BENQ FP731 PF751 3A fuse Q

[Frankenstien]

Hey all ...

I've got one of these BENQ's that seems to have a backlight failure ...
I have proper power button LED function but can see only a dim desktop image using a flashlight or in overhead sunlight.

I searched 'FP731' (then 'PF751' within certain resulting found threads) and read up on the capacitor and cold solder joint issues with these displays ...
ie. 'benq fp731 backlight problem'

Q. does the PF751 fuse protect only the CCFL lamps, or, the entire PCB assembly? ie. would I get the dim image with a blown fuse?

From what I've read so far, I think I'm in a little over my head, however, I would be game to crack this thing open if all it needs is a fuse.

Also, I was unable to view any thread images earlier so don't know the fuse location, appearance or type (clip-in? / soldered?).

Any info / advice would be appreciated ... thanks, FFF

[Krankshaft]

Re: BENQ FP731 PF751 3A fuse Q

Probably dead C5707 transistors and blown pico fuse.

Yes the fuse only protects the inverter that drives the backlight and no replacing the fuse almost never fixes the problem.

Replace all four C5707 transistors touch up any cracked solder joints replace the pico fuse and you should be all set.

The pico fuse will usually have a board designation of F101 for example F stands for fuse. It won't look like a standard glass fuse it will be cylindrical probably black and will have the amperage printed on it for example 3A.

[Frankenstien]

Re: BENQ FP731 PF751 3A fuse Q

Thanks Krankshaft ... I shall delve into this display and see what I can see.
From what I understand, those caps may exhibit obvious physical overheating and/or leakage ... I just need to look at a couple of images attached to threads here to get more familiar with what I'll be looking at / for.
FFF

[retiredcaps]

Re: BENQ FP731 PF751 3A fuse Q

Just got one of these FP731 today. I checked the infamous C5707 and they test okay with the diode test.

The PF751 fuse (3A) shows 30 ohms. I know a good fuse measures around 0.03 ohm.

Is this considered a "good" fuse or a marginal fuse? I another Benq board and its picofuse reads 0L so I know without a doubt it is shorted.

I'm going around checking all the other components for a shorts and checking solder joints, but would like to know if the fuse is considered good or not with a 30 ohms measurement.

The original owner of the monitor says there is power, but no backlight, but you can see the picture faintly. It sounds like the infamous 2 seconds to black.

[alexanna]

Re: BENQ FP731 PF751 3A fuse Q

Well I have to ask this question.What ohm reading do you get when you thouch the two leads together ? most if not all dvm meters are not going to read 0.
also Digital meters have such high resistance built into them{ 10meg ohm} that they do not flow alot of current in the circut. It is possible to get fooled by the ohm reading,Say like having a verry slight contact that the ohm reading is good but when current flows thru the circut it opens up. The fool proof way to test would be with a load on the circut.
Al.

[retiredcaps]

Re: BENQ FP731 PF751 3A fuse Q

Two probes together = 0.00 ohms.

I'll put everything back together sometime later this weekend and test for secondary voltages.

What ohm reading do you get when you thouch the two leads together ? most if not all dvm meters are not going to read 0.

[retiredcaps]

Re: BENQ FP731 PF751 3A fuse Q

Scratch that. If I hold together the probes for a good 3 or 4 seconds, I get 0.03 ohms. I get 0.00 ohms if I only hold it for 1 or 2 seconds. It seems this multimeter takes a bit longer to "lock in".

Secondary voltages all look good and solid.

Two probes together = 0.00 ohms.

[alexanna]

Re: BENQ FP731 PF751 3A fuse Q

Probably dead C5707 transistors and blown pico fuse.

Yes the fuse only protects the inverter that drives the backlight and no replacing the fuse almost never fixes the problem.

Replace all four C5707 transistors touch up any cracked solder joints replace the pico fuse and you should be all set.

The pico fuse will usually have a board designation of F101 for example F stands for fuse. It won't look like a standard glass fuse it will be cylindrical probably black and will have the amperage printed on it for example 3A.

I would like to build just a little on what Krankshaft said. I have been blessed by one of the local recycling centers here in the Midwest. I got a whole bunch or the dell H.P. Benq monitors.What I have been finding is if you can see faint image and have good power light the pico fuse is good and replacing the c5707 transistors along with Removing the inverter coils and reordering connections will most likely fix the problem. If you have no image and no power light the pico fuse is bad and there will be a couple of other transistors that will need to be replaced ,There are a couple of possibilities depending on your board I know one number is j598 but the other number escapes me at the moment, These two are more toward the middle and to one side of the board .They look just like the c5707 transistors .As far as testing these I think to get an accurate test on these you should remove them from the board, I do not think they like heat to much and they are really cheep I think you would be money ahead to just replace all six of them. After you get your monitor up and running set your computer to go into the sleep mode frequently and then wake it up many times if it is going to fail that will be the test.
Al.

[rivette65]

Re: BENQ FP731 PF751 3A fuse Q

i have 10 Nec lc19m lcd monitors
the problem is that the screen goes dim after 30s .and i found that in every one there is a bad backlight bulb : that's why it goes dim i
i have tried to put an external bulb instead of the bad and the monitor does not go dim . and i have realise that with only one bulb the brightness of the monitor is good . so instead of changing a second new bulb i want to use only one . but the problem is the feedback circuit : with only one bulb the screen shuts down .
ihave you any idea about what to connect in the bulb connector on the inverter to bypass the detection of the bulb or to bypass the feedback circuit
thank you

[retiredcaps]

Re: BENQ FP731 PF751 3A fuse Q

Just answering my own question. I replaced the fuse today and it works. I tested all the other components before I tried the new fuse and found nothing wrong.

So 30 ohm for a picofuse is bad. A good fuse should be 0.3 (not 0.03 as I wrote earlier).

The PF751 fuse (3A) shows 30 ohms. I know a good fuse measures around 0.03 ohm.

Is this considered a "good" fuse or a marginal fuse?