Need help to repair dead monitor power supply

[Agnatorian]

Hey guys,
I need urgent help to repair my moitors power supply. Over the last few days it just died on me a few times and needed a lot of time to turn back on until it died completely today. Only the power button is flashing once when I start it and nothing else happens. Then it needs to take around 2 minutes untill I can enable the power again to get the flash again.
Already found out that the primary capacitator needs to discharge for the light to flash. But on testing it it worked just fine.

I also checked all the other Capacitors and replaced them with new ones in the hope it might fix something but I had no luck. Also checked the fuse but only got a reading of 0 Ohm so that one should be fine too. Same for the Rectifier (Checked for the correct values) and the main Transformers.

I could really need help to find out what could be causing the problem. Images of the board are attached.

 

[shrtrnd]

Is that a shadow, on 2nd picture (back side of board lower left corner, larger three-terminal device), or is that component burned?

 

[Harald Kapp]

This one?

Doesn't look burned, no signs on the pcb.

 

[Agnatorian]

Is that a shadow, on 2nd picture (back side of board lower left corner, larger three-terminal device), or is that component burned?

If you mean that one it was me installing a new capacitor and burning the board a bit. This part is still working though.

I really cant figure out what it might be

 

[73's de Edd]

Sir Agnatorian . . . . .

¿ Whyforyounogivemonitorsbrandandmodelnumber ?
¿ Whyforyouhavenewcapacitorswithfulleadsandyoutheninstalltheminthatcrudeandslipshodmanner ?

Guess that leaves me to relying on my "reading of the board" and guess that the unit is a KB monitor using a TUO8Q164 P.S. board.

With you mentioning of it working sometimes, but lately . . . that is being iffy.

I have the board markup sectioned into the AC line "HOT" portion being all within the YELLOW markup that also connects into the BLUE star power transformer within the RED rectangle, with the custom RED star switch mode driver IC # IC602 that drives this transformer.
Come back with its marked on numbering . . . as it might be needed later on, in analysis..
PLUS I have never seen THAT specific surface mount 12 pin unit being used.
On that transformers secondary, there are being two isolated / cojoined secondary windings that are marked up with BROWN and VIOLET rectangles.
They create two different DC supplies.
The BROWN feeds the D711 diode with it outputting its DC into the C711/A capacitor.
The VIOLET feeds into the D701 diode and it outputs DC into the C701 /C, C705 /D and then passes to C703/E beside the CN701 connector down at the end of the board..

TESTING . . . .
With the board dormant and having had no power applied to it for some time, initially do an ohms test across the PINK boxed mark up of the parallel-o-series connection of R609+R610 and R607 + R608 " bump up " surface mount resistors. The end to end reading should be ~1 Megohm.
(No problems being expected there . . . .)
Then plug in all connectors and power cord and set metering to DC voltage and be expecting that there will be ~320-340VDC present across the main raw DC power supply capacitior C605 / YELLOW F
. . . I'm expecting it to be there.
Then you move over to the safer lower voltage portions of the supply, first to the . . . now easily accesible "high riding" terminals of C711 A capacitor OR you can use meter ground to bare tinned Jumper Wire 3 and meter + probe to the cathode / silver banded end of D711.
Then you power up the set with its power switch to see what DC voltage you hopefully might have there now. I'm sort of expecting it to be there.
The other supply voltage to now check for will be derived from the VIOLET D701 diode.
Check for it being down at the end of board connector CN701, with either of the two black wires being used for meter DC negative and the meter DC positive lead can go to either of the 2 RED or 2 ORANGE wires . . . as they are being jumpered together... . . that is being your main DC supply, is it being present ?
IF you find both supplies outputting report back their voltages.
Plus, with both of those supplies present and operating, that will also confirm what I suspect might be your units problem.
The ORANGE boxed in component area with its 8xx series parts numbering will use the RED oval IC801 + the ALSO RED oval Power FET to supply a feedback controlled voltage, derived from the " BROWN " power supply, to the separate LED strips that provide backlighting for your frontal transmissive LCD screen.
One white LED has opened up and disabled a whole strip and the controller detects this fault and shuts down all of the strips.
IC801 pinout seems to suggest that 3 (maybe 4 ) LED backlighting / illumination strips are being used.
Your forthcoming IC 801's ID #'s will confirm.

Standing by. . . . . . .

Awaiting for your findings + supplementally needed info of . . . . .
LED's Power FET # . . . .IC801 BL controller # and main PS SMPS IC IC602 #

Curiosity question ? . . .
At the very right front bottom corners quadrant, there is being a stand alone / totally isolated circuitry path between CN702 and M501 beside the AC power plug, what is that M501 . . . . . on the sets back . . . being labeled as ?



Thaaaaaassssit . . . ..

Your . . . PHOTOGRAPH(s) . . . . .is / are now being all marked up . . . .Ladyva sez . . .Me Too !
(

)

73's de Edd . . . . .



Remember this . . . . . you cannot achieve the impossible without occassionally attempting some of the the absurd.

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[Agnatorian]

Thank you so much for answering my question. I took all the measurements you wanted

the parallel-o-series connection of R609+R610 and R607 + R608 " bump up " surface mount resistors. The end to end reading should be ~1 Megohm.

Jup both of them read 1 Megohm

Then plug in all connectors and power cord and set metering to DC voltage and be expecting that there will be ~320-340VDC present across the main raw DC power supply capacitior C605 / YELLOW F

The reading from the main raw DC power supply capacitior is always around 340V. So no problems there.

OR you can use meter ground to bare tinned Jumper Wire 3 and meter + probe to the cathode / silver banded end of D711.
Then you power up the set with its power switch to see what DC voltage you hopefully might have there now. I'm sort of expecting it to be there.

Measuring between the JW3 and the end of D711 gives me around 17,4 V.

Check for it being down at the end of board connector CN701, with either of the two black wires being used for meter DC negative and the meter DC positive lead can go to either of the 2 RED or 2 ORANGE wires . . . as they are being jumpered together... . . that is being your main DC supply, is it being present ?

When reading from one of the red cables to the black ones it gives me a reading of around 5,4 V. But from orange to black its almost 0 so that didnt work.

Awaiting for your findings + supplementally needed info of . . . . .
LED's Power FET # . . . .IC801 BL controller # and main PS SMPS IC IC602 #

I know that the LED's Power FET is a c200y transistor (Has k137 c200 y written on it) right next to the LM431 Regulator. Actually mistook that one for the Transistor.

Could you elaborate what you mean with the other 2? Its a quite new field in electronics for me but I really want to learn more about it !

At the very right front bottom corners quadrant, there is being a stand alone / totally isolated circuitry path between CN702 and M501 beside the AC power plug, what is that M501 . . . . . on the sets back . . . being labeled as ?

That whole circuit there controls the Audio output of the monitor. No labels there or anything to check what exactly is being used there.

Thanks a lot. I will try to respond asap to every answer

 

[73's de Edd]

Sir Agnatorian . . . .

I'm ab-soo-LOOT-ley, having no luck in finding a schematic for your BenQ unit, but in searching for similars, it seems like Ben-Q and Acer just might sleep in the same bed.
Since, I am finding that some of their 22 and 24 inch monitors are using the same two I.C. 's in some of their power supplies.
Of course, I am referring to the 12 pin power controller I.C. being assigned the designator of . . . . .IC 602 . . .with the RED star Mark up.
And also the RED Oval marked 16 pin IC 801 LED backlight controller.
If you can pass back those numbers . . .we will have those , I .C. numbers being identified for our needs AND for anyone in the future that might need use of that info here.
( I never have seen one of the IC 602's being used in anything else ! )
Knowing the now all important IC801, will then let me research it against its data sheet and ascertain its connections to the LED strips.
With you now have given me the info of the Ben Q's model number, relates it to being a 22 " size display. That should need only two LED illumination strip, with one at its very top and very bottom
All of the LED's then face into a Lucite strip that tapers towards its mid screen point . . . . much in the manner of a linear Fresnel lens . . . such that the LED's initial pinpoints of light are dispersed apart laterally as well as optically / lensatically creating equal levels of illumination that will match, both at the very top edge and very bottom edge AND at center of screen by having its total flat side pressed against a frosted thin plastic sheet ( think of a ground glass sheet as is used by photographers) that then faces, mates to and illuminates through the LCD pixel panel back..

In "reading" the ORANGE line outlined in portion of your board , what I am seeing now is the just barely visible GRN-YEL-RED RED-BRN-BLK wires that come from the two Backlight Illumination LED strips into connector CN861. On the board they connect together the two REDs at pins 3 & 4. That leaves us to perceive that the R-Y and G and the separate R-Br-Blk are the LED strip connections.
With R being for pulse width regulated power input . . . then . . . Y and Brn, of each, connected to a LED cluster closect to ground return for sampling voltage and current present on that cluster to be fed back to IC801inputs to then be used for the power supplying / regulation POWER FET . . . .being Pale BLUE Oval marked. That is then leaving G and Y wires being used for ground returns.
Without a data sheet the best I might now visually surmise is that Brn wire feedback info routes from CN861 thru a dead end path to the left or to the right thru a VALID path up thru JW-6 and into pin #8 of IC801 . . . . . . . I can't be as sure on the Yel wire , but sure looks like it will dead end downward but validates if going upwards via a 0 resistor and to JW-5 and into pin #7 of IC801.
The sampling / inputting of both LED strip V an I conditions will then let IC801 set pulse width modulation compensatory characteristics, as gate drive to the Pale BLUE Oval power FET that passes up from from IC801's pin # 4.

Now for the ground returns . . . .which the black photo masking clips off the far left foil paths of.
Now it looks like the Black wire initially goes to the right and then upwards and then eventually into the # 9 of IC801 which must also subseuently be tied into the 18 vdc supply ground.
As for the GREEN wire , consider it going upards and then into JW-9 and then into pin # 6 of IC IC801 and I can't confirm its ground path from there unless that pin also ties into the ground plane / foil heat sinking that is running up and down the longitudinal center of the IC801.


With consultation of the relevant DaDa sheet of IC 801 we can confirm all of this .

That then lets you take 2x 9V batteries and serial end clip them together to get an 18VDC supply with an instant capability of power bursts of 4-5 amps.
We then compute the value of series voltage dropping / current limiting resistor so that we can then take the combo and do a short length connection of its power outputting leads into one LED strip and then the other to see which one will light up and the other nonresponsive bad one with an open LED(s), located somewhere in its series cluster.
The controllers sampling of the bad unit is giving a NO_GO fault condition . . . and its like shutdown response.

Now . . .giv us-es-es dose Eye Sees numbahs !

Tha a a a a s s s s s s s it !

REPEAT / CONVENIENT DISPLAY OF MARK-UP . . . (already being site hosted)

73's de Edd . . . . .


Even from a early age . . . . . I could almost sense that my parents hated me . . . . . my bath tub play toys were being an electric toaster and a plug in AC-DC radio.
( But, maybe that is why I now can handle a 120V AC wire in each hand. . . . but . . . . then turn the water hose on me and its game over ! )





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