# benq fp91g+ (q9t4) monitor not turn on



## paulmars

benq fp91g+ (q9t4) monitor. Mostly wont turn on. Once in awhile it does 
and works fine. However, that's very seldom. The on off switch hot side 
is 3.3vdc. The switch grounds that when its pressed and my dvm then 
reads 0vdc. Every time. Ive traced it to the display circuit board 
plug, gray wire. So, it goes from 3.3vdc to 0vdc when the switch is 
pressed - Every time. Over and over I can do this and every once in 
awhile, the unit powers up and it works fine. however, the next time, 
it wont power up. 

Any ideas what could be wrong? All the caps look fine. Ive also 
inspected all solder joints.

tks,
pa


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## octaneman

Hi paulmars


Test the p-channel MOSFET it should have an FQ prefix, also look at cold joints re-solder the high voltage transformer pins. 



post back your findings.


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## paulmars

all solder joints look good. I dont have equip to test transistors.


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## octaneman

To continue troubleshooting you need to pick up a multimeter to test components.


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## paulmars

I have a DVM.


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## octaneman

OK, The first step is to locate the mosfets that are on the PCB, these are usually on heat sinks and are easily recognizable. Testing them is a simple process. First, the pins have to be desoldered off the board to get an accurate reading off the meter. You will need the data sheet to locate the pin designations of the component. Some mosfets have other components built into them like triacs or diodes so they need to be tested according to what the data sheet diagram tells you.


Here's a picture of a typical power mosfet that has a triac and zener diode built into it. The meter is set on diode test and the leads are placed accordingly. Meters that do not have the diode setting are set in the 1X and 10X resistance scale for testing the points. If you have an analogue meter instead of a digital the testing procedure is different.


Please take a picture of your motherboard to begin troubleshooting. 


post back your findings.


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## paulmars

they labeled D701 and D702. No other markings on them or the board.

more info: Benq monitor rarely turns on - Badcaps Forums

and this, too bad I cant read it: Forum for Electronics


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## paulmars

so, i cant get the datasheet. can i still test them?


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## octaneman

Yes you could, just remember to desolder the pins before you take readings. The readings on regular mosfets from gate to source and gate to drain are infinity. Readings from drain to source should have a reading but very small. When leads are reversed your readings should all be infinity. If any readings do not match up then the mosfet is gone..


Before you test out the mosfets you need to test your trasformer for the secondary voltages. You mentioned you are getting the 5v and the 3.3v but the rest of the pins are dead. You should have 15v or higher on some of the pins on the molex connector. 


Are you sure you cant see the numbers ??


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## paulmars

all this applies even though the unit sometimes works? When its on, it wont turn off. When its off, it wont turn on. When the power switch is pressed enough times it will change states. That usually means pressing the power button several hundred times, or just a few dozen times. Once in awhile, all I have to do is press it once or just a few times for it to turn on or off. However, its usually tens or hundreds of times.


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## paulmars

ref: Are you sure you cant see the numbers ?? 

yes, neither on screen or printed. i can see them, just cant read them. Bad resolution.


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## octaneman

paulmars said:


> all this applies even though the unit sometimes works? When its on, it wont turn off. When its off, it wont turn on. When the power switch is pressed enough times it will change states. That usually means pressing the power button several hundred times, or just a few dozen times. Once in awhile, all I have to do is press it once or just a few times for it to turn on or off. However, its usually tens or hundreds of times.





So the issue is that the on/off times vary, this changes the dynamics of the problem. It could be that there is just a leaky capacitor on the on\off OSD board or a bad micro switch that makes intermittent contact. Take your meter hook it up to the switch while doing a continuity test, press on the switch on/off constantly to determine any irregularity. Micro switches have 4 pins so use the meter to see which pins make contact. Do another continuity test by also taking the output from the switch to the molex connector, somewhere in between contact gets lost.


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## paulmars

Ive already done all that. please refer to the other forum. The switch is fine and Ive followed it all the way to the PCB.


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## octaneman

Have you done the voltage drop test on the switch with power on ?


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## paulmars

yes (the message u entered is to short)

.awnrf we riofjweio awe hjc urhjwe4 fcn wfu w weuio;fcn ukzxwdfi o

I hope that is long enough.


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## octaneman

The photocoupler acts as an isolator but it also switches the mosfet on to give power to the rest of the board. If its faulty it can give the intermittent on/off problem. You will need to test the coupler's diode and transistor by setting your meter on diode scale. Your meter should get a reading on one direction on the diode side of the coupler. On the transistor side of the coupler the meter should read one direction but when leeds reversed it shouldn't show any readings. If you get confusing readings on the diode scale of your meter set it to the resistance scale. On the resistance scale set it to 10x for the diode side and for the transistor side set it to 1x. 


post back your findings.


What in the king's english is this ?! ------> .awnrf we riofjweio awe hjc urhjwe4 fcn wfu w weuio;fcn ukzxwdfi o


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## paulmars

on diode side readings; infinite and 0.748. 

On trans side readings: 0.9 and infinite. 

Meter diode scale.


trouble posting back to this site. Keep getting:
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because the token has expired.
Please reload the window.


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## paulmars

reloading the page did not fix. I needed to close and open browser


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## octaneman

paulmars said:


> on diode side readings; infinite and 0.748.
> 
> On trans side readings: 0.9 and infinite.
> 
> Meter diode scale.
> 
> 
> trouble posting back to this site. Keep getting:
> Your submission could not be processed
> because the token has expired.
> Please reload the window.





Readings you took are ok. 


Take voltage readings on all your transformer secondary windings because something isn't right. 

post back your voltage readings.

Question:

Do you have the 5v standby ?


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## paulmars

It might take me awhile to get it to turn on. I been pressing the pwr sw all day.


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## paulmars

Your submission could not be processed 

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## paulmars

again I had this issue and nothing but closing and opening browser will allow me to post. When it displays this message it also deletes all the text i entered in the quick reply.


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## paulmars

It turned on this morning on the 1st press. Voltages on transformer are not steady and it turned to standby mode before i finished reading all 5 points. I turned it off and back on and the readings were again different.

left most pin: 0.06, 2.4, 0.5, 2.5, 20, 18, 25, 16, 1.4, 11.6
next pin: 22, 25, 2, 20, 3, 1.8, 4, 0.7

other pins varied likewise. Right 2 most were negative voltage.

3 times in a row, I pressed switch and it turned off, I pressed again and it turned on. Its like this. switch will sometimes be totally unpredictable, otherwise it keeps turning off and on. Actually its really not too unpredictable. Most times it consistently turns off and on or consistently will not turn on, unless its on, then it will not turn off.


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## octaneman

The readings you took on the transformer are confusing. There shouldn't be that many as you posted. One coil of the transformer represents the number of pins that are present at the molex connector, the voltage coming out from the transformer have to correspond to the connector. 

Please re-do the test again because the voltages that are coming out from the transformer* Only * is A/C not DC. The voltages that are *On * the pins is D/C.


Next to the optoisolator from what I see is a transistor, this should be the driver for the coupler. Have you tested it ? If this overheats it sends wild signals to the coupler, when the set is on does it get hot ?.. ( please be careful when touching it) 


Also look at the capacitor C602 these capacitors leak all the time, set your meter to the high ohms scales (desolder the pins first) see if it charges and discharges at a consistent rate. If you get erratic readings then something is wrong with it. 



On the next posting the next test you'll do is the ripple test on ALL the capacitors. 


post back your findings.


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## paulmars

the readings vary at each transformer pin. The transistor is not hot or warm. The unit is powered up on standby.


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## octaneman

Ripple test.


Power up the board. Take your meter set it on A/C scale. Test * ALL * polarized capacitors (no exceptions). Since you are limited to test equipment, this is the only test that can determine leaky capacitors. Keep in mind that even if capacitors look in pristine condition, it doesn't mean that they are still good. Set your meter to A/C to the capacitor voltage value. Your meter readings should be 0.00 there should be absolutely no leakage. If you get any readings then the capacitor leaks. Capacitors block D/C but allow A/C to pass, this test is critical you need to know if they are doing their job. 



post back your readings.


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## paulmars

only 2 caps that I can test from top side PCB. One 23 volts ac across the leads (c606), the other 0 volts. The remainder, I will need to remove and turn over the board.


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## octaneman

After re-reading your posts I'm convinced its from the power mosfet. Due to the high voltages that run through it, it must be shorting out causing the power/on off issue by overheating. Using cold spray on it would give us a better a picture, but since you dont' have it we'll test it old school. 

Some of these things have built in damper diodes and some do not. Post the number or the datasheet so I can show you how to test it.


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## paulmars

even though the unit never turns itself off? Another words, once its on, it stays on even for days. In standby mode that is. So that one cap w/23v is ok? I was going to remove and flip the boards to test the remaining caps. 

datasheet? Here is the schematic: Forum for Electronics


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## octaneman

The MOSFET is a 2SK3264 which has a damper diode built into it. You will need to desolder the pins to get accurate readings. 
Here's how to test it (pic).
(thanks for the schematic)


Addendum.

Look for R701 on the board its a thermal resistor. Thermal resistors give readings ONLY they get too hot. When it conducts it acts as a thermal shutdown when voltage or heat is too high. When you take readings of it when the board is dead cold it should read open on your meter. If you do get readings then R701 could very well be your culprit.


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## paulmars

Your submission could not be processed because you have logged in since the previous page was loaded.

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that DONT work !


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## paulmars

i decided to check R701 1st. However, I cant find it. Its not on top. On bottom, many of the component markings are not readable. 

This forum requires that you wait 30 seconds between posts. Please try again in 17 seconds.


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## paulmars

of note:

my transformer readings were not right. i tested the leads coming out of it on the top side PCB. however, looking on bot side, i see these are not the same points that are electrically connected to the foil underside of PCB.

I can now read the schematic.

Im reinspecting all solder joints and i see that 2 terminals on a small IC on bot of PCB. the two terminals are bridged together with solder. See pic.


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## paulmars

pics solder bridge


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## paulmars

Im sure of the solder bridge above. its not even touching the board. Look at this next pics. Several places where it looks like the solder bridged 2 or 3 terminals, but in not sure.


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## octaneman

paulmars said:


> pics solder bridge






Doesn't look to be like a solder bridge its more like debris stuck in between the pins.


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## octaneman

paulmars said:


> Im sure of the solder bridge above. its not even touching the board. Look at this next pics. Several places where it looks like the solder bridged 2 or 3 terminals, but in not sure.





Its bridged but thats how the pcb is made. There's nothing out of the ordinary that I see.


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## octaneman

On the schematic there is a NCP1200 Pulse Width Modulation chip that turns on the power mosfet Q601. Its Vcc is 15v (are you getting 15v?) but since you don't have an oscilloscope we cant see what frequency its switched or if its pulsing. You need to look carefully at IC601 and its surrounding circuits IC602 the push/pull transistor circuit Q602, Q683 on the diagram. IMHO the key area is between the power mosfet and that circuit because the data sheet on it if there is no pulse the power mosfet won't switch on.


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## paulmars

octaneman said:


> Its bridged but thats how the pcb is made. There's nothing out of the ordinary that I see.


Solder is not even touching the board. Its between two leads on that IC, above the PCB. hanging between the two leads.


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## paulmars

IC601 Vcc 0.00 volts w/unit powered up.


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## octaneman

paulmars said:


> IC601 Vcc 0.00 volts w/unit powered up.




Something is not right. You can't get 0v with unit powered up unless you placed both your leads on the same voltage rail. The I.C601 needs the 15v to fire up because its clamped at 15v. The output is pin 5 is what drives the power mosfet, pin 5 has got to have a base minimum of 5v to turn on the power mosfet. 


On the diagram I've marked in red trace of what the voltage on the rails should be. In blue I've circled the push/pull transistors, the NPN amplifies the top wave of the oscillator and the PNP the bottom wave. When you press the on/off button I.C 602 turns on. Once its on the 2 transistors conduct, now if either one of these transistors is faulty you have the on/off problem. When they work properly the transistors then turn on IC601, which at pin 5 is the oscillator output that switches on the power mosfet which powers the whole board.


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## paulmars

your right. I had wrong IC pin. IC601 pin 5; now i get...well, it was 4.73 but it keeps climbing. i stopped at 29.7 volts DC. When the meter is off the lead for a few seconds, the value keeps climbing. leave meter off for a few minutes and the VDC is less then last reading, but still above 4.x and is climbing again.


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## paulmars

after a few:

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## paulmars

correction. those readings on pin 5 only present and climbing when my body was touching the circuit ground. When Im not touching the case gnd, the voltage is zero (0.00)


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## paulmars

to reiterate. IC 601 pin 5 0.00 volts with unit either ON or STBY.


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## octaneman

paulmars said:


> correction. those readings on pin 5 only present and climbing when my body was touching the circuit ground. When I'm not touching the case gnd, the voltage is zero (0.00)



Ok, typically on the motherboard there are 3 types of grounds. Circuit ground, floating ground and chassis ground. When you touch the board your body is creating the return path thats how you got the voltage readings first time around. When your body was not touching the motherboard and getting "0" volts, you have a "floating ground". The mosfet needs 35v to kick in, so when your meter was climbing it was because of the capacitor C611 so essentially your voltage readings was correct. 


2 Questions; 

1) Have you tested the push/pull transistors ? 

2) Is your meter equipped with a frequency setting ?


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## paulmars

I was touching the metal that the board is mounted to. So, when not touching it. lead 5 = 0.00v

no and no.


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