Benq Q7t4 / Fp71g+

[scb10]

Hi All,

I have 5 of the above LCD's with all the same problem (and three OK). Lucky to have so many hey?

On turn on the LED goes green and the BENQ logo is displayed, then the desktop is displayed and 'nonpreset mode' is flashed up before the screen goes blank. The power led then goes amber (stanby). Changing the resolution on the PC (I hooked two LCD's up on a dual video card to do this) will briefly get the BENQ to display and then die again, but the 'nonpreset mode' does not always flash up. No resolution mode will make the BENQ stay on.

I can tell you it is not the Inverter Power Board but it is the Interface Board (M170EP01 V2). I have swapped the IP Board and it works on a known good screen.

Any ideas would be appreciated.

Jamie

[PlainBill]

Re: Benq Q7t4 / Fp71g+

Hi All,

I have 5 of the above LCD's with all the same problem (and three OK). Lucky to have so many hey?

On turn on the LED goes green and the BENQ logo is displayed, then the desktop is displayed and 'nonpreset mode' is flashed up before the screen goes blank. The power led then goes amber (stanby). Changing the resolution on the PC (I hooked two LCD's up on a dual video card to do this) will briefly get the BENQ to display and then die again, but the 'nonpreset mode' does not always flash up. No resolution mode will make the BENQ stay on.

I can tell you it is not the Inverter Power Board but it is the Interface Board (M170EP01 V2). I have swapped the IP Board and it works on a known good screen.

Any ideas would be appreciated.

Jamie

Good job of troubleshooting.

Just the usual suggestions. Post a picture of the logic board (AKA interface board; control board). Look for bad caps with an ESR meter. Hope it's bad caps; if the main control chip is bad you are probably SOL.

PlainBill

[scb10]

Re: Benq Q7t4 / Fp71g+

Thanks PlainBill,

Well as you can see it is all Surface mount components. I was hoping that the fact that I have 5 of these all with the same fault that there maybe somone out thier that has had the same fault and fixed it! I know I will be pushing it up hill to get BENQ to give me an answer.

The black heatshrink on the board is my attempt to change the crystal from a known good board. Unfortunatly the tracks went well under the crystal and resulted in the ripping of tracks to get it out. Even with a good lead-free soldering iron. There is one fuse that is OK. I have checked diodes and transistors and a few caps and all seem OK. I am getting to the stage where it may be one of the control IC's but it also seems like a crowbar circuit is operating as it will stay on for at least 2-3 seconds!

Cheers
Jamie

[bgavin]

Re: Benq Q7t4 / Fp71g+

Look for bad caps with an ESR meter.

Can this be done with the cap in-circuit?

[PlainBill]

Re: Benq Q7t4 / Fp71g+

Thanks PlainBill,

Well as you can see it is all Surface mount components. I was hoping that the fact that I have 5 of these all with the same fault that there maybe somone out thier that has had the same fault and fixed it! I know I will be pushing it up hill to get BENQ to give me an answer.

The black heatshrink on the board is my attempt to change the crystal from a known good board. Unfortunatly the tracks went well under the crystal and resulted in the ripping of tracks to get it out. Even with a good lead-free soldering iron. There is one fuse that is OK. I have checked diodes and transistors and a few caps and all seem OK. I am getting to the stage where it may be one of the control IC's but it also seems like a crowbar circuit is operating as it will stay on for at least 2-3 seconds!

Cheers
Jamie

A picture of what you refer to as the inverter / power board would be appreciated. The circuit board you are showing in the picture is what I usually refer to as the panel circuit board.

So far every LCD monitor I have worked on consists of two or three boards. The power supply, the inverter, (these are often combined onto one board) and the logic board. The logic board has the VGA and DVI connectors, the video conversion controller, and the connectors to the buttons and to the LCD panel.

I can't think of any reason why the logic board couldn't be integrated with either the power supply / inverter or the panel circuit; it's just that I have never seen one - yet.

PlainBill

[PlainBill]

Re: Benq Q7t4 / Fp71g+

Can this be done with the cap in-circuit?

Yes. Understand that often the caps are in parallel, so if a cap is supposed to have an ESR of less than .1, three in parallel should read about .03.

PlainBill

[scb10]

Re: Benq Q7t4 / Fp71g+

Sorry I am lost as to where you are going with the caps! I have placed the Inverter Power Board into a know good monitor and it works, so that board is OK. This model has two boards only. The Power/Inverter board and the main Interface Board (M170EP01 V2) which I have proven to be the fault. Only two circuit boards, no more.

"logic board has the VGA and DVI connectors, the video conversion controller, and the connectors to the buttons and to the LCD panel." Yes this is the board I have a fault with PlainBill.

I am still checking the componenets but as you can see they are all surface mount components! I would have thought that if I had 5 with exactly the same fault that someone out there may have had a fix or heard from BENQ on this issue.

Thanks again

Jamie

[scb10]

Re: Benq Q7t4 / Fp71g+

I will get a photo of the board up for you. Oh and it is an Australian model, not that I would think that that would make any difference!

Cheers
Jamie

[scb10]

Re: Benq Q7t4 / Fp71g+

Here is a photo of the power board.

[PlainBill]

Re: Benq Q7t4 / Fp71g+

Well, they have definitely combined the logic board with the lcd panel board. That makes it much more interesting. The only thing that seems strange is that while I can see what appears to be voltage regulators on the logic / panel board, I don't see any capacitors.

PlainBill

[tmwalsh]

Re: Benq Q7t4 / Fp71g+

Mr Plain,
I don't get it. What is showing on my screen is the power board. I didn't understand why it was being looked at, as the problem was determined to be in the QUOTE:

"logic board has the VGA and DVI connectors, the video conversion controller, and the connectors to the buttons and to the LCD panel." Yes this is the board I have a fault with PlainBill."

IMHO, what is needed is a picture of the logic board, not the power board.

tom

[PlainBill]

Re: Benq Q7t4 / Fp71g+

Mr Plain,
I don't get it. What is showing on my screen is the power board. I didn't understand why it was being looked at, as the problem was determined to be in the QUOTE:

"logic board has the VGA and DVI connectors, the video conversion controller, and the connectors to the buttons and to the LCD panel." Yes this is the board I have a fault with PlainBill."

IMHO, what is needed is a picture of the logic board, not the power board.

tom

This fooled me too. Look at the pictures of the 'panel board'. Look at the connectors and the ICs on that board. Notice anything strange?

Benq has combined the logic board and the panel board into a single board. The two connectors are for the DVI input (which can handle both digital and analog signals) and the front panel switches.

PlainBill

p.s. It's PlainBill. No Mr Plain, definitely not Mr. Bill, and salutations such as Sir, or Your Highness are undeserved.

pb.

[scb10]

Re: Benq Q7t4 / Fp71g+

Thanks Your Highness & tmwalsh

There are no more boards in this LCD, just two, and that the fault is on the Interface Board or could it be the fluro's?

Just thought someone may have come across it.

Cheers
Jamie

[PlainBill]

Re: Benq Q7t4 / Fp71g+

Thanks Your Highness & tmwalsh

Hey!!! None of that!!!

There are no more boards in this LCD, just two, and that the fault is on the Interface Board or could it be the fluro's?

Just thought someone may have come across it.

Cheers
Jamie

I don't think so, but it bears investigation.

Usually there is one-way communication between the logic board and the inverter. The logic board sends two signals to the inverter - On / Off and Brightness. The inverter monitors the voltage across the CCFLs and the current through them. If either is out of spec for any CCFL the inverter controller shuts all of them down. That information is not relayed to the logic board. The fact the power LED goes orange after the desktop is displayed for a few seconds indicates either the logic board is not sensing a signal, OR Benq has added a signal which informs the logic board when the inverter shuts down.

Are there any markings on the power supply / inverter or the logic board which indicates what signals are on the cable that connects the two?

PlainBill

[scb10]

Re: Benq Q7t4 / Fp71g+

I did find this service 'guide' that may help others in future.



About 2.4Meg. The lodgic board is not to similar though to mine.

My thoughs are the same in that something is telling it to shut down into sleep mode.

[PlainBill]

Re: Benq Q7t4 / Fp71g+

I did find this service 'guide' that may help others in future.



About 2.4Meg. The lodgic board is not to similar though to mine.

My thoughs are the same in that something is telling it to shut down into sleep mode.

A quick way to do this is to check the voltages on the cable connecting the power supply / inverter to the logic board. If the voltages remain the same with lamps on or off as you switch resolutions, it is a problem with the inverter or CCFLs.

Most of my experience is based on repairing Viewsonic, Acer, and Gateway monitors. In my (limited) experience they did not blank the backlights if the resolution changed, but did pop up a message announcing the new resolution (that message can be suppressed by changing a menu setting). This weekend I repaired a Dell E172FPb (built by Benq). On that monitor if the resolution changed the power button would go from green to orange and back again, (and the backlights may have turned off).

A quick summary of how the backlight system works on most monitors. The logic board sends signals to the inverter to turn it on (and set the brightness.) Initially the inverter turns the CCFLs on at full power. After a few seconds the inverter controller reduces the brightness to the set point and begins monitoring the voltage across and the current through each CCFL. If any of these are out of spec, it shuts down all CCFLs but has no way to tell the logic board this happened.

On those monitors if a CCFL fails the symptoms are a steady gerrn power light but a black screen. If you use a very bright light you can see the desktop (or whatever).

PlainBill

[tmwalsh]

Re: Benq Q7t4 / Fp71g+

Upon re-reading, I noted this: "desktop is displayed and 'nonpreset mode' is flashed up before the screen goes blank."

Being ignorant, I will ask. Does "nonpreset mode" mean that the Benq is trying to operate in an odd video display mode, i.e., instead of 1024X768 (which I assume is a 'preset mode') it is operating in 1099X994 (just made up..) mode, and doesn't like it?
If that is the case, it may be shutting down to protect its 'drivers' from blowing up, or the screen from being 'burnt' by some weird combination of numbers, such as when you have vertical failure in a crt set, you can burn a line across the screen relatively easily.
For what it is worth, I would try to set up the resolution in the 'menu' system of the Benq versus letting it 'learn' from the h/v rates coming into on the video connecter.
A quick test would be to tak a working monitor and pull up the menu w/o connecting to a video source. If it is possible, it would also be possible on the failing machines. Set the resolution to a proper pre-set value combination and give it a whirl. I am leading to the point of you may be having an EEprom or NVRAM failure where it has been written to so many times it won't work any more, or is just not working. Some computers 'discover' their HD and CD/DVD configuration every time they are booted. The information should be saved in the 'setup' screens, but isn't. So, the memory is being cycled(written) on every boot, which can add up. Just thinking outside the box.
FWIW, I have a KVM switch on my desktop so I can switch between operating systems on two computers. Now and again, it will boot up with a message along the lines of "not anywhere near close to a valid HxV configuration". [well, I made up the words cuz my old brain doesn't remember] The only fix is to shut down the computer, blindly, and reboot. At which time, it normally works fine...??? I think it is a glitch in the KVM sending or blocking a signal that the monitor is expecting, or sending cr*p instead of a valid signal. But, what do I know?
tom

[scb10]

Re: Benq Q7t4 / Fp71g+

FIXED!

One of the techs here found a posting in another forum that led him to belive that the fluro's were the issue.

BINGO.... The ends short with the chasie and cause the LCD to shut down. We got replacement fluro's from the USA on eBay.

SO PLAINBILL you were barking up the correct tree. It was the CCFLs.

Thanks all for your inputs. We now have 6 of these so I would imagine this to now be a common fault.

Cheers
Jaden

[Bobdee]

Re: Benq Q7t4 / Fp71g+

This thread would be a great asset to the TROUBLESHOOTING COMPUTER DISPLAYS forum (where it should have been posted in the first place)
It has a Panel/Logic Board that I believe has never been seen on the Forum before
Also it is a first for any Panel/Logic board to the Forum
More importantly it seems as if there is a signal sent back to the logic board from the inverter telling it to go into stanby mode when there is a fault found in Inverter or CCFLs (I believe thats a first as well)
Please post your views
Bob

[scb10]

Re: Benq Q7t4 / Fp71g+

Just a bit more. It is a big job to replace them. Look for blackend ends of the CCFLs (PHOTO'S ATTACHED). They must break through the glass. When they were replaced they are extreamly easy to break.

Sorry for posting in wrong forum. Did not realise there were others! But more the merrier.

Jamie