LCD monitor VT17W7X not powering on

[Welchs101]

Hi,

A friend of mine brought me his off-brand monitor. Model #VT17W7X. He said it flickered. I could not get it to power on.


1) I opened up the monitor and immediately saw the "oozing" stuff from several capacitors (see enclosed pic "Overall_with_badcaps_2" and Mag_of_badcaps")

2) I replaced ALL 6 of the 680uF/25V caps with new 680uF/25V caps. I tried turning on the monitor........nothing happened.

See enclosed pics "Badcaps replaced with good".

The inductors are kinda sandwiched in with the caps.........the inductors dont "ooze" do they?

I measured the voltages at the connectors between the power board and the logic board and i measured voltages of 4.97V and I think one was even 13V but i cant recall exactly.

3) I researched this monitor online at badcaps and found a link:
https://www.badcaps.net/forum/showthread.php?t=8737

This information was very helpful in finding out what MIGHT be wrong with my friends monitor.

So i looked at the back of the board and as was indicated in the original badcaps forum posting the DK QR transistors were bad....or at least looked bad two of them.

(see enclosed pics "Overall_backboard_2" and "Mag_backboard_blown_transistors_2")



Questions:

1) So i know i probably need to order replacement transistors that are blown. From the badcaps link i found I AM GUESSING the parts i need to order are 2SC4672T100QCT-ND (this is from digikey). Is this right?

2) Because i like to learn......lets say i did not have the badcaps forum as a reference telling me to look at the transistors. What should I have looked at to identify that the transistors were bad. What kind of trouble shooting should I have done to narrow down the problem. To be honest after measuring the connector voltages my first reaction was "OK it must be a logic board problem.". Note: I can remeasure the voltages and state them here if this helps anyone.

3) If you look at the pic "Overall_with_badcaps_2" i show 4 areas of "concern". (1) and (4) are the 680uf caps and the transistors respectively. But what about (2) and (3) .....should i be concerned about these? If so how would i tell........ie...what should i measure to determine they are ok or not.


I realize i am asking a lot of you guys but you guys are great! I have learned a lot from you guys........just trying to keep the learning process going.

Thanks.

If there is any info you need or anything you want me to measure just let me know.

[PlainBill]

Re: LCD monitor VT17W7X not powering on

Good job of troubleshooting, let's see if I can recall my process the few times I've seen this happen. If I recall correctly, in each case the monitor had the same symptom - the power LED would flash for a fraction of a second. Opening it up would reveal bulging caps. Replacing the caps wouldn't correct the problem. I BELIEVE at that point I would notice that the power supply outputs were stable if the logic card was not present, but were unstable if the logic card was present. That implied some sort of an overload.

The other thing that screamed at me was the components in the inverter. The inductor (black cylinder) and poly capacitor (brown device) near each transformer is a sure indication of Royer oscillators. From that it is a quick leap to the familiar problems with the Benq, Dell, and HP monitors. In the first two cases I saw, one leg of the capacitor had a bad solder joint. This would cause the oscillator to run too fast, increasing current and overloading the transistors. I believe the other condition required is a power supply with very little excess capacity, so an overload would cause the power supply to shut down.

Part of the detective work involves working backward to explain the symptoms. When a monitor is operating normally it's 'last known state' and other settings are stored in serial eeprom. When AC power is applied it resumes that state. So the sequence goes like this: AC power is applied, the SMPS starts. The processor on the logic card starts up and loads 'last known conditions' from eeprom. It turns on the power LED and the backlights. Because the transistors are shorted, the power supply overloads and shuts down. The power supply will try to restart. Since the inverter is now off the overload is gone, it restarts successfully, the processor comes up, etc, and the process repeats.

One other point. Replace the brown poly cap, resolder the transformers, and check the diodes in the inverter. All of those could cause the original failure.

PlainBill

[Welchs101]

Re: LCD monitor VT17W7X not powering on

Hi, Thanks for the quick reply.

I had a few follow up questions:
1) The brown poly cap you suggested i replace. I am enclosing pics (not really good pics but the best i can do). I think the part number on the brown cap is as follows:
F184J G
250MPH1

What would be the replacement part for this? Any suggestions?

Using my voltmeter i measured the following characteristics of this brown cap.
Resistance: Open.......infinite.........which is good.
Capacitance: I measured 177nF.........not sure what its supposed to be


2) You also mentioned the "diodes" in the inverter........which diodes are you exactly referring to. I am not sure.

3) Any good schematic/diagram of a Royce Oscillator circuit i could use as a reference? Instructional material if you know it.

Actions still to be taken:
a) order new parts for the DK QR transistor...........is the part number i listed correct?
b) resolder inductor and transformers in the high voltage inverter circuits.
c) replace/order brown poly capacitors
d) check diodes........once i know which ones to check?

[alexanna]

Re: LCD monitor VT17W7X not powering on

Here is another monitor to look at.
Looking at this power supply it looks similar to this one,
https://www.badcaps.net/forum/showth...580#post139580
If it is, I noted how the inverter behaved with up to two ccfls not working .
I also had a problem with D611 being cracked on one of my P/S inverter board
I also had a problem with what I think were the small start and run capacitors.
Al.

with only only one ccfl hooked up the monitor would give a very brief flash of the ccfl.

[PlainBill]

Re: LCD monitor VT17W7X not powering on

Hi, Thanks for the quick reply.

I had a few follow up questions:
1) The brown poly cap you suggested i replace. I am enclosing pics (not really good pics but the best i can do). I think the part number on the brown cap is as follows:
F184J G
250MPH1

What would be the replacement part for this? Any suggestions?

Using my voltmeter i measured the following characteristics of this brown cap.
Resistance: Open.......infinite.........which is good.
Capacitance: I measured 177nF.........not sure what its supposed to be


2) You also mentioned the "diodes" in the inverter........which diodes are you exactly referring to. I am not sure.

3) Any good schematic/diagram of a Royce Oscillator circuit i could use as a reference? Instructional material if you know it.

Actions still to be taken:
a) order new parts for the DK QR transistor...........is the part number i listed correct?
b) resolder inductor and transformers in the high voltage inverter circuits.
c) replace/order brown poly capacitors
d) check diodes........once i know which ones to check?

Information on capacitor labeling. The poly cap is reads 184J which translates to 180000pF or 180nF. J indicates 5%. Based on the reading you obtained, it appears to be good. Still, a replacement will cost less than the transistors.

My mistake. No diodes in this circuit.

Google Royer oscillator, also look at schematics for the Dell E172FPb, E173FPb monitors.

PlainBill

[Welchs101]

Re: LCD monitor VT17W7X not powering on

I am just now getting around to placing my order with digikey.......i typically dont place an order until i get a lot of things to order.........

This brown poly cap........i am looking on digikey for a replacement.........polycaps are not a selection to chose from.......any ideas?

[PlainBill]

Re: LCD monitor VT17W7X not powering on

I am just now getting around to placing my order with digikey.......i typically dont place an order until i get a lot of things to order.........

This brown poly cap........i am looking on digikey for a replacement.........polycaps are not a selection to chose from.......any ideas?

AKA Film. As in Polysomelongunpronounceableword film.

PlainBill

[Welchs101]

Re: LCD monitor VT17W7X not powering on

what voltage rating should i use for this 184J cap?

[Welchs101]

Re: LCD monitor VT17W7X not powering on

From the picture of the cap i see a 250MPH1.......does this mean use a 250V rated cap?

[Rtech]

Re: LCD monitor VT17W7X not powering on

They usually are 250v

[Welchs101]

Re: LCD monitor VT17W7X not powering on

thanks.

Is there a way to tell the voltage rating from the part number on the unit itself. I enclosed a pic in an earlier post.

Not sure what all the numbers mean on the part.

[Welchs101]

Re: LCD monitor VT17W7X not powering on

is this a good replacement


http://search.digikey.com/scripts/Dk...name=P14243-ND


i selected the 250V DC voltage rating..........there is also a 250V AC rating.........i assume the 250V mentioned is for DC and not AC.....is this right?

[Rtech]

Re: LCD monitor VT17W7X not powering on

Yes, thats correct,I presume the 250 on the cap indicates that.

[Welchs101]

Re: LCD monitor VT17W7X not powering on

thanks.

[Welchs101]

Re: LCD monitor VT17W7X not powering on

So i finally got around to putting in the replacement ICs that had blown.

I removed the blown IC shown in the previous pic.......just the two that blew. Did not remove the other two.

I also resoldered the transformer pins. Why? Because this has helped with things in the past and it did not take long.

While looking at the back of the power supply board i actually found a cap which looked soldered into place but actually was not......you could simply remove the cap.......i have pics. Never saw this before.

Recall, in my original post I had 4 areas of concern.
1) Lots of oozzing caps.......i replaced all those.
2) Cap beside transformer.........i replaced both of these
3) Electrolytic cap laying on its side looks like it oozed ........replaced this cap.
4) ICs blown on back of power supply board. I replaced these two ICs.

I did all of these things put everything back together and NOTHING. Still NO Led light.

I measured the voltages (with the LCD panel removed ie....LCD panel not connected to logic board) on the logic board at the big connector. If you look at the back of the logic board the connector actually has what the voltages "should" be. THe results i measured are below: (again lcd panel not connected)
Board / What i measured:
12V / 12.48V
GND / 0V
5V / 5.07V
5V / 5.07V
GND / 0 V
ADJ / 0.281V
VOL / 0.056V
GND / 0V
ONB / 0.068V
MUT / 5.06V



Anyone have any ideas?

If you need additional measurements made i can do this just let me know what you want.

[newbie1]

Re: LCD monitor VT17W7X not powering on

This could be irrelevant but just happen to see that there is a 'solder'/metal wire shorting R4 & D18 on the logic card. Is that deliberately done?. or....

[Welchs101]

Re: LCD monitor VT17W7X not powering on

i think so. I know i did not do it. I saw this as well. But i guess i have seen similar things on other boards before so i just dismissed it. Was this a bad idea?

[PlainBill]

Re: LCD monitor VT17W7X not powering on

1. You are correct, the power supply is providing the correct voltages.

2. The signal (logic) board is also providing a reasonable 'backlights off' voltage.

3. One thing I did not hear is your stating you had hooked up the front panel and pushed the button (Yah, I'm grasping at straws).

4. I notice two voltage regulators on the signal board. Are they outputting the correct voltages?

PlainBill

[alexanna]

Re: LCD monitor VT17W7X not powering on

1. You are correct, the power supply is providing the correct voltages.

2. The signal (logic) board is also providing a reasonable 'backlights off' voltage.

3. One thing I did not hear is your stating you had hooked up the front panel and pushed the button (Yah, I'm grasping at straws).

4. I notice two voltage regulators on the signal board. Are they outputting the correct voltages?

PlainBill

I have several of these monitors sitting around, If you need help in identifying any correct voltages. I will do my best to help.

[Welchs101]

Re: LCD monitor VT17W7X not powering on

yeah, i tried the button with everything hooked up except a computer. Do you think i need to hook up a computer to the monitor before the LED will come on?

How did you know the backlights off voltage was ok?

I will check the voltage regulators and let you know what i find.