benq fp731 backlight problem

[haku]

hi, i'm new here, but did a bit of searching and found many others with the same problem I have.

The backlight on the monitor seems to be busted ( I can see a faint image if I shine a flashlight to it).


So everyone seemed to narrow it down to either the 4 (Q740/Q739/Q759/Q760) 2SC5707 transistors or the pf751 fuse 3amp.

When I was taking the board out, I accidently ripped out a leg on the item circled in red, it's labelled "L603", can anyone tell me where I can source (or the name of it) another part out??


I'm familiar with soldering (audio cables), but not PCB soldering. So what reading am I looking for when I test the transistors/fuse of the board to see if it's fried or not? And since there are 3 legs/pins on the transistor, which is which


thanks for your time



PS: these transistors will work 'just as well' as the originals? http://cgi.ebay.com/MJE3055T-MJE3055...2em118Q2el1247

[Spacedye69]

Re: benq fp731 backlight problem

I just bought some 2sc5707 from this guy. On the transistors, your more than likely looking for a short between 2 of the legs. The legs are labeled E C B on the board. The fuse just check for continuity.

I cant see any red circle.

[haku]

Re: benq fp731 backlight problem

hi oops, i reuploaded the pic


i found that ebay seller too unfortunately they want 7$ to ship to canada


thanks


ps: is it just me or can you not edit a first post?

[MikeM]

Re: benq fp731 backlight problem

You can get 2sc5707 in Australia for A$1.75 ea. Won't be far different in value in C$'s. L303 just seems to be a wire wound thru 3 times and exiting as 2 leads. Should be easy fix unless you have damaged the ferrite 'casing'.

[c_hegge]

Re: benq fp731 backlight problem

I have the same problem with the same monitor. I'll try replacing these parts and post back with results.

[c_hegge]

Re: benq fp731 backlight problem

One other question before I order parts. would this be a suitable fuse?

http://australia.rs-online.com/web/s...t&R=2509674449

[George Ortiz]

Re: benq fp731 backlight problem

I think the power supply in your Benq is that same power supply that's in my
(free) Dell E173FPb. I troubleshot and found two shorted transisters. I ordered mine transistros off of ebay. Should have them here tommorrow. Spent $5.98 to get the 4 transistors, 2 other semiconductors and the small fuse. I was getting the power light only flashing when turning on the monitor

[c_hegge]

Re: benq fp731 backlight problem

Fixed it.

I replaced the four 2SC5707 transistors and PF751 and it works now. I am amazed since the mounting of the transistors was entirely emprovised. I ordered them from RS Components and they arrived with the middle leg very short (like on a VRM MOSFET), so I used wires to connect to the pins on two of them and was able to mount the other two on the underside of the PCB. I also damaged a couple of the pads, so I joined the wires to the other parts in the circuit.

I also replaced all the caps, except for the primary, with Panny FC

[smason]

Re: benq fp731 backlight problem

hi oops, i reuploaded the pic


i found that ebay seller too unfortunately they want 7$ to ship to canada


thanks


ps: is it just me or can you not edit a first post?

Where in Canada are you?

[charliea]

Re: benq fp731 backlight problem

Fixed it.

I replaced the four 2SC5707 transistors and PF751 and it works now. I am amazed since the mounting of the transistors was entirely emprovised. I ordered them from RS Components and they arrived with the middle leg very short (like on a VRM MOSFET), so I used wires to connect to the pins on two of them and was able to mount the other two on the underside of the PCB. I also damaged a couple of the pads, so I joined the wires to the other parts in the circuit.

I also replaced all the caps, except for the primary, with Panny FC

Bought the kit but haven't put it in yet. Noticed one of the 470uf caps is bulging on top so I ordered 5 to replace all plus the 3 other 1000uf caps.

Ran a continuity test on the fuse and it was .5 ohms on my cheapie old needle type multimeter.

Since only the capacitor showed visible signs of a problem I was going to replace them first before replacing the transistors and fuse.

Sound like the right approach??

[retiredcaps]

Re: benq fp731 backlight problem

Ran a continuity test on the fuse and it was .5 ohms on my cheapie old needle type multimeter.

Since only the capacitor showed visible signs of a problem I was going to replace them first before replacing the transistors and fuse.

Sound like the right approach??

The fuse is good.

You can test the transistor for shorts using your analog multimeter. Just set it to ohms and test 1-2, 1-3, 2-3 on c5707. If it reads fine and the fuse reads fine, replace the caps and try the monitor.

On this board you may also find a FU9024. Check these for shorts too.

PS. Make sure to resolder ALL the transformer pins before turning on the monitor. They have reputation for being bad due to cold/dry/bad solder joints.