Benq Q7T4 / FP71G+

[jt3204]

Hey all, I have a FP71G which is turning off about a second or so after it is brought out of standby.
Mains power to monitor > monitor standby
Push on button > power led turns green and display turns on (Benq logo displayed)
2 seconds later > monitor returns to standby.

Interestingly, there has always been a slight high pitched whine from this monitor whenever it was on standby. Not really knowing much about the internals of a computer display I ignored it. It seems that this has come back to bite me on the ass.

Pulled the thing apart and it appears that there is nothing visually out of order (solder, connections etc all look perfect). The only thing I have to go on is the sound.

Scenario 1:
Plug mains in > high pitch to slightly lower pitch (is this the sound a bad capacitor makes as it attempts to charge?)
Unplug mains > (if video driver board is attached (the one with the input ports)) the pitch goes up and down as it appears something is discharging.
(If video driver board is detached) the pitch goes up and up over a period of about 20 seconds, then all over the place for about 3 until it “dies”

Scenario 2:
Plug mains in
Push power button (whether the boards are connected to the LCD or not does not appear to matter as far as this sound is concerned, so all this testing was carried out with only the two boards and the bezel buttons connected.
Quick (less than a second) discharge sound, followed shortly by two similar sounds, then silence for a second, then back to standby noise/condition.
This can be repeated over and over with the same outcome.

The charging/discharging noise appears to be coming from capacitor C607 or somewhere very close nearby. I wouldn't suspect the resistors (they have been tested and are fine), the diode is working, but there are two small silver cylindrical components (L605 and L604) which I cannot identify. I believe it is C607, but sound can only get you so close. Who knows, it could be coming out of the side of nearby transformer(?) L602.
Please see the attached pictures of the nearby circuitry.

Any ideas? I don't have an ESR meter, so I can't test the capacitor.

jt

[retiredcaps]

Re: Benq Q7T4 / FP71G+

While you didn't show a picture of the entire board, we have seen enough of these Benq boards to know that

a) you must resolder all the inverter transformer pins
b) resolder the c5707, fu9024 pins

A high pitched whine could be the first warning sign of a capacitor going bad. My own guess would be C511. I had a horrendous whine and it was due to the small cap even though it wasn't bloated.

[PlainBill]

Re: Benq Q7T4 / FP71G+

Good pictures, but your description isn't clear. You could be describing normal operation, two seconds to black, or a monitor which is not recognizing the video signal. A SMPS power supply will generate some noice; this is normal.

I would suggest supplying several additional pieces of information.

1. Attach top and bottom pictures of the power supply and the logic card. Make the resolution as close to 2000 x 2000 as possible. We've spent too much time troubleshooting monitors with 'no visible bad components', then seeing several obviously bad caps when pictures were supplied.

2. Carry out the following test scenario.
A. With power off, hook the monitor up to a known good computer, which is also turned off. With the computer still off, plug in the monitor's power cable. Does the power LED Turn green, then turn orange after a few seconds or does it come on and stay orange, or does it come on briefly and turn back off?
B. Now turn on the computer. Does the monitor power LED turn green? (If it was initially off, does it turn on when you hit the power button?) Does the monitor display the start-up (post) screen? Does the screen stay on as the computer boots?
C. If the screen goes blank does the power LED stay green? Does it turn orange? Does it go black?
D. If the screen goes black can you see activity on the screen if you shine a flashlight on it?

PlainBill

[jt3204]

Re: Benq Q7T4 / FP71G+

Retiredcaps: I shall try that if nothing else comes up, but it would be appreciated if you could point out which of the transformers is the inverter (I am assuming the largest one in the middle but correct me if I'm wrong). I have found C5707, but cannot seem to locate FU9024 or C511 – am I going blind? Where about on the board should I be looking?

PlainBill: It is not the standard “no signal” response, but I will run thought your tests. Also attached are some high def pictures.

I should note that during these tests I realised that my wording was misleading in my initial post – the monitor never goes to standby, ever. Only off (no LED) and attempt to turn on (brief green LED) when the power button is pressed. Sorry about that.

A: Plug monitor into working computer. Plug monitor power in. Power LED black. I assume you missed the part where I need to press the monitor power button to turn it on. Essentially the same thing as before happens. If I press the monitor power button it turns on for a second (LED green), the benq logo comes up for under a second (less than it usually would), then it turns back off (black LED).

B: Same as above

C: Power LED turns off (black).

D: No activity, it's definitely dead.

Next steps?

Thank you both for quick replies
jt

PS. Ignore the dark patches in two of the photos - that's just me darkening the overexposed part of the photo in Photoshop so it can be seen.

[sabre504]

Re: Benq Q7T4 / FP71G+

Hi i have had the same problem on a board exact same model
Did the usual and replaced all 4 5057 +2 x fu9024

Noticed on your board pics a burn mark same place i had on mine
have circled in red on mine i used another fu9024(as i couldnt read numbers on it ) to replace this as i was informed that did the same function
Obviously i had to trim middle leg but Plainbill or Retiredcaps may be able to tell you correct part number

[sabre504]

Re: Benq Q7T4 / FP71G+

Hi sorry just read your other post
The 9024's are circled in yellow

[PlainBill]

Re: Benq Q7T4 / FP71G+

Retiredcaps: I shall try that if nothing else comes up, but it would be appreciated if you could point out which of the transformers is the inverter (I am assuming the largest one in the middle but correct me if I'm wrong). I have found C5707, but cannot seem to locate FU9024 or C511 – am I going blind? Where about on the board should I be looking?

PlainBill: It is not the standard “no signal” response, but I will run thought your tests. Also attached are some high def pictures.

I should note that during these tests I realised that my wording was misleading in my initial post – the monitor never goes to standby, ever. Only off (no LED) and attempt to turn on (brief green LED) when the power button is pressed. Sorry about that.

A: Plug monitor into working computer. Plug monitor power in. Power LED black. I assume you missed the part where I need to press the monitor power button to turn it on. Essentially the same thing as before happens. If I press the monitor power button it turns on for a second (LED green), the benq logo comes up for under a second (less than it usually would), then it turns back off (black LED).

B: Same as above

C: Power LED turns off (black).

D: No activity, it's definitely dead.

Next steps?

Thank you both for quick replies
jt

PS. Ignore the dark patches in two of the photos - that's just me darkening the overexposed part of the photo in Photoshop so it can be seen.

Darn good pictures. Sabre504 has provided some information. I would suggest also measuring the resistance across the leads of Q808, Q809, Q815 and Q816. Odds are one or more of them are shorted.

A common scenario is shorted transistors in the inverter cause the power supply to overload and shut down. Usually the supply will try to restart, but that is not always the case.

PlainBill

[retiredcaps]

Re: Benq Q7T4 / FP71G+

b) resolder the c5707, fu9024 pins

In my haste to leave yesterday, I forgot one key action in my sentence

"check for shorts and resolder c5707 and fu9024 pins"

Also check pf801 for a blown fuse.

edit: If any of these are bad, there are "kits" sold on ebay for roughly $5. Just search for "c5707" and sort by lowest price. I have purchased these kits to fix similar Benqs boards and they work fine.

[jt3204]

Re: Benq Q7T4 / FP71G+

I get a resistance of around 1 ohm over the emitter and base of Q808 and Q809, as compared to much more (5k ohm) over Q815 and Q816. I guess that’s a short in 8/9 if ever there was one! Q812 and Q805 also check out alright (25k ohm E --> B).

All the other similar transistors appear to be ok and seem to have good connection (visual and electronic check). Retiredcaps, PF801 = 1ohm. If it’s a fuse, I’m assuming that’s ok?

Sabre504, IC701 tests ok – plenty of resistance between pins. I think it’s some sort of adhesive on it rather than a burn mark?

So are these transistors that are shorted “c5707”’s, or
“fu9024”’s? Or are they actually the same thing? My guess is the 4 x Q808/9/15/16’s are c5707’s and the two Q815 and Q816 are fu9024’s. But it would be a real pain to get the wrong part!

Once I can ID replacement transistors I will get some off eBay, as well as a cheap 60W soldering iron, some wick, and rosin core solder (found a nice low cost store in Taiwan). Gosh it’s been a long time since I’ve soldered anything now that I think about it!

So how exactly am I supposed to avoid soldering transistor pins together, seeing as they are so close?

jt

[retiredcaps]

Re: Benq Q7T4 / FP71G+

I get a resistance of around 1 ohm over the emitter and base of Q808 and Q809

Definitely shorted. Replace both.

Retiredcaps, PF801 = 1ohm. If it’s a fuse, I’m assuming that’s ok?

Yes that fuse is good if it reads 1.0 (unit) of ohms. That 1.0 ohms should be on the right hand side of your multimeter.

"1" on the left hand side of the multimeter means out of range or infinite. That means the fuse is blown => bad.

Both my autoranging meters using 0L to indicate out of range. Manual meters using "1" on the left hand side of the display.

So are these transistors that are shorted “c5707”’s, or
“fu9024”’s? Or are they actually the same thing? My guess is the 4 x Q808/9/15/16’s are c5707’s and the two Q815 and Q816 are fu9024’s.

So how exactly am I supposed to avoid soldering transistor pins together, seeing as they are so close?

If you look at the chips, they are written right on there. If not, the Q812 and Q805 are the fu9024 fets. The Q808/809/815/816 are your c5707 transistors.

I have done this repair and you need a fine tip on the soldering iron. It isn't that difficult as it seems.

I found this great soldering video by CuriousInventor on youtube. See

http://www.youtube.com/watch?v=I_NU2ruzyc4

I will remind you again that you must resolder all the inverter transformer pins to prevent future malfunctions.

[sabre504]

Re: Benq Q7T4 / FP71G+

IC701 there is definite scorching on the opposite side of the board
So maybe it has already been replaced once

[drabara]

Re: Benq Q7T4 / FP71G+

My psu board is not the same as displayed, and Q805 and Q812 are some sort of GK1C1545, are this an equivalent of the FU9024N ?

[retiredcaps]

Re: Benq Q7T4 / FP71G+

My psu board is not the same as displayed

Please post a clear focused picture of your power board using manage attachments. Please do not post inline pictures.

[drabara]

Re: Benq Q7T4 / FP71G+

Here is the board, i replace almost all the caps, and the fuse and the 5707, but i desolderd the Q805 and it is shorted. so i need to replace it.

[retiredcaps]

Re: Benq Q7T4 / FP71G+

Q805 and Q812 are some sort of GK1C1545, are this an equivalent of the FU9024N ?

I can't find a datasheet for GK1C1545, but it seems that Benq changes the components quite a bit depending on the board revision.

See this thread

http://www.jestineyong.com/?p=534

It asks similar questions about Q805 and Q812 so I'm guessing fu9024 will work based on replies there?

Many sellers on ebay are selling c5707 kits and they come with the picofuse and fu9024s.

[drabara]

Re: Benq Q7T4 / FP71G+

i will ask him what he thinks ! thanx

[driftwoodsound]

Re: Benq Q7T4 / FP71G+

I stumbled across this searching out the reasons for the exact BenQ monitor issue mentioned here. I have four of the same monitors all with the same problem.
This will be my first real attempt at this level of electronics so I'm trying to cover all my bases and making sure I do everything right.

I've numbered some points in the attached images to make this a bit easier.

I ran the test scenarios suggested by PlainBill with the following results:

A: Computer off, monitor off, VGA cable in, IEC out.
When the IEC is attached the monitor does nothing.
When the power button is pressed the LED lights up green, the POST screen flashes up for half a second, then the screen and LED go black.

B: Computer powered on
When the computer is switched on the monitor does not respond at all. After a minuet I pressed the monitor power switch, which provided the same results described in 'A'

C: LED state with blank screen.
See 'A'

D: Shining a flashlight on black screen.
No image is visible.

Sabre504 mentioned scorch marks and having to replace IC701, labelled '2' in my pictures. What is this part and are the scorch marks significant? There is a second scorch mark on my boards, labelled '1' in the pics. What could have caused this one, and is it something to worry about?


PlainBill suggests measuring the resistance across the leads of Q808, Q809, Q815 and Q816. I see these components have three leads. Which two of these should I measure the resistance across?


Retiredcaps suggested to check and resolder C5707 and fu9024, as mentioned above, I'm not sure which leads to check.
I have checked PF801 with a reading of 0.01. From what I understand that bad and needs to be replaced, correct?



Driftwood

[PlainBill]

Re: Benq Q7T4 / FP71G+

Scorch marks at 1 & 2 - common for the Benq monitors from this time.

Cap at 3. Should be replaced. This may be responsible for other problems.

Transistors at 4, 5, 6, 7. Note the transistor at 4 has the leads labeled E C B. While they are not visible in the picture, the leads of other transistors are labeled the same

Test 1. Set the DMM to the 200 ohm range. On each transistor measure the resistance E-C, C-B, E-B. Repeat for the other three transistors. Report the measurements.

Transistors at Q805 and Q812. Make the same measurements.

The reading indicates PF801 is good.

PlainBill

[retiredcaps]

Re: Benq Q7T4 / FP71G+

If you need to replace any of the transistors, fets, or picofuses, search for "c5707 kit" on ebay. I purchased some myself for Benq boards.

You must also resolder the inverter transformer pins.

[sabre504]

Re: Benq Q7T4 / FP71G+

The chip 2 i mentioned looks to have burned out i have had this on one of these boards before , this one is not in any of the kits i have seen.
But this can be replaced with an FU9024 but you will have to trim the legs
unless you can find the correct replacement