• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

BenQ FP547 15" BLACK SCREEN

Status
Not open for further replies.

tofutofu

New Member
hello everyone.:)

trying to found the problem in the Q5x3 power bord of my benq FP547 monitor.

the problem started with black screen every few minutes . but soon become black permanent.
conecting to power and a signal, giving a green light (recognize the signal).
i changed 2 puffed cap's. with no chang.

checking the voltage on bord:
the V-inverter is 13.3v (is that OK?). but no power is geting in to the 2 small transformers (cant see any Voltage after the fu9024n mosfet-[ Q751 and Q743 on the diagram]) .

I changed the 2 couple of transistors [Q759,Q760 & Q730,Q740 on diagram] (should be 2SC5706. but I got only the 2SC5707 that was available at store ) and replaced the 2 fu9024n mosfet. but no chang.

I'm working with the diagram, but don't have any clue what to do next.

any help or idea?


the q5x3 diagram :
 

duffy

Well-Known Member
What is the voltage at BLT_ON?
 

tofutofu

New Member
BLT_on voltage

What is the voltage at BLT_ON?

tanks for replay..

the BLT_on voltage :

1)when starting, (no connection to vga source)- the green light is turning on for few seconds and the voltage is 3.3v as well (and then go to 0.0v)

2)when connection to vga source- the green light is turning on permanent- voltage is 0.0v

is that all-right?

just found out that when no source is connected, the back-light is turning on for few seconds. with the message "NO SOURCE" on screen.
 
Last edited:

duffy

Well-Known Member
When BLT_ON goes to 0.0V then Q761 will shut off and disconnect Vcc from IC761 which controls the fluoro inverter. That might be the issue. It would also explain why there's no voltage downstream of Q751 and Q743.
 
Last edited:

tofutofu

New Member
that's make sense. so the problem is at the source of the BLT_ON.
that will be the processor board (with the dsp chip)

is their any way to check that out?
 

tofutofu

New Member
I tried connecting 3.3v to the BLT_ON from other source. the back-light is turning on, then I can see the picture blinking on the screen every second (wile the back light is continually turned on).

anything to do from hear?
 

duffy

Well-Known Member
Is it the backlight that's blinking or the LCD? Do both the CCFL's look equal brightness? Problem might be on the processor side, the IC761 controller does the shutoff for the CCFL tubes itself. Maybe check for a fluctuation on the various power supply voltages that is in sync with the blinking - could be something is loading the power supply down, and if so you might want to check the current draw.
 

tofutofu

New Member
hi duffy.
thank you for your help

1) does the IC761 controler you'r talking about can be the IC751 on my diagram & board?

2) it is not the backlight that's blinking. as I can see the bulbs turned on all the time, wile the picture is blinking.

3) I'm not sure if I understood u correctly (as my English is not perfect..) - Do u mean checking the voltages on random terminals to see if it steady ? or maybe the transform output?
 

tofutofu

New Member
checking the transform T601 onboard (not connecting to power), I could see that terminal 1,2,3,4,5 are all shortcut between each other. (making "beep" on the multimeter).
(is that normal or mean the transform is bad?)


checking T601 output voltage and "playing" the start button, I could see changes when green light off/on (not connected to VGA source):

terminal1=0.2 /0.7
terminal 2=0 /0
terminal 3=0 /0
terminal 4=3.0 /6.7
terminal 5=2.5 /5.6

(*I'm not sure if the unit are volts... as the multimeter is set on " ACV , 200")


checking the Transform Output (TO)(assuming it's AC) related to BLT_ON voltage (DC) wile connecting/disconnecting VGA source:
* checked the TO on the D702 terminal

1) NOT connected :TO=0.2 ,BLT_ON=0.0V

2) connected: TO=0.4. BLT_ON=0.0v

3) disconnecting:
TO: first no change (0.4) then raise to 0.8 and go down to 0.2 (when the green light shut down)
BLT_ON: first no change (0.0v) then raise to 3.3v and finely drop back to )0.0v (wile the green light shut down)

( in last case, when the BLT_ON raise to 3.3v' the backlight turn on shortly )
 
Last edited:

duffy

Well-Known Member
1) does the IC761 controler you'r talking about can be the IC751 on my diagram & board?
Yes, sorry.

3) I'm not sure if I understood u correctly (as my English is not perfect..) - Do u mean checking the voltages on random terminals to see if it steady ? or maybe the transform output?
I meant the labeled supply rails - 3.3V_MCU, V-INVERTER, V-AUDIO.

In particular, 3.3V_MCU. You can see now the inverter circuit is working, so it must be something resetting the MCU. Might be the power supply - could be dropping out, or just has noise from those bad caps you mentioned. Might be something else - what kind of signal are you sending to the monitor (VGA, svideo, etc.)? Have you checked for a bad connection in the connector or cable? Can you swap cables?
 

tofutofu

New Member
hi duffy

the 3.3v_MCU not changing (stable at 3.4V)

V-INVERTER is stable 14v when not connected to signal (VGA)
BUT when connected to signal (VGA) the V-INVERTER is moving up & down from 13.5V to 12V at frequency that might be same to the LCD blinking (hard to know exactly... )

I'm using VGA signal ,but have no other cable to change. (the connection looks good).

I cant measure the V-AUDIO, as my LCD has no speaker (and missing parts like D703 on power board )
 

duffy

Well-Known Member
Are you seeing that with BLT_ON tied high or with the backlight blinking? If the former, it could indicate a short somewhere else - but if not, that sounds perfectly normal.

Need to try a different video input source, with a different cable. At the very least, try different video settings - change the resolution and refresh rates. The backlight circuit itself seems to be ok, it's something on the MCU side that is resetting. Something on that VGA input might be the culprit.
 

tofutofu

New Member
when source conected, the back light is off. only when im giving 3.3v to BLT_ON from another source (lets say from 3.3_MCU) ,the back light terns on. then, It's not the backlight that blinking, it's the picture itself (the LCD bord). the back light is continually on .
another thing that happened- the pic is very big and the left side oriented (the left edge is cut).

(Does the BLT_ON need to be 3.3v when connecting to source? )

the VGA input is ok. as I tried few of them, and oconnecting them to other screens seams to work all right.
a bad cable is a possibnility. I will try to find another one and see. (can't I check it with multimeter?)

About MCU malfunction, what are the possibility's?
 
Last edited:

duffy

Well-Known Member
What happened when you changed the resolution and refresh rates?
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top