Problem with Viewsonic VG920 lcd monitor

[mrpd]

Hello-

Found this site and hope someone can help, I have a Viewsonic VG920 lcd monitor and it won't power up at all. I have taken apart the monitor and looked at the PSU (part# 715G1492-1) and it all looks good, no bad caps that I can see. Is there a fuse that could have blown, I don't find anything that looks bad. I have been reading/searching the forum but have not found any problem with this particular monitor on here so I decided to post and see if someone had an idea of what is going on.

Picture of the PSU is below.

Bigger PSU Picture



If you would like to se any other part of the PSU or any pictures please let me know and thanks for any help with this.

PD

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

For some reason my picture didn't come through and I can't seem to edit my post to fix it. So I will put it here.

Bigger PSU Picture

[Toasty]

Re: Problem with Viewsonic VG920 lcd monitor

The fuse is the brown rectangular block next to the power plug. You can use a small "pigtail" type fuse (one with leads on it) of the same voltage & ampere rating there.

Check the solder connections of those transformers. Cold joints are a common problem. They will look gray and rough, not shiny like they should.

If a bad tube is causing an overload, it -could- be the reason the fuse failed. There are a multitude of other reasons.

For starters, report back with the manufacturer(s) of the caps on there.

Be very careful when working on this "live" as several components will present with household current on exposed surfaces. "One hand in your back pocket rule" applies if measuring voltages. The large cap can have near 400 volts across its leads. Measure that caps voltage and make sure its zero before working on anything.

DO NOT run it without the lamps connected.

Toast

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

Hello-

Found this site and hope someone can help, I have a Viewsonic VG920 lcd monitor and it won't power up at all. I have taken apart the monitor and looked at the PSU (part# 715G1492-1) and it all looks good, no bad caps that I can see. Is there a fuse that could have blown, I don't find anything that looks bad. I have been reading/searching the forum but have not found any problem with this particular monitor on here so I decided to post and see if someone had an idea of what is going on.

Picture of the PSU is below.

Bigger PSU Picture



If you would like to se any other part of the PSU or any pictures please let me know and thanks for any help with this.

PD

In the case of a totally dead power supply, one quick test is to measure the DC voltage across the large capacitor. Note Toasty's comments on safety; those are minimum precautions. The voltage across the cap should be roughly 1.4 times line voltage. For the US you should read about 165 volts DC.

You appear to be having a problem attaching the pictures. Below the 'Submit Reply' and 'Preview Post' buttons is a box with Additional Options. Use the 'Manage Attachments' button there.

If the proper DC voltage is present, check the output voltages from the supply. The board is often labeled on either the top or bottom near the multi-colored cable in the upper right corner.

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

Thankyou Toasty and PlainBill for your quick responses. Yes I saw the attachment area below the 'Submit Reply' and 'Preview Post' buttons after I posted, I am however unable to locate the where I can edit my posts I wanted to try and fix my post but was unable to. I have taken better pictures and labeled the manufacturer(s) and info on the caps. (I used the attachment feature this time)

I am a complete noob when it comes to trouble shooting on the electrical level, I have dabbled a little with things here and there. Repairing desktop and laptop MB but never to this degree. I do have a DMM but could you explain how to check

DC voltage across the large capacitor

I really don't want anything bad to happen (to me that is)


I have located the brown rectangular block fuse next to the power plug and tried to read the info but can't read what is on it until I remove it.

Toasty you stated

You can use a small "pigtail" type fuse (one with leads on it) of the same voltage & ampere rating there

like the kind found in desktop computer power supplies?

Toasty you also said to

Check the solder connections of those transformers

I am just checking but would the transformers be the 2 rectangular black items on the left of the board or the one covered in yellow, or both?

Lastly ...

If a bad tube is causing an overload, it -could- be the reason the fuse failed.

What is the tube?

Again I appreicate all of your help and bearing with me being that I am a greenhorn, I enjoy trying to fix things if they can be, instead of throwing away stuff that might be usable if not even fixable.


PD

[Toasty]

Re: Problem with Viewsonic VG920 lcd monitor

1) yes
2) all 3
3) CCFL's (cold cathode fluorescent lamps) - the back light bulbs of the display - http://en.wikipedia.org/wiki/Cold_cathode

Toast

[EDIT: fuse is a 2 amp, 250v]
[EDIT # 2: is that cap actually spelled "Rubicon" ?? ]

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

Thankyou Toasty and PlainBill for your quick responses. Yes I saw the attachment area below the 'Submit Reply' and 'Preview Post' buttons after I posted, I am however unable to locate the where I can edit my posts I wanted to try and fix my post but was unable to. I have taken better pictures and labeled the manufacturer(s) and info on the caps. (I used the attachment feature this time)

I am a complete noob when it comes to trouble shooting on the electrical level, I have dabbled a little with things here and there. Repairing desktop and laptop MB but never to this degree. I do have a DMM but could you explain how to check I really don't want anything bad to happen (to me that is)

I have similar feelings. If you died of electrocution I might feel guilty and loose some sleep.
I've slightly edited the topside picture you uploaded and attached it. The pins of the large cap have been circled. Put the board back in the monitor and fasten it in place with at least two screws. Hook up all of the internal leads, but don't hook up to AC power yet. Set your DMM to the highest DC scale (probably 1000 VDC) and turn it on. If you have test leads with J hooks or alligator clips, hook them to the legs of the large cap (red to the leg circled in red, black to black). If you don't, practice touching the probes to the leads without the probes slipping. Now plug the monitor in (touch the probes to the leads of the large cap if necessary), and note the reading. Unplug the monitor. If you measured a voltage approximately 1.4 times your AC voltage (US is 120 VAC), the fuse is good. If you used alligator clips or J-hooks wait a few minutes before disconnecting them.

I have located the brown rectangular block fuse next to the power plug and tried to read the info but can't read what is on it until I remove it.

Don't bother removing it. If you read the proper voltage above, the fuse is good. If you didn't, check continuity on the fuse AFTER you have unplugged the monitor and the large cap discharges. Set your DMM to the 200 Ohm scale and probe the two leads on the bottom of the board.
Toasty you stated like the kind found in desktop computer power supplies?

Toasty you also said to I am just checking but would the transformers be the 2 rectangular black items on the left of the board or the one covered in yellow, or both?

Lastly ...

What is the tube?[/QUOTE]
Yes, both, the backlights (CCFLs)

Again I appreicate all of your help and bearing with me being that I am a greenhorn, I enjoy trying to fix things if they can be, instead of throwing away stuff that might be usable if not even fixable.


PD

There is a saying in electronics: Given enough time and parts, we can fix anything.

Do the test, then we'll carry on from there.

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

Yes I agree with you on the electrocution thing except in my case I would be taking a long sleep, which is not a good thing which is why I am asking questions that might seem kind of straight forward but it is better to be safe then sorry.

I will check the readings and post back with the results.

Toasty-
Yes that is my bad spelling the "rubicon" cap is actually spelled rubycon. Sorry about that. I did go ahead and do what you suggested. I checked and just to make sure everything was good I resolder the connections for the transformers, mosfets, fuse, and the large cap. I attached some picks to show the results and see if I missed anything. Also when hooking the CCFL wires back, does it matter which one goes in what socket? On the top set of sockets the white & pink wires are in the top socket and the blue & white is below them but in the bottom set of sockets the blue & white wires are in the top socket and the pink & white are below. I don't think the wires are long enough to change them around, but I just thought I would ask.

thanks again,

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

what did you mean by

If the proper DC voltage is present, check the output voltages from the supply. The board is often labeled on either the top or bottom near the multi-colored cable in the upper right corner?

I don't see anything labeled on the board near top or bottom near the multi-colored cable in the upper right corner. I am not sure what I am supposed to look for. Please explain, if you would.

PS Why can't I go back and add (edit) a previous post that I did? Am I missing something? Is is that you can only edit the post you are working on at the moment and not one that was submitted previously?

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

Yes I agree with you on the electrocution thing except in my case I would be taking a long sleep, which is not a good thing which is why I am asking questions that might seem kind of straight forward but it is better to be safe then sorry.

I will check the readings and post back with the results.

Toasty-
Yes that is my bad spelling the "rubicon" cap is actually spelled rubycon. Sorry about that. I did go ahead and do what you suggested. I checked and just to make sure everything was good I resolder the connections for the transformers, mosfets, fuse, and the large cap. I attached some picks to show the results and see if I missed anything. Also when hooking the CCFL wires back, does it matter which one goes in what socket? On the top set of sockets the white & pink wires are in the top socket and the blue & white is below them but in the bottom set of sockets the blue & white wires are in the top socket and the pink & white are below. I don't think the wires are long enough to change them around, but I just thought I would ask.

thanks again,

It doesn't really matter. Notice that you have two transformers, but 4 pairs of wires. The output from each transformer drives two CCFLs.

EDIT: You can only edit a post within 10 minutes of posting it.

Sometimes a board will be labeled with the function of each pin. 5V 5V 5V GND GND GND BL_ON BRI is an example.


In this case, starting from the black wire the pins would be GND GND 5V 5V GND 12V and either BRI BL_ON or BL_ON BRI

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

I hooked everything up and check the voltage from the large cap it is reading 169-170. So I guess that means the fuse is OK. The monitor still doesn't turn on and not sure if I mentioned it but I don't get a power light either. So what would be the next step to determine what could be the problem? Checking if the proper DC voltage is present, check the output voltages from the supply? If you would please explain just how to do this?

thanks again,
PD

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

I hooked everything up and check the voltage from the large cap it is reading 169-170. So I guess that means the fuse is OK. The monitor still doesn't turn on and not sure if I mentioned it but I don't get a power light either. So what would be the next step to determine what could be the problem? Checking if the proper DC voltage is present, check the output voltages from the supply? If you would please explain just how to do this?

thanks again,
PD

You are a genius!! BTW, I am working another thread with the same problem. If you haven't already done so, put in the screw closest to the output connector. Reset the DMM to the 20 volt DC scale, hold the black probe on the screw, and probe each pin of the connector (monitor plugged in, of course). Report the voltages you get (even on a good monitor some of the pins will read 0 volts).

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

You said that you are working another thread with the same problem, is it with the same lcd also? Should I check out the posts?

Anyhow here is the voltages of the wires

Starting from the top wire (can also view attached picture to see color code with voltages)

Black
0.01

Brown
4.49

Red
14.45

Orange
0.00

Yellow
4.97

Green
4.97

Blue
0.00

Purple
0.00

Also now that I can hear myself think around here it is alittle more quite and I could hear a slight humming or squealing, I believe coming from the transformer that is covered in yellow. Could that be the culprit to my problems?

Thanks again for your help,

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

You said that you are working another thread with the same problem, is it with the same lcd also? Should I check out the posts?

Anyhow here is the voltages of the wires

Starting from the top wire (can also view attached picture to see color code with voltages)

Black
0.01

Brown
4.49

Red
14.45

Orange
0.00

Yellow
4.97

Green
4.97

Blue
0.00

Purple
0.00

Also now that I can hear myself think around here it is alittle more quite and I could hear a slight humming or squealing, I believe coming from the transformer that is covered in yellow. Could that be the culprit to my problems?

Thanks again for your help,

Probably not. The results are the same in both cases. The voltages you are getting are what I would expect from a good power supply. It is possible replacing all the capacitors would help, but I really doubt it. If the 5 Volt lines were lower or if the power led on the monitor were flashing it would be worth trying.

As I requested from the other person, post good pictures of the top and bottom of the logic card.

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

PlainBill-

So it seems the PSU is good after all, hmm. I also get nothing on the monitor at all no power indicator nothing. I have a couple of questions for you if you don't mind answering them. 1.) This one is really stupid I know but I have been trying to turn on the lcd without having a computer hooked up to the lcd, would that cause it not to turn on? Maybe some kind of signal that comes from the vga or dvi when hooked up? I am pretty sure I know the answer to this dumb question, I mean it should at least power up the backlight and power button even it is not hooked up, but I thought I would ask. 2.) Just curious (not doubting you in the least) but how/why would logic board prevent the lcd from atleast powering up (backlight, green/amber power button)? Again, please don't take this question the wrong way I AM NOT DOUBTING YOU IN ANY WAY SHAPE OR FORM I just kind of like to know things, and you know more then I do so just trying to pick your brain. The pictures you requested are attached.


thanks,
-PD

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

PlainBill-

So it seems the PSU is good after all, hmm. I also get nothing on the monitor at all no power indicator nothing. I have a couple of questions for you if you don't mind answering them. 1.) This one is really stupid I know but I have been trying to turn on the lcd without having a computer hooked up to the lcd, would that cause it not to turn on? Maybe some kind of signal that comes from the vga or dvi when hooked up? I am pretty sure I know the answer to this dumb question, I mean it should at least power up the backlight and power button even it is not hooked up, but I thought I would ask. 2.) Just curious (not doubting you in the least) but how/why would logic board prevent the lcd from atleast powering up (backlight, green/amber power button)? Again, please don't take this question the wrong way I AM NOT DOUBTING YOU IN ANY WAY SHAPE OR FORM I just kind of like to know things, and you know more then I do so just trying to pick your brain. The pictures you requested are attached.


thanks,
-PD

Every monitor I have ever seen, even the earliest CRT ones, will at least power up even if it is not hooked up to a computer. The behavior of LCD monitors varies. Most flash some kind of a logo or warning ('No Signal'). Some Dell's feature a logo marching across the screen for many minutes if the video cable is not hooked to a computer. ALL of them will turn on the power LED if the power button is pushed.

The answer to your other question is slightly more complex. In essence, a LCD monitor is a tiny computer. The backlights, the display, and even the led in the power button are controlled by the controller IC - the big IC on the right with the heat sink in your picture. If it isn't working, you won't have anything,not even a power LED.

If you haven't done so, look at the other thread - the guy has two dead Acer monitors. Look at the logic card in his and compare it to yours. Yours is 715G1558-1-2, his are 715G1558-1.

Next, I am of the opinion that the voltages both of you found indicate the power supply is working perfectly. And I've got a perfect record - I'm either right or not right 100% of the time. So if either of you want to go ahead and recap the power supply, by all means do so. Use good quality caps, and if you prove me not right, I will be happy for both of you.

Lastly, the other gentleman found a service manual for his monitors. I've glanced at it, and it includes schematics of the logic board and a troubleshooting tree. Someone else is going to have to work with you on that. Some person close to me gave me a head cold and I'm rapidly losing interest in electronics. My wife had it two weeks ago and is still recovering.

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

Yes I found the other thread and was looking at the posts. I went ahead and did the same diagnoses on the voltage regulators

This is the usual drill; put the logic card into the monitor, hook the monitor up to power. The black lead would go to the screw in the hole circled in black (or directly to the solder area). Measure the voltages at all three pins of each of the two voltage regulators circled in blue.

My reading are almost the same as his

Starting from the top voltage regulator :
Left - 3.30
Middle - 2.47
Right - 2.63

Top - 4.66
Middle - 3.30
Bottom - 0.00

The difference I see in the logic boards is mine has DVI and the sound board and that is about it (to me atleast)

I will see about doing a recap. Here is another one of those stupid questions but can bad caps take down the entire lcd functionality, or should something atleast still work? I also have found the manual for my lcd, as well as the one he found for his and just like he said

unfortunately I don't really know what any of it means

I am sorry to hear you are taken ill, hopefully you will make a fast recovery. As far as needing this fixed ASAP that is not the case with me, would you be willing to help trouble shoot after you regain your health? Or is that not something you do? In any event you have helped out already, I got a bit of an education and I thankyou for everything you have done so far.

-PD

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

Yes I found the other thread and was looking at the posts. I went ahead and did the same diagnoses on the voltage regulators

My reading are almost the same as his

Starting from the top voltage regulator :
Left - 3.30
Middle - 2.47
Right - 2.63

Top - 4.66
Middle - 3.30
Bottom - 0.00

The difference I see in the logic boards is mine has DVI and the sound board and that is about it (to me atleast)

I will see about doing a recap. Here is another one of those stupid questions but can bad caps take down the entire lcd functionality, or should something atleast still work? I also have found the manual for my lcd, as well as the one he found for his and just like he said


I am sorry to hear you are taken ill, hopefully you will make a fast recovery. As far as needing this fixed ASAP that is not the case with me, would you be willing to help trouble shoot after you regain your health? Or is that not something you do? In any event you have helped out already, I got a bit of an education and I thankyou for everything you have done so far.

-PD

I discovered there are only so many hours a day that one can sleep. At some point the mind starts running around in circles and I have to do SOMETHING. (Emptying a box of Kleenex every two hours does not qualify as something). As far as recovery, I have COPD. If I can prevent this from settling into my lungs, I will be fine. Unfortunately, somewhere in the design process of the human body a decision was made that the sinuses (sini?) should drain into the lungs. Perhaps the fact that sitting hunched over a keyboard transfers the drainage to the nose IS a sign of intelligent design.

I'm basing these comments on the manual for the Acer AL1916v manual. Chapter 4 is a series of troubleshooting trees. Page 31 covers 'No Power'. Notice the third block down "Measure U701 Pin2=3.3V, U702 Pin2=3.3V". Looking at the excellent picture you provided I note that U701 is not present. The schematic seems to indicate U701 and U702 are alternate components. And your test verifies 3.3 volts is present at U702, pin 2.

Now we have a problem. The next step is "X401 oscillate waveforms are normal". X401 is the crystal oscillator, an oval metal can below and to the right of U702. Checking this requires an oscilloscope.

If that is good, the next step is to replace U401, which provides it's own set of complications. This site indicates they have the part in stock, at a cost of $10. However, replacing this requires advanced reworking skills and some special equipment.

I would suggest you see if you can find a hobbyist or professional in your area who has an oscilloscope and can check the crystal.

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

Glad to see your well enough to get back on here, again I thank you for your effort and time trying to trouble shoot this. Sadly I don't know of any hobbyist or professionals in your area and even if I did it would probably cost more $ to have them check it out then just to buy a new monitor. I noticed that you live in PHX so do I now I don't know if you are up to this or even do this kind of thing or not buy I would gladly pay you if you would like to see what you could find out. I don't expect something for free and would rather pay you then trying to find someone. If it sounds like something you would like to do just let me know.

Again thankyou very much for your help thus far and the education on LCD.

-PD

[Toasty]

Re: Problem with Viewsonic VG920 lcd monitor

14.45v doesn't sound correct. There's something not right on that output and the high voltage may be inhibiting the circuit in the logic board from coming on. The squealing you're hearing may be a bad output cap on the 12v line.

There's a 13v zener (ZD904) down below in that Acer schematic that must be firing; it being 1.45v above its rating...?

There's also one across the 12v & 5v output rated at 9.1v. If the difference between the two goes beyond that it conducts, acting as a crowbar of sorts. If that zener exists in this PSU, then we are past its rating by .38 volts. (14.45 - 4.97)

Isn't there a way, after disconnecting this from the logic circuit, to get the backlights to fire? Applying a few volts to On/Off line which goes to Q201?

Curiosity @ PlainBill - On the logic board & the "side board" - There are a whole slew of tiny little SMD's marked FB###. Are those fuses? I noticed them on my Dell's and everyone of them reads 0 ohms. Just curious if any of them are reading open.

Or, maybe I'm just Nuckin' Futz...

Toast