Problem with Viewsonic VG920 lcd monitor

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

mrpd, here's some information from the other thread. The gentleman followed my advice (scary) and shorted the output of the reset controller to ground. At that point the monitor power LED came on. So it APPEARS U406 may be the culprit.

Toasty, they use a fairly 'loose' regulation on the other output voltages of these supplies. The backlight system is a major part of the power consumption of the monitor. I would expect the backlight supply to rise somewhat with the backlights off.

The 'side board' (I would call it a daughter board) is the audio amplifier. Unlike back in the days of tube amplifiers, modern audio amplifiers have a very high frequency response. High frequency interference would degrade the amplifier output. I very confident FB8607 is a surface mount ferrite bead.

PlainBill

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

MRPD, look at the other thread. I just noticed he appears to have a couple of caps in the 12V supply that may be bulging slightly. Would You take a picture of the power supply from the same side view.

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

Attached are the pictures you requested, I didn't see any bulging caps in them though. I tried the other method of testing which you suggested in the other post, unforunatly I didn't get the same results as robertoisrad. My monitor did nothing. Just to go over the sequence to make sure I did it right, I assembled the lcd boards and cables, I found a screw driver just wide enough to span the two bottom leads of U406, I plugged in the monitor, I shorted the two bottom leads of U406 with the screw driver and tried to press the on/off button, is this all correct? Sadly I guess this is where robertoisrad and my trouble with this board differ. I am still open to do more trouble shooting if you have any other ideas to try.

thanks again for your help,
-PD

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

Attached are the pictures you requested, I didn't see any bulging caps in them though. I tried the other method of testing which you suggested in the other post, unforunatly I didn't get the same results as robertoisrad. My monitor did nothing. Just to go over the sequence to make sure I did it right, I assembled the lcd boards and cables, I found a screw driver just wide enough to span the two bottom leads of U406, I plugged in the monitor, I shorted the two bottom leads of U406 with the screw driver and tried to press the on/off button, is this all correct? Sadly I guess this is where robertoisrad and my trouble with this board differ. I am still open to do more trouble shooting if you have any other ideas to try.

thanks again for your help,
-PD

Maybe, maybe not. Time will tell.

PlainBill

[mrpd]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

As far as my pictures did the caps look good to you? Even though I know that a cap can look good physically but still can be bad. Is there a way to test the cap itself to check if it is good or not? Was my order sequence right for testing U406? I am still wanting to trouble shoot if you think of anything.

Thanks again for your help, I appreciate it very much.

-PD

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

Plainbill-

As far as my pictures did the caps look good to you? Even though I know that a cap can look good physically but still can be bad. Is there a way to test the cap itself to check if it is good or not? Was my order sequence right for testing U406? I am still wanting to trouble shoot if you think of anything.

Thanks again for your help, I appreciate it very much.

-PD

Physically, the caps look good. There was definitely no sign of bulging tops. An ESR meter will give a good test of the state of the caps. Your test sequence was correct. I don't see any additional troubleshooting that you can do with the equipment you have available.

PlainBill

[Toasty]

Re: Problem with Viewsonic VG920 lcd monitor

>>surface mount ferrite bead.<<

Heh! I just don't know why that never occurred to me. DOH!

Mental block or just mental maybe? It's not easy being this nutz.

Thanks!
Toast

[djbush2000]

Re: Problem with Viewsonic VG920 lcd monitor

Toasty, Plainbill, Et all,
I know this is an old post, but I have a ViewSonic VG920 with exactly the same symptoms. I'm guessing there is a capacitor or other device leaking that is causing the crowbar circuit to immediately shut it down. Any advice before I 'shotgun' the power board capacitors?
Thanks,

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

Toasty, Plainbill, Et all,
I know this is an old post, but I have a ViewSonic VG920 with exactly the same symptoms. I'm guessing there is a capacitor or other device leaking that is causing the crowbar circuit to immediately shut it down. Any advice before I 'shotgun' the power board capacitors?
Thanks,

Actually, you are wrong. there was nothing wrong with the power supply on mrpd's monitor, nor on the power supplies of the monitors in the other thread. In that case, replacing the reset controllers on the logic boards fixed two monitors. That thread can be found HERE.

The first step is to measure the output voltages of the power supply. If they are correct, try shorting the reset controller to ground. There are no guarantees, but it is possible that there were a bad lot of reset controllers. The part is quite inexpensive.

PlainBill

[djbush2000]

Re: Problem with Viewsonic VG920 lcd monitor

Thanks for the update PLAINBILL. I have ordered the CPU reset device for U406 and will report if it works.

[koppit]

Re: Problem with Viewsonic VG920 lcd monitor

Hi Everyone! My name is Brendan. I work for an IT company and I've been replacing capacitors for a little while, but only recently moved into actual troubleshooting for LCDs, which is a little more involved for me than just looking to see if there are any blown caps.
Let me know if this cpu reset worked - I am running into the same issue on an Acer AL1916 - it has the same 715G1558-1 Controller, but there were two caps that weren't reading voltages properly - the two large ones, reading 25v 100uF, in the row of 5. Actually, mine is a slightly different model, with 2 in a row, 1 missing, and 2 in a row. All capacitors and components read correctly on the PSU end of the LCD.
Thanks!

edit: looks like there are more capacitors than I thought on this controller board not functioning correctly. I will replace and advise.

[12ax7a]

Re: Problem with Viewsonic VG920 lcd monitor

Let me know if this cpu reset worked

yes, i can tell you first hand that after going through a bunch of these repairs last fall that the supervisory cpu reset chip causes more problems than they're worth. when they fail (and they seem to fail easily - possibly a bad batch and / or possibly a misunderstood design) they end up holding the graphics cpu in 'eternal' reset. the schematic for these monitors would indicate that acer's june grad engineers got confused over the meaning of logic active low / high.. or someone was just sleeping on the job when they 'designed' the 'enhanced' reset. originally this just appeared somewhere as a simple r/c delay for a smooth power up but then it looks like someone got the bright idea to incorporate a 50 cent part to destroy 1000s of monitors all over america. the fact is this pin can almost be left floating and still work just fine as evidenced by the picture attached. once removed or a couple pins lifted, these monitors are fine & i have yet to witness any problems related to unstable cpu initialization or anything else that would otherwise latch up the chip on startup. acer will tell you they've never heard of this & that you're the first to call of the problem... yeah sure.. i can't even imagine how many of these units just ended up in landfills across the planet... ~

[moam]

Re: Problem with Viewsonic VG920 lcd monitor

yes, i can tell you first hand that after going through a bunch of these repairs last fall that the supervisory cpu reset chip causes more problems than they're worth. when they fail (and they seem to fail easily - possibly a bad batch and / or possibly a misunderstood design) they end up holding the graphics cpu in 'eternal' reset. the schematic for these monitors would indicate that acer's june grad engineers got confused over the meaning of logic active low / high.. or someone was just sleeping on the job when they 'designed' the 'enhanced' reset. originally this just appeared somewhere as a simple r/c delay for a smooth power up but then it looks like someone got the bright idea to incorporate a 50 cent part to destroy 1000s of monitors all over america. the fact is this pin can almost be left floating and still work just fine as evidenced by the picture attached. once removed or a couple pins lifted, these monitors are fine & i have yet to witness any problems related to unstable cpu initialization or anything else that would otherwise latch up the chip on startup. acer will tell you they've never heard of this & that you're the first to call of the problem... yeah sure.. i can't even imagine how many of these units just ended up in landfills across the planet... ~

Guys, I had the same problem, viewsonic VG920 LCD monitor screen wont turn on.
After some reading, I did what 12ax7a suggested and took the Transistor U406
off the board. Also looking under the miscroscope I found U406 to be burnt out.

After putting everything back to gether and powered on the monitor, everything worked fine.

[smason]

Re: Problem with Viewsonic VG920 lcd monitor

What ever happened to using a simple RC time constant circuit to generate a reset pulse? I guess these reset controllers are smaller than the size of the requred cap for an RC circuit?

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

What ever happened to using a simple RC time constant circuit to generate a reset pulse? I guess these reset controllers are smaller than the size of the requred cap for an RC circuit?

There are a number of variables here. A 'simple' rc circuit will require at least two, preferably three components (a diode to prevent the reset input going above Vcc when power shuts down). Also, the 'simple' rc circuit lacks the ability to compare Vcc to the minimum voltage the processor requires, lacks the ability to reset the processor when the voltage drops momentarily, and is subject to noise (the last can be accomplished by a schmidt trigger on the processor reset input. It also presents difficulty if you want a long reset delay.

I note that the gm2621 processor has integrated the reset controller function into the processor. Of course, that makes it difficult to synchronize the start up timing of a support processor.

PlainBill

[moam]

Re: Problem with Viewsonic VG920 lcd monitor

yes, i can tell you first hand that after going through a bunch of these repairs last fall that the supervisory cpu reset chip causes more problems than they're worth. when they fail (and they seem to fail easily - possibly a bad batch and / or possibly a misunderstood design) they end up holding the graphics cpu in 'eternal' reset. the schematic for these monitors would indicate that acer's june grad engineers got confused over the meaning of logic active low / high.. or someone was just sleeping on the job when they 'designed' the 'enhanced' reset. originally this just appeared somewhere as a simple r/c delay for a smooth power up but then it looks like someone got the bright idea to incorporate a 50 cent part to destroy 1000s of monitors all over america. the fact is this pin can almost be left floating and still work just fine as evidenced by the picture attached. once removed or a couple pins lifted, these monitors are fine & i have yet to witness any problems related to unstable cpu initialization or anything else that would otherwise latch up the chip on startup. acer will tell you they've never heard of this & that you're the first to call of the problem... yeah sure.. i can't even imagine how many of these units just ended up in landfills across the planet... ~

By removing that component from the circuit Will that cause any problem to
the monitor or any functional aspect of the monitor?

[PlainBill]

Re: Problem with Viewsonic VG920 lcd monitor

By removing that component from the circuit Will that cause any problem to
the monitor or any functional aspect of the monitor?

It's hard to tell without a full datasheet and application note for the processor. I would expect problems would be most likely to show up if the monitor was used in an electrically noisy environment, or if the power supply caps have deteriorated.

Leaving the reset input floating is always questionable practice (it may not actually be floating; there might be an internal pull-up resistor). In general, I take the attitude that most components were added to improve the design. At the cost of the reset controller (about $.75 IIRC), I would install a replacement.

PlainBill

[smason]

Re: Problem with Viewsonic VG920 lcd monitor

Also, the 'simple' rc circuit lacks the ability to compare Vcc to the minimum voltage the processor requires, lacks the ability to reset the processor when the voltage drops momentarily, and is subject to noise l

Ahh, some good points. Thanks for enlightening me.

[ohmed_out]

Re: Problem with Viewsonic VG920 lcd monitor

Noob. Cant believe Iv never found this site befor. I wanted to toss this out there. Alot of similar pieces. Im working with a Viewsonic IBM 17in monitor. We have several in shop all doing the same thing. You power it on only to have a blinking green front LED and a brief flash of the "viewsonic" screen saver. Acting like its not getting good power. I have to pinpointed to the power board but for the life of me cant seem to find out what. Iv tested every component along with many others in my shop. We bought a new monitor off ebay to compare, only to find a revision in the new board that makes it non-compatable with the video board. The P/N of the bad powerboard we have is 715G1492-J. The unit we bought offline has a powerboard of 715G1492-1. If anyone has some information I could try it would be greatly appreciated. The google machine seems to point me twards a few places online to pickup new ones for not to much but would rather try and fix what I got.

ninja edit***
There is an obvious heat mark in the center of the board. No swollen caps. Nothing blown off. I have done a basic reflow with no luck.

[ohmed_out]

Re: Problem with Viewsonic VG920 lcd monitor

Norry noob forgot to attach pictures;