Troubleshooting Benq FP757

[atrain]

Hi, I've been reading the forums for a little while now, and finally got around to re-capping my monitor. It is a Benq FP757, and originally would quickly blink the BenQ logo dimmly, then display nothing, and the power light would go from blue to blinking orange (standby?) after a few seconds. Also, it made a high-pitched noise when off. The inverter + general power supply are one board on this unit. I recapped all the electrolytic caps, even though none were visibly defective (but I have no ESR/cap meter so no way to test, just replaced all 19).

Now, the backlight does not work at all. I can still see the benq logo if I shine a flashlight directly on it, but doesn't appear to have any image after that. And it still makes the whining noise. How do I proceed? I'm guessing it's the inverter side of things that's causing the problems, how do I go about diagnosing it? How to I test transistors? Anyone have any experience with this model?

The old caps were a combo of Elite, Lelon, and a few Jamicons. The new ones are Arks, as best I can tell. (a letter A in a rectangle, it was all the local store had.)

[atrain]

Re: Troubleshooting Benq FP757

[could not find edit button]
After turning off my lights, and shining my flashlight at a specific angle, I noticed the screen is working, so the backlight/inverter is indeed the problem.

But the question remains: How do I test the inverter circuit? How do I test the bulbs themselves?

[Edit] This post has edit button, but not first post... why is that?

[Gianni]

Re: Troubleshooting Benq FP757

First of all welcome

I'm not a guru here but AFAIK the brand Arks is not in the list of good caps.

Good brands are Panasonic, Sanyo, Samxon, Nichicon, Rubycon, UCC... read some thread on The GOOD Capacitors Thread!! [url]

You need also to replace caps with same grade/type because there are some kind that even if are good brand are not suitable for some application like monitor, MOBO and PSU.

If you can post some pictures of your board, maybe some guys here can identify something wrong on it.

For edit button: you can see it only for 10 minutes after posting your message, you can't edit old post.

Ciao
Gianni

[atrain]

Re: Troubleshooting Benq FP757

Thanks for that.
1) I know about the good + bad lists. I wrote most of them down on my list of caps to buy, but the store only stocked one brand. And just because the cap isn't highly recommended, doesn't mean it is going to fail. Also, even bad caps will last for a certain while.

2) Everything was replaced with the same voltage rating or better, and same capacitance value. (I replaced some 10V with 16V cause I was lazy, but I cannot imagine any harm done there.) I did not have the foresight to check if any were special (eg: low ESR), and after checking now, it seems as if some of the elites were low impedance, everything else was fine.

I'll post some pics soon, the next time I rip it apart to find out what capacitors I bought.

[PlainBill]

Re: Troubleshooting Benq FP757

The easiest way to test inverters and backlights is with a case mod light kit - the type that has two CCFLs and an inverter (costs about $10). Use the kit inverter to test the monitor CCFLs, use the kit CCFLs to test the monitor inverter.

PlainBill

[atrain]

Re: Troubleshooting Benq FP757

At this point I'm pretty sure it is the inverter, as there are 2 CCFLs on each side, and I doubt all 4 would have died at once. I'm kinda thinking of being lazy and using LEDs/a case mod light to illuminate it.

As for testing them: You can also wire them to laptop/other desktop screen, no need to go out and buy a case light.

[atrain]

Re: Troubleshooting Benq FP757

I have a friend who had a broken CCFL bulb, and asked him for the inverter. I was thinking of just finding 12V on this board and connecting this inverter because I was too lazy to solve the issue. But he couldn't find it, so I trudged on.

It seems benq reuses the same circuit, or at least part labeling, as I am finding many posts with different supply boards, and similarly labeled parts. One part which keeps popping up is pico-fuse PF751. I tested this fuse, and found it to be broken (reading ~5Kohms.) I cannot imagine it is a good idea to just bypass this fuse, as it must have blown for a reason, and it must be there for a reason, so I will wait until I can find a proper replacement before confirming this is indeed the problem with the unit. (I also noticed I had ripped the pad of a cap right next to it, I've added a proper jumper now, could that have done it?)

[Demix]

Re: Troubleshooting Benq FP757

This BenQ power/Inverter board has been the bane of my life the past few days, as im new here and ive been having luck with other monitors ive repaired i was given one of these to repair, it would turn on and just have a green light.. no backlight at all. I replaced the 4 Transistors which are known for failing (Q759, Q760, Q740 and Q739) and now when i switch it on the blacklights flicker on and off for a second then its back to using a torch to see the screen lol. Im waiting for PF751 - Pico fuse to arrive in the post now and i guess i shall see. Looks like BenQ have many many many problems with the amount of posts ive seen about their power/inverter boards

[lucky13]

Re: Troubleshooting Benq FP757

From my experience, if one of the lamp is gone, you should at least see the pix flashes on for a second before the inverter's open-lamp detection shuts it off.

So, if you don't even get a flash, then it is the inverter itself.

Pico fuse is just another type of small fuse. In a pinch, you can wire in a whatever similiar current size fuse to do a quick test.

[PlainBill]

Re: Troubleshooting Benq FP757

This BenQ power/Inverter board has been the bane of my life the past few days, as im new here and ive been having luck with other monitors ive repaired i was given one of these to repair, it would turn on and just have a green light.. no backlight at all. I replaced the 4 Transistors which are known for failing (Q759, Q760, Q740 and Q739) and now when i switch it on the blacklights flicker on and off for a second then its back to using a torch to see the screen lol. Im waiting for PF751 - Pico fuse to arrive in the post now and i guess i shall see. Looks like BenQ have many many many problems with the amount of posts ive seen about their power/inverter boards

If the backlights flicker on the pico fuse is good. I would suggest resoldering all the leads on all the transformers in the inverter area.

PlainBill

[atrain]

Re: Troubleshooting Benq FP757

No, the pico fuse was definitely shot. I've replaced it, but it still doesn't run. I didn't have a chance to pick up better caps though. (The guy at the store didn't know what ESR was.)

I just picked up a new multimeter (MST 2325), and will start testing the transistors. But if I can't figure this out, I already have a 12v CCFL inverter waiting to be picked up, and I'll just poke around until I find 12v and be done with it. (Or do these things take considerable power, do I need an external supply?) (hops on the benq hating wagon.)

[Krankshaft]

Re: Troubleshooting Benq FP757

C5707s short they blow the pico fuse. You replace the pico fuse C5707s blow it again. They sometimes take the a backlight off FET with them too but we won't go there until the backlights are working again.

Replacing a fuse blindly is never I good idea mechanical fatique (a fuse blowing from old age) isn't as common as it sounds.

Replace the fuse and the C5707s when the backlight comes on press the power switch if the LCD shuts down but the backlights still run then you need to replace one of the backlight supply FETs.

[atrain]

Re: Troubleshooting Benq FP757

Well, you were right about the C5707s. Not 100% sure how to test them effectively, but it seems 2 of 4 are shorted right through. The other side seems fine, though I'm not 100% clear on the procedure for in-circuit transistor testing. (is there a good tutorial somewhere?)

Are there any alternatives that are better suited for this board? Or should I just stick with a C5707?

[Krankshaft]

Re: Troubleshooting Benq FP757

Probably only one is shorted since they are wired in circuit together it looks like that they're both shorted.

At minimum replace them in pairs even better replace all 4.

Touch up the solder joints on the inverter transformers and the tuning caps (gray boxes near the C5707s) and everything should be fine.

If the monitor then works but the backlights won't shut down when the power switch is pressed. We'll deal with it later. Sometimes this failure shorts an On / Off FET sometimes not.

I am using a repaired C5707 based monitor (HP 1955) on my primary PC since DEC 07 still no repeat failure. I replaced the caps too since they were Elite (didn't want to dig into it later).

I've personally seen more repeat failures from people trying to sub the original C5707s for something else. Not to mention the slicing and dicing needed to get a TO-220 package to fit through the backplate.

[atrain]

Re: Troubleshooting Benq FP757

I've read on google about hfe differences with older and newer C5707s. (the ones in the board have 350-400, and some newer ones are 150-200 sometimes) But if you say they are ok, I'll order some + some proper low-esr caps and see what happens.

Can't seem to find any 2SC5707 on future electronics or digikey. Who sells them?

[PlainBill]

Re: Troubleshooting Benq FP757

I've read on google about hfe differences with older and newer C5707s. (the ones in the board have 350-400, and some newer ones are 150-200 sometimes) But if you say they are ok, I'll order some + some proper low-esr caps and see what happens.

Can't seem to find any 2SC5707 on future electronics or digikey. Who sells them?

There are several sellers on eBay.

PlainBill

[EGuevarae]

Re: Troubleshooting Benq FP757

There are several sellers on eBay.

PlainBill

I second that. They even sell kits with the 5707s, the FUs and a fuse.

[tirigliu]

Re: Troubleshooting Benq FP757

Hi guys,

I'm having real issues with my monitor...I followed closely your advices but the outcome is not the one I've expected.
the problem: Power on...No light, but I can see the image on the lcd using a flashlight.
The board looks similar to this one:
invertor
I don't have a camera right now so I'm using the image I've found on the forum.
Q760 was shorted, I've replaced Q759 and Q760 with new ones.
PF751 was open, I've used some fine wire as a temporary fix.
Soldered all transformer's leads to make sure there's no cold soldering.

Now when powered on the lcd turns white, I see a logo, and then it turns black.
Also the board is getting extremely hot around T751-T753.
Also I've noticed that C734-735 and C754-755 are shorted measured on the board but when taken out they're fine.

Can you help me please ?

Thanks.