LG Flatron L1942T-PF monitor no power

[mikey5791]

Hi all,
Got this LG Flatron L1942T-PF LCD monitor free from a friend but the problem is it does not have display power (power board model LGP-002L) and lacks a main transistor at Q101. After searching thru service manual, I installed a mosfet SSS7N60B at Q101, replaced all the secondary filter caps. Then I connect a 60W light bulb in series at F101,switched on mains 220vAC power,the light bulb flash a second,then lights off. This indicating there is no shorted component.

FYI, I have a digitalmultimeter(DMM), capacitor meter and a 40W soldering
iron with basic skills in replacing bulging capacitors and revive dead LCD monitors.

First,on the powerboard, I need to be sure there is poweroutput from P201 white connector where I perform DC voltage test by touching red probe to label 5V and black pin to cold ground (screw pad) but see no volt reading(zero volt) on DMM, meaning the 5V standby is not present.
Later on I perform the same DCvoltage test(red probe to cathode) on the following with the corresponding reading:

C101 main cap=286vDC
Q101(middle pin)=108VDC
D102 diode= -109vDC( negative reading?)
D107 diode=109vDC
D106=109vDC
D101=109vDC
D105 & D104= no DC at all

On power IC at U101 it is label LAF0001,pin 1=Vstr,pin 2=CS/FB,pin 5=Ground,pin 7=Vcc Putting my red probe to pin 7,black pin to cold ground yield a DC reading -109vDC(negative?)

The opto at PC201 is label 817C,on diode test,one end on left facing D102 reads "1"= infinite while the other end reads 1335. Is the opto good?

Refer to inverter transformer at T301, I set my DMM to resistance test on 2k ohms scale. Reading on one end (1&2) is 1028 while pin 3&4 reads 1024. Guess the trans is good.
BTW, how do I test if the yellow transformer beside maincap is good with my limited DMM. I also attached some photos.
Hope someone can help me in solving the no power issue. Many thanks in advance.

 

[mikey5791]

One More photo of inverter transformer on power board LG Flatron L1942T-PF.

 

[HellasTechn]

Can you confirm that the CCFL lamps are not bad ? Even if one of them is bad then the monitor will not turn on or it will for a second and will turn off again.

You can only test them on an other inverter known to be good.

I would also replace the components in red circle with new and check u201.

The yellow transformer as well as the inverter are by 90% ok.

 

[73's de Edd]

.


Sir . . . . . . .mikey5791

(Hmmmmmmmm . . .seems like I have typed that name before . . . .

If you had a CPU missing you would know it by its HUGE pin count and its quite large accompanying footprint .
Instead, you are just dealing with a missing power mosfet used as the units principal switch mode POWER switching device. . . . its schematically designated as Q101.
With the fuse missing and the Q101 missing , might that last repair person just have pulled that device and then found that it was not just some easy thing that he was going to readily find at the nearest "Radio Shack.
So, he just abandoned the project.
On the other hand, you must be in downtown Kuala Lumpur? to have so readily obtained that part . . . . and not being way out in Kota Bharu.


In consideration of two different routes of evaluations:

One is to see if solder conditions on the joints or actual electrolytic capacitors are showing of them having been replaced at the C203-C206-C202-C207 positions and down at C204-C212-C205 positions that are being wired to the pins 6-7-8-9-10 connections of the secondary of the POWER switch mode transformer T101.

Also of a lesser degree of significance would be one at pin two of the primary winding, off terminal 2 to C103.
Also one down near the switch mode controller IC of U101 as C105 on its pin 3.

We are VEWY interested as to these caps having possibly warmed up and progressively boiled off their internal sealed off electroltye in their earlier life..
That condition will result in a progressively increased operatonal temperature and accompanying internal pressure build up and a puffing out of the capacitor.
This predominantly shows as either a bulging out of the bottom seal or a doming of the top of the capacitor. Look at the top of the capacitors and you will typically see a single stamping of an indented ridge line or a cross X in the top of the case.
These specific areas of selective weakening of the case permits metal rupture at that area to release internal pressure before the case go KAAAAAA-BOOOOOOM .

That is one method of knowing of a deteriorated and / or stressed electrolytic capacitor . . . . . . just by using visual inspection.
If you can do that initially, you will have taken your first evaluative step.

I'm going on . . . . . to give the next operations:

On your voltage measurements, take note of the central POWER switch mode transformer and then visually draw a vertical line to separate the schematics left and right circuitry.
That circuitry on the left is AC line associated / connected and referred to as HOT circuitry.

That circuitry to the right of the division line is being ISOLATED by the transformer secondary winding and is thereby referred to as COLD circuity.
HOWEVER, you might find a couple cases of the two different grounds wandering over to an opposite side of the board.
But that situation will be for an isolating agent such as a capacitor or high value resistor being connected between them.

Now in your metering ground on the left side of the circuitry, you need to use one of the triangle designations of a HOT ground, when measuring that HOT circuitry.
In your metering of the right side circuitry, place your metering ground to the conventional ground symbol.
(That should explain some of your given "weird / unexpected" voltages that you experienced before . . . .due to cross ground references.)

l see that you were able to fire up and got the normal "safe" glow-dim test lamp response, but now go over and find the top right corners C207 DC reservoir supply capacitor for your units 22VDC output
Also , drop down and locate the C205 equivalent capacitor for the 5V supply.
Check both to see if there is any retained /stored voltage ?
I'm expecting none, but if so found, short them out , to drain them down to zero volts.

Then you plug in the set, still using the inline test lamp for its fuse, and see if there is any degree of a then created voltage being on either caps of those +22 and +5 V supplies .
If SO . . .we then know that there is being an initial complete power loop thru the system, but its just not maintaining a SUSTAINED action.

That then signifies that we need to check out the U101 pulse mode controller IC.
It produces run drive to the gate of the Q101 Power Mosfet from its pin # 6 output .

Your voltages passed to me tell us that the sets AC line derived raw DC supply is present, adequate and present on the drain of Q101 and its ready to go, pending drive pulses from U101 pin # 6 to its gate.

There are three aspects of the power supply for the U101.
There is a parasitic power supply for its voltage derived by the use of winding at SMPS transformer terminals 1-2 feeding to R111 dropping / fusible resistor to D102 to C103 electrolytic to then feed to pin 7 VCC of the
U101 chip.

I now have to see if your power board layout is being like this supplied reference below:

http://assets.shopjimmy.com/media/c...5774693fb5d/s/h/shopjimmy-eay41830902-top.jpg


On the boards top near the U101, I am finding there is being no C104- C105- C106- R112- R103-R102-R118-R117 . . . . any chance that they are being mounted on the other side of the board, as being surface mount
components ?

Waiting for your feedback on that info.


73's de Edd


.

 

[mikey5791]

HellasTechn,

It is unlikely to confirm if the ccfl lamps are not bad as I do not have a known good inverter to do the testing. Referring to your comment on "I would also replace the components in red circle with new and check u201."

In fact,one of the red circle item at Q101(on heat sink) is a newly placed mosfet SSS7N60B, while U201 is a UTC TL431K (3 terminal regulator). How do I check to see if U201 is good? Please kindly tell me which DMM probe goes to which pin.....as I am still a new learner in this. Appreciate your input.

 

[mikey5791]

Sir 73's de Edd,

Indeed you have typed my name before and advise on few of my earlier repairs.

Your detailed explanations on the circuit workings helped a lot on further understanding of this monitor power board which I will need time to absorb. To answer on C207(22vDC supply and C205 as 5vDC supply), I put my DMM scale on DC setting to check for any DC, red DMM probe to positive side of cap and black DMM probe to negative side of cap while off mains power. Too bad there is no retained or stored voltage. I assume power has not been able to reach the caps yet.

Then, with inline light bulb on fuse area, I switched on mains 220vAC, the light bulb quickly dims, but sadly there is still no DC output on the two caps. Which part could have gone bad and stopping power to reach the secondary caps.

Earlier on, I have replaced all secondary filter caps with new one; eventhough on visual sighting there is no bulging or top doming. BTW,near U101 on those caps & resistors C104- C106- R112- R103-R102-R118-R117 . . . . they are being mounted as tiny SMD components on the back of the power board.

I look forward to your kind guidance on what to do next....like putting what color probe to which pin to check for voltage, etc....be patient with me as i consider myself a new learner.

 

[mikey5791]

Hi,

Done a DC voltage test by red probe to each of the pins on U101 (PWM controller
LAF0001) while black probe to ground screw pad on right side of the board. On pin 1 (with a circle denote pin1),which is Vstr the DMM reads 74vDC then slowly drops to zero. On pin 5 Grd,the DMM reads -112vDC, i got same reading -112vDC on pin 6,7,8 as well.

Does the above result conclude the U101 is bad since according to Fairchild pinout diagram, Vcc(pin 7) absolute maximum ratings should be at 20V while pin8(Vref) should be 5V. Also I did DC voltage test on diode D102, putting the DMM red probe to diode cathode(diode end with black strip) while the black probe to negative leg of main cap C101. The DMM reads zero. Is the diode D102 bad?

 

[73's de Edd]

Sir mikey5791 . . . . . . .


That's a relief, with those parts being found on the other side of the board . . . so I actually wasn't using the wrong schema or board layout for my referencing.

Initially . . . . . your Answers:

On power IC at U101 it is label LAF0001 . . . . . 7=Vcc Putting my red probe to pin 7, black pin to cold ground yield a DC reading -109vDC(negative?)

BECAUSE . . . . that I.C. . . . . . is being associated with the boards HOT circuitry, therefore, your meter negative probe referencing point SHOULD have been on that I.C.'s pin 5 . . . which is a HOT ground.

The opto at PC201 is label 817C,on diode test,one end on left facing D102 reads "1"= infinite while the other end reads 1335. Is the opto good?

That test is being logical, for its internal LED being good, while the internals photo transistor sensor on its C-E junction reading, is also being in order, with that "open" reading . . . those units are usually quite trouble free.

. . . . . . while U201 is a UTC TL431K (3 terminal regulator). How do I check to see if U201 is good? . . . .

I wouldn't even suspect U201 . . . . at this early stage of analysis.

U201 is being an adjustable shunt Zener regulator that has the capability of being adjusted for a Zener engage threshold of anywhere from 2.5 V through 36 V, it’s all in accordance to the voltage division input being set at its central R terminal.

Refer to inverter transformer at T301,

I don't have the CCFL supply circuitry being shown on this page . . . . but I hope that it will not even be involved

Hope someone can help me in solving the no power issue. Many thanks in advance.

That’s the route now planned . . . . .


On your post #7, in BOTH cases, you are IMPROPERLY using a COLD ground for your metering ground reference . . . it should be a HOT reference ground.


In the case of the transformer derived supply voltage, which is being derived from terminal winding numbers 1 and 2 of the SMPS transformer T101 and then their diode feed . . . to that mentioned D102.

That voltage supply’s reservoir capacitor is C103 and you do need to note, that its negative terminal IS being connected to a HOT ground.

A CONTINUANCE . . . . .

Let’s cover three aspects of the units further troubleshooting / analysis this time around.

Procedural testing for the supply voltage for U101.

Which would be the supply mentioned JUST above

But do note that voltage is not even generated or being available, unless Q101 is being driven to energize T101 power transformer.

That requires a look at my second referencing provided, for the U101 chips internals.

Take note of pin 6 of the unit where you see that the output drive of he unit is being provided by two series arranged transistors that are being directly across the full power supply voltage of the chip.

Normal design utilizes a NPN and PNP transistor in series and is called a “totem pole” output.

In this case they are using two series NPN units due to the more robust reliability of NPN units.

Then,it might be named a “Quasi totem pole” output

The bottom transistor is receiving normal drive from the base drive being supplied by the bottom output of the 4 input 2 output or gate.

While the top transistor is receiving a mirrored drive from the base drive being supplied by the top output of the 4 input 2 outputs nor gate, due to its inverted output. (So designated by the small circle)

Now with this type of drive output, one can have a very HIGH drive level output and both transistors to alternate in the drive signal presented at pin 6.

Then the series R122 and R105 reduce down to the desired gate input level of the Q101.

Since you received the unit without Q101 and with the assumption that the fuse was blown. That would have been a HARD failure and some of that high voltage present at the drain of the transistor could have shorted out and came in via the gate and into pin 6 of this IC and cause damage to one or both of those transistors. They certainly can’t handle 180 + VDC across them.

If their failure mode was a short, it could be detected with an ohmming out from pin 7 to 6 or 6 to 5.

If junctions were blown open, It just wouldn’t be simple to detect . . . expect on replacing the IC to confirm for sure.


Now the last thing that you need to know about that IC’s operation is its powering up procedure.

It is initially dependent upon RAW DC voltage from the power supply coming down my marked in RED ARROW path.

There is the 47k resistor that drops down that high voltage to the level needed at pin 1 Voltage Start terminal of the IC.

There is an additional like voltage, coming down the R117-R118-R107 path.

Those series resistors, along with C104 provide a charging time constant, so that when you initially power up the supply, there will be duration of about 200 repetitive drive pulses being supplied, to feed to Q101gate.

If all is well in the circuitry related to the T101 transformer . . . specifically . . . neither shorted parts nor excessive loading down of output supply lines. The output supplies should come alive, ALONG with the GREEN ARROW path which then keeps U101 powered up, after that initial “kick start” drive input, just explained.

TESTING:

Have the unit without power and then check across main filter C101 to see if it is discharged, if not, bleed it down.

Then take an ohmmeter and check out high resistance resistors R117-R118-R107 to see if they are on value and then step over to R108 to see if it is being 47K.

BELLS and WHISTLES:

Since you are having minimal test equipment . . . . particularly with you being less a scope.

Let’s take a page out of the Industrial-Military-Aerospace-NASA complexes and slip in an infinitesimally simple, in this case, yet VERY-VERY EFFECTIVE application of BITE technology . . . . . ( Built–in-Test Equipment).

Let’s tack in 3 constant monitors for the units 5VDC, 22VDC, and the ~8-10 VDC run voltage for the U101 IC.

All it will take is 3 resistors and 3 LEDs.

Use an approximately 330-470 ohm dropping/current limiting resistor in series with the LED used for the 5VDC supply.

Use an approximately 2K ohm dropping/current limiting resistor in series with the LED used for the 22VDC supply.

Use an approximately 1K ohm dropping/current limiting resistor in series with the LED used for the 8-10VDC supply.

Solder tack them together and test each of the three sets for proper operation with a 9V transistor battery or? DC Supply.

That confirms functionality plus assures their proper polarity of install, for their add on, yet to come.

Then track down those three voltage supply points and tack solder in the monitors properly. . with a double checking.

NOW, when you initially power up for testing of the power supply, you will immediately see if there is being a voltage presence.

OR MOST IMPORTANTLY . . . you can see a voltage come up for as short of a time period as 200 milliseconds, even if it then drops back down or off.

It’s now TRYING to work . . . . . but you certainly wouldn’t have known that before with the S L O W, one place at a time, monitoring that you were using earlier.


Thasssit for now . . . . .


Attached referencing:

73's de Edd

 

[HellasTechn]

HellasTechn,

It is unlikely to confirm if the ccfl lamps are not bad as I do not have a known good inverter to do the testing. Referring to your comment on "I would also replace the components in red circle with new and check u201."

In fact,one of the red circle item at Q101(on heat sink) is a newly placed mosfet SSS7N60B, while U201 is a UTC TL431K (3 terminal regulator). How do I check to see if U201 is good? Please kindly tell me which DMM probe goes to which pin.....as I am still a new learner in this. Appreciate your input.

I think you got the answers !

about the TL431K regulator i dont think there is a way to test it besides buildig the actual regulator circuit as suggested in it's datasheet.

Hope i that helps.

P.S. Dont worry i am a learner also, slow one but still learner

 

[mikey5791]

Hi Sir 73 de Edd,

Done the dc voltage test, resistor R117 is smd 244 reads 220k, R118 smd 244 also reads 220k.
Reisitor R107 reads 840k and still keeps increasing while i measure it on board.
Resistor R108 reads 45.9k
I also measure the ic LAF0001 pin 7 & pin 5 ground, dmm reads 64.8 ohm

Not too sure what you meant by tack solderthe three resistor, then test with 9 v battery. Where do i hook all
These resistor? Can pls draw simple diagram, how to connect those resistor and where?
Sorry i m a real beginner in this. Thank inadavance

 

[73's de Edd]

Sir mikey5791 . . . . . . .

I also measure the ic LAF0001 pin 7 & pin 5 ground, dmm reads 64.8 ohm

That certainly sounds bad . . . . . but measure again from pin 6 to 7
And then from pin 6 to 5 and pass those readings back to me

Will give you a "solder tacking" (a fast /temporary connection ) explanation picture tomorrow . . .I am on small laptop now.

73's de Edd

 

[mikey5791]

Sir 73's de Edd,

Done resistance measurement on IC LAF0001 by setting my DMM to 200 ohm scale.
Recheck and probe on pin 7 vcc & pin 5 Grd= 64.8 ohm (may I know why is it bad ?)
Pin 6 out to pin 7 vcc= 104.6 ohm
Pin 6 out to pin 5 Grd = 49.9 ohm

All the above are done while the IC is still on board. Does it say anything about the condition of the IC? I wonder if it is necessary to unsolder all the 8 pins and check resistance on IC off circuit. I can do it if needed to but surely this will take some time.

If it help, I will look forward on your "solder tacking" (a fast /temporary connection)
on the three resistors/LED explanation picture. Hopefully this power board and
monitor can be rescued and saved from the landfill or recycle centre.
Appreciate and my sincere thank you for your generous help.

 

[73's de Edd]

Sir mikey5791. . . . . . .


My suspicion of your IC, is because when you are testing those connections you are ohmming through two series arranged Collector to Emitter transistors junctions inside of the IC, which should not read that resistance.
Just try that test , using a single silicon transistor that you have, in the measuring / ohmming from its collector to emitter. Two series junctions would be even more isolated.

This illustration below fully explains the quick and easy assembly or dis assembly of simple circuitry by using drops of solder for " solder tacking" connections together.

I even considered your having to use salvaged LEDS from other equipment, where you would not have a RED dot lead or longer / shorter lead length for the identifying of LED polarity.

You just need to be able to see the Gold Cup and Gold Wire inside of the LED.

When you get those LEDS in circuit, lets continue the earlier mentioned tests and not worry about the Switch Mode IC just yet, until we see if that IC's IC supply voltage tries to appear, even if its only for a moment. . .which that LED blip would show.


Illustration:

"Solder Tacking" / Temporary connecting

73's de Edd

 

[mikey5791]

Sir 73's de Edd,

Out of suspicion, i measured resistance of those tiny smd resistor at the back side of power board and noticed three resistors worth further checking.
Setting my DMM to 200 ohm scale, I got the two following readings (the resistor marking in bracket):
R122(4R7)= 80.8 ohm (4R7 actual value is 4.7 ohm)
R106(472)= 100.1 ohm (472 actual value is 4.7k ohm)

Both the above two resistors are slightly below Mosfet Q101 and the resistance value is way off. Does this affect any output power?
Another resistor is at R102 which i can't see the marking but the schematic list it as
5.8k ohm, my DMM reads 6k ohm. Guess this is still ok.

Is my earlier reading
Pin 6 out to pin 7 vcc= 104.6 ohm
Pin 6 out to pin 5 Grd = 49.9 ohm
on the IC LAF0001 prove the IC is bad?

Today I managed to get some resistors and LED from my junk parts collection as per
image 1826. Got 1 pc resistor of (brown,black,red,gold) 0.98k, 1 pc resistor of
(brown,red,red gold) 1.18k, 1 pc resistor of (red,red,red,gold) 2.17k, 1 pc resistor of
(brown,grey, brown,gold) 179 ohm & 1 pc resistor of (brown,violet,brown,gold) 176 ohm. To get your recommend 330-470 ohm resistor, I decide to connect in series the last two resistors to get a value around 353 ohm.

Sorry I am still unsure of your earlier posts "Solder tack them together and test each of the three sets for proper operation with a 9V transistor battery or? DC Supply.
That confirms functionality plus assures their proper polarity of install, for their add on, yet to come. Then track down those three voltage supply points and tack solder in the monitors properly. . with a double checking.
NOW, when you initially power up for testing of the power supply, you will immediately see if there is being a voltage presence. "

I understood your picture of the LED "solder tacking", the end of - & + to connect to a 9V battery or DC supply. But I do not know where and which voltage point(5V,22V,8-10V) to solder tack..... Are you referring to temporary solder the separate three resistor to P201 output white connector area? or somewhere else?
Sorry I am really novice on this. Hope you can patiently point out or mark on the power board area how to "solder tack". Thanking in advance.

 

[73's de Edd]

Sir mikey579. . . . . . .

Of ALL of the photos that we have, I find it hard to find ONE that shows the components good enough to point out the connection points of all three test LEDS in one photo.

Let me just describe them:

For the 5V supply LED see if you can connect across the R211 resistor with the LED ground side lead connected to the COLD ground side.

For the 22V supply LED, connect the LED positive side to either D201 or D205's cathode and get the ground from the nearby C203 or C206 filters COLD ground terminal.

For the10-12V supply LED, connect the LED positive side to D102's cathode and its ground to the closest . . . .C103's HOT ground lead or pin 1 of T101 nearby power transformer.

NOW . . . .what I suspect happened to this unit and why repair was abandoned on it.

You know about the fuse F101 being blown and missing .
You know about the Q101 power FET being missing, assuredly being bad also.
Related:
On your post # 15 and in photo . . . # IMG_1829-powerboard back.JPG . . . the Q101's areas discoloration of the PCB shows either / both some extensive use of this unit and possibly some hot running during its final days.



Q101 being not just bad . . . . . but BAD- BAD - BAD . . . in respect of it having shorting between Drain and GATE and SOURCE.
That condition temporarily let that ~176 VDC from the main power supply pass into the Q1 gate and then into
R122 which it damaged in shifting it up to 80 ohms by overload. Also that 1N4148 right beside it might now be shorted.
PLUS there is a R105 resistor across that pair which you also need to check.
When that high voltage reached pin 6 of U101 it connects into a series arranged NPN and PNP transistor at their shared collector and emitter connection .
That excessive voltage blasts both of those internal transistors of the IC U101, with you confirming 104 ohms across one transistor and 49 ohms across the other.

When you test across a silicon transistor from its collector to emitter, you don't expect that low reading.

The last thing to confirm is if the last part in the power loop . . . . the source resistor R109 was affected.

After you have checked those areas and have a new U101 to install and the gate drive parts checked out and corrected, then we can try a Load Lamp Limited power up test.




73's de Edd

 

[mikey5791]

Sir 73's de Edd,

It seems like the IC at U101 being LAF0001 needs a replacement; which is not easy to get at our part of the country. I have checked the local store but they do not have it.
So i checked online at utsource.net, the cost for 1 piece is USD 1.00 but shipping is USD 20.00; this is way too much a bomb and over my expected budget !

I am thinking "ghetto way" of connecting an external power supply and re using the power board (let it "as is" condition without fuse) as a connecting board to the vga display board.
My choice of external power supply is either a 12 V(6000mA) power adapter or Liteon 19V (3.42A) laptop power adapter or a switching ATX 250W power supply supplying at DC 12V (9A). Pictures of the power board and external power as attached. Which one is the best option as external power supply and which part or position I need to connect to provide the required 22V and 5V.
Will connect the external power to C207 (desolder the cap at C207,then connect external power positive to cap positive position, negative power to cap negative position) be suffice ?

This is my last desperate option to supply power to the board. If this way is not workable, the power board will go to the bin.
Hoping to hear any suggestion. Thank you.

 

[73's de Edd]

Sir mikey5791 . . . . . . .


It seems like the IC at U101 being LAF0001 needs a replacement; which is not easy to get at our part of the country. I have checked the local store but they do not have it.
Have you checked for its real Fairchild part number of FAN7601.
I was surprised with how easily you were able to come up with that Q101 FET.

Yes, a logical work around on the power supply would be to use an ~ 1995-200 vintage laptop computers power block where some of them generate in that range of voltage output.
Then you just have to find or make another 5 VDC at 2 amp supply for the other portion of this monitor.

That should confirm if the rest of the monitor is working, by you then having fulfilled its 22 and 5 V supply needs.

Insert the 22 VDC power just past the rectifier diodes at the anodes of D205 / D201.
Insert the 5 VDC power just past the rectifier diodes at the anodes of D202 / D203.
Grounds at C203-C205 and / or C204-C212


73's de Edd

 

[Hשמשמ]

Dear Sirs Mikey5791 and 73's de Edd,

Ive just received an LG Flatron monitor of another model but exactly the same schematics: Same part numbers and same parts, though with another layout.
I am afraid of the HOT ground and parts- therefore I use a UPS unit because it can be really float, without any reference to ground: I charge it most of the time from LINE power, and disconnect it from line and then connect it to the module under test. The test must be done in a few minutes (well, up to 15 ) before the UPS shuts down. However, this is most important when I want to use an oscilloscope!
As to the results: With the oscilloscope I can see on U101 regulator (LAF0001==FAN7601) that Vstr at pin1 is charging the Vcc capacitor on pin7, but then sloping down as the result of the internal fuction of the start-up circuit {latch voltage at pin3 is rising}. This is a repetitive cycle. I can see the Vref on pin8 coming up shortly thus enabling oscillations at pin4 but with all this activity, I can not see any pulses at pin6 output.
My guess is that some latch gets activated and then blocks the output.
I have to do some measurements with a storage scope and try to see what happens at the initial microseconds after switching ON.
Regards

Hanan

 

[73's de Edd]

.


Sir Hanan . . . . . . .

You are doing just fine with he UPS, as you are then assured of a ground isolation when operating totally free of the AC line. (If no grounded RF cable / antenna is connected . )

There are two actions being made at power up of that IC, there is your initially mentioned use of the RAW DC from the power supply coming down through R108 and into IC pin 1 for INITIAL power up BUT thre is also the same voltage supply line feeding through R117-118 and 107 series arranged resistors.

With time and use, one of these high value resistors can open up and the initial strike up voltage to start /shock that chip into its initial start up operation never occurs.

If that start up DOES occur, then there is output drive from that U101 chip, out of its pin 6 and THEN IF all is well in the Q101 circuitry, that power FET will start power drive to the T101 transformer at the very top.

The terminals 1 and 2 winding from the T101 will get rectified at D102 and filtered with C103 and fed down to U101 pin 7 and all of the time it is running, THAT will then be the PRIMARY source of power for that chip up until POWER turn off time .

Soooooooo . . . . . in a no power applied situation check all 3 of those resistors ohmmages, to see that none of them are open, nor have they drifted on up to a much higher value.

Mmmmmmmmm very nice reference schematic !



RELEVANT SCHEMATIC REFERENCING:

73s de Edd

.








.

 

[Hשמשמ]

Dear 73s de Edd,

Thank you very much indeed for the prompt response.
Using the UPS is not only for "doing fine" but is necessary when using an oscilloscope which is usually ground-referenced.

Now to the good news: The monitor is working!
Apparently all circuitry was working fine except for the output capacitors on the 5V and 22V supplies! These capacitors (C202, C203, C204,C205, C206, C207 and C212 if they are all installed) have two functions:
a- as every one knows: charging through the respective rectifier diodes at the peaks of the voltage from the output transformer and supplying this energy to the load till the next peak.
b- as we sometimes forget: to absorb spikes and extreme peaks at voltage transitions.
Apparently the controller IC (FAN7601==LAF0001) was triggered into "blocking" mode by the first rising edge of the output voltage (5V I guess) and unless measured within about one second after ON no output voltage could be noticed
The ability of the capacitors to perform this high frequency filtering reduces with time ! It can be improved externally by soldering high value ceramic SMT capacitors (in the order of 0.1uF or more) across the pins of the above mentioned capacitors.
In any case, when you take the trouble of trying to repair a monitor, it might be useful to replace all these capacitors (or their equivalents in other designs) they cost much less than your time...!
It seems my first "suspect"= the regulator IC is well protectd and also protects all other circuitry controlled by it.

Thanks again,
Hanan