazardous time, particularly if the bios is being changed. One precaution therefore would be to turn the monitor OFF after pressing F10 and Enter following a bios change. Then ON again once Windows boot is well in progress.

The 2SC5707 transistors can be found on eBay.

[davmax]

Re: Benq 17" Q7T3 fix

For those that may be interested. BLT_ON = backlight ON

I have attached the circuit that ensures the BLT_ON signal cannot be reapplied within one second.

The circuit uses a 4047 monostable to generate a one second duration when BLT_ON is removed. During this period a transistor prevents a return of BLT_ON getting to transistor Q757.

Unfortunately the 15volt supply is not highly regulated and can destroy the 4047 so zener protection is required, adding four more components. The zener can be between 12 and 15 volts with other values unchanged.

[Krankshaft]

Re: Benq 17" Q7T3 fix

It would seem that the computer post is the most hazardous time, particularly if the bios is being changed. One precaution therefore would be to turn the monitor OFF after pressing F10 and Enter following a bios change. Then ON again once Windows boot is well in progress.

The 2SC5707 transistors can be found on eBay.

Thanks for all the help Davmax.

One final question does this same hazard exist while the computer is coming out of standby or only during POST?

Because 90% of the time I just standby my PC when I'm not using it anyways.

[davmax]

Re: Benq 17" Q7T3 fix

From what I have seen this hazard is only during Post and so far it seems to be only if there is a change to the bios. During Standby the conditions are much simpler, the conditions are restored, the video is simply switched ON.

[atDucat]

Re: Benq 17" Q7T3 fix

Impressive investigation, must've been lot of fun.

I have the same problem with my Benq. Now, when I have localized blown transistors 2SC5707 (one in each inverter, unfortunately) and fuse I put two working transistors into one inverter, replaced fuse and – miraculously I can see what's on the screen but only for half a second (and half bright). While I wait for new transistors, should I look for more problems or will it be solved when I place new ones in second inverter.

One more thing, when I switch video of and on again – I can see screen again for another half a second – BLT_ON signal is switching off for some reason – any idea?

Regards.

[Krankshaft]

Re: Benq 17" Q7T3 fix

It is possible that the protection circuit is kicking in since one of the inverter circuits is not working due to the transistor swap.

If this continues after replacing the transistors definately check out the secondary capacitors on the power supply you will probably find that they are bulged.

This is a classic symptom of failing caps the caps don't do their job and filter out the AC ripple. This allows too much AC ripple to get into the VCC pin of PWM IC the IC becomes unstable and shuts down.

You should always check over the caps when doing this transistor repair and at least replace ALL of the secondary caps if ANY of them are bulged.

I usually replace every cap on the board if its a bad brand just so I don't have to go in there again.

I replaced those stupid Fukyuu caps in my Antec PSU about 7 months ago with Panasonic FCs and not a single bulger. Unlike Fukyuu which would already be bulging by now. Those Panasonic FCs are wonderful. I haven't had a single failure with any of those caps.

[Flavior]

Re: Benq 17" Q7T3 fix

Hi, I am new here of!
Have changed all the transistors that thin to have now been recommended, but now when I give current to the monitor, the 2 C5707s smoke!! If I remove the Q743 the problem it is not verified but obviously the lamps he turns on and they extinguishes later immediately him.
when the backlight worked he didn't extinguish him to the turning off of the
monitor

Besides all the components of the inverter burned!
Does someone have some hypotheses about it?
I hope for him!!

Thanks!!

[Krankshaft]

Re: Benq 17" Q7T3 fix

If windings on the inverter transformer were shorted that could cause the new transistors to smoke.

If the inverter got toasted and smoked it could only happen due to user error is sounds like what happened is someone jumpered a blown fuse with a length of wire (or a fuse failed to open) and the board fried as a result.

NEVER jumper a fuse if you install a new fuse and the new one blows there IS A PROBLEM. Without the fuse the shorted components will DRAW EXCESSIVE CURRENT until they cook themselves at which point they will open.

[davmax]

Re: Benq 17" Q7T3 fix

Yes if the transformers etc smoked then the fuse must have been jumpered. I suggest that the transistors failed as usual, the fuse was jumpered the transformer/s were then destroyed. Fix would require replacement of transformer/s, switching transistors and most likely the MOSFETs that control the voltage to the switching transistors. Sounds like a mess.

[Flavior]

Re: Fix for Benq FP737s 17" (Q7T3)

Thanks thousand x your interest

Then as you recommend me to proceed?
is there something that I can measure?

the transformers I can measure them?

Thanks!!

[Flavior]

Re: Fix for Benq FP737s 17" (Q7T3)

P.S.
Is there way of alone usar the other wing of the inverter considering that that is perfectly working?

Thanks!!

[davmax]

Re: Fix for Benq FP737s 17" (Q7T3)

It is good that the one inverter section is working. This powers only two of the four lamps.

If you have burned or smoked the other transformer you will need to replace it. Forget measuring it. The transformer requires special testing eg insulation and shorted turn testing.

Unfortunately you may not be able to fix the monitor unlees you can get the right parts. Correct 3 amp fuse, 2SC5707, transformer and FU9024 MOSFET.

Better to geta replacemnet board. Try this site: http://www.lcdrepair.us/e172-e173fpb...ter-power.html

This page shows that same inverter board, but you need to ask supplier if it will match Benq FP737s. I think it will.

[Krankshaft]

Re: Fix for Benq FP737s 17" (Q7T3)

Hi again here is a status report on the HP (Benq Monitor). I replaced the transistors and the monitor came on and worked flawlessly.

However I found another problem as soon as power is applied one of the two inverter circuits come on and when the power button is pressed the other inverter circuit comes on along with the video. In other words when I press the power button to power the monitor down only 2 of the 4 CCFLs shut off.

So I take the board out again and find the problem a blown SMT MOSFET IC. The IC package has a small bubble in it which is never good. I saw it while working on the board but I denied there was a problem .

So I measure the SMT FET that is not bubbled from Source To Drain (the side of the inverter that was shutting off) and I get a voltage drop.

Then I measure from Source To Drain on the bubbled FET and its Open Circuit.

For future reference I have circled the FETS that shut down the inverter circuits there are two one for each circuit which drives 2 CCFL lamps.

Well now I've found my shutdown problem now here is my question.

I think I've found a replacement MOSFET IC but would like an opinion. I have plenty of experience soldering these things so its no biggie for me.

The IC has the following numbers on it.

4431

BP (Triangle) Probably Vishay's Logo

L47C (Probably A Lot Number)

Here is a link to the datasheet of what I think is a suitable replacement:



What do you guys think?

Thanks for any help in advance.

[Krankshaft]

Re: Fix for Benq FP737s 17" (Q7T3)

Also please note that the Source To Drain diode next to each FET IC checks out good.

And yes that is a lifted trace above the defective FET I scratched off some solder mask off of the board and soldered the new caps leg onto the trace itself.

I have never lifted a trace while recapping before the traces on this board must really be crap.

[Krankshaft]

Re: Fix for Benq FP737s 17" (Q7T3)

Well it seems like the Si4431ADY is hard to obtain but I have found the Si4431DY would that work?



I intend to replace both FETs so that they will be the same type. It probably doesn't make a difference but it would make me feel better.