ViewSonic Model VG920 - No Display/No Power Light

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I have a ViewSonic Model VG920 - No Display/No Power Light.

Looking at the only other VG920 thread on these forums I did the first troubleshooting step, measuring the voltage on the large capacitor. No voltage.

"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." by PlainBill

I then unplugged the monitor, remove the PSU and tested the fuse. It is blown. I now know I will have to replace the fuse, but is there anything else I can look for before I order the fuses?

I live in the middle of nowhere Kansas. I am aware more often then not a bad fuse is caused by other bad parts. Nothing really pops out at me looking at the board, but I'll include pictures, maybe you will see something I missed. I would like to order everything I might need at once. Any advice would be appreciated.

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

I have a ViewSonic Model VG920 - No Display/No Power Light.

Looking at the only other VG920 thread on these forums I did the first troubleshooting step, measuring the voltage on the large capacitor. No voltage.

"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." by PlainBill

I then unplugged the monitor, remove the PSU and tested the fuse. It is blown. I now know I will have to replace the fuse, but is there anything else I can look for before I order the fuses?

I live in the middle of nowhere Kansas. I am aware more often then not a bad fuse is caused by other bad parts. Nothing really pops out at me looking at the board, but I'll include pictures, maybe you will see something I missed. I would like to order everything I might need at once. Any advice would be appreciated.

Thank you. This is fun. Better than finger painting.

I've identified the components in the power flow path. The most likely cause of the fuse blowing is the bridge rectifier. Other possible suspects include the power FET, and the main filter cap (the large black one lying on it's side.)

Check the bridge by measuring resistance from each lead to each of the other three leads. If you use the 200 ohm scale on a DMM it should read open between all leads with the possible exception of the two end leads.

PlainBill

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Re: ViewSonic Model VG920 - No Display/No Power Light

Thank you for your response.

I tested the bridge rectifier with my DMM and all leads read open. I assume this means it is good?

If I remember my basic electronics class from more then a decade ago FETs are tested with an Analog MM, all I have is a DMM. I am not sure why I remember that, and I may be totally in left field because I cannot think for the life of me why it should matter. Either way I do not remember how to test the FET. Do you suggest that I just replace it? Or is there more troubleshooting you wish me to preform first?

Also will badcaps.net have all the parts I will need?

Thanks again for your help.

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

Thank you for your response.

I tested the bridge rectifier with my DMM and all leads read open. I assume this means it is good?

Well, it means it probably isn't shorted. The next step would be to repeat the test using the Diode Test range of your DMM.

If I remember my basic electronics class from more then a decade ago FETs are tested with an Analog MM, all I have is a DMM. I am not sure why I remember that, and I may be totally in left field because I cannot think for the life of me why it should matter. Either way I do not remember how to test the FET. Do you suggest that I just replace it? Or is there more troubleshooting you wish me to preform first?

Check the resistance between the three legs of the FET, first with the DMM on the 200 ohm range, then on the diode test range.

Also will badcaps.net have all the parts I will need?

Thanks again for your help.

Badcaps may; however he specializes more toward the motherboard repair niche. I prefer to use Digikey.

PlainBill

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Re: ViewSonic Model VG920 - No Display/No Power Light

Testing the bridge rectifier in diode mode leads, numbered 1-4 from left to right (by your picture)...

Black 1 Red 2 - 589
Black 1 Red 3 - 581
Black 1 Red 4 - 572
Black 2 Red 3 - open (reverse leads still reads open)
Black 2 Red 4 - 576
Black 3 Red 4 - 584

Testing the fet in ohms 200 range leads, numbered 1-3 from left to right (2 being the raised lead, I assume that is the gate)...

They all read open.

Testing the fet in diode mode leads, numbered 1-3 from left to right....

Black 1 Red 2 - open (reverse leads reads 1800)
Black 1 Red 3 - 661
Black 2 Red 3 - 572

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

Testing the bridge rectifier in diode mode leads, numbered 1-4 from left to right (by your picture)...

Black 1 Red 2 - 589
Black 1 Red 3 - 581
Black 1 Red 4 - 572
Black 2 Red 3 - open (reverse leads still reads open)
Black 2 Red 4 - 576
Black 3 Red 4 - 584

"What we've got here is a failure to communicate". (For three points, what movie?)
The way I want it done:
Hold the black probe on 1. Readings with the red probe on 2, 3, 4.
Hold the black probe on 2. Readings with the red probe on 1, 3, 4.
Hold the black probe on 3. Readings with the red probe on 1, 2, 4.
Hold the black probe on 4. Readings with the red probe on 1, 2, 3.
QUOTE=browork]Testing the fet in ohms 200 range leads, numbered 1-3 from left to right (2 being the raised lead, I assume that is the gate)...

They all read open.

Testing the fet in diode mode leads, numbered 1-3 from left to right....

Black 1 Red 2 - open (reverse leads reads 1800)
Black 1 Red 3 - 661
Black 2 Red 3 - 572[/QUOTE]

Ahh, I think that's OK.

If the above readings are OK, try this test. Take a 40 - 100 Watt 120 volt light bulb and use it to bridge the fuse. Plug in the monitor. How bright does the light bulb glow? Measure the voltage across the large capacitor.

PlainBill

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Re: ViewSonic Model VG920 - No Display/No Power Light

Finally a question I can easily answer - 1967 Cool Hand Luke. I think there is another more modern film it is in too, but nothing is popping into mind at the moment.

By the way, my Army education just taught me basic electronic theory and how to pluck and chuck. No wonder the government is broke, they throw away millions of dollars in repairable circuit cards (and that was just me personally).

black probe on 1. Readings with the red probe on 2(595), 3(587), 4(578).
black probe on 2. Readings with the red probe on 1(open), 3(open), 4(582).
black probe on 3. Readings with the red probe on 1(open), 2(open), 4(589).
black probe on 4. Readings with the red probe on 1(open), 2(open), 3(open).

Give me a few on the other test (about an hour)... I have some other work to do first. And do you really want to know how bright the light is?

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

Finally a question I can easily answer - 1967 Cool Hand Luke. I think there is another more modern film it is in too, but nothing is popping into mind at the moment.

By the way, my Army education just taught me basic electronic theory and how to pluck and chuck. No wonder the government is broke, they throw away millions of dollars in repairable circuit cards (and that was just me personally).

black probe on 1. Readings with the red probe on 2(595), 3(587), 4(578).
black probe on 2. Readings with the red probe on 1(open), 3(open), 4(582).
black probe on 3. Readings with the red probe on 1(open), 2(open), 4(589).
black probe on 4. Readings with the red probe on 1(open), 2(open), 3(open).

Give me a few on the other test (about an hour)... I have some other work to do first. And do you really want to know how bright the light is?

Well, sorta. The idea of the bulb is to limit current. If the bulb glows at full brightness, there is a problem that we have not identified yet. If the bulb varies in brightness; or if it starts off very bright, then dims, there may not be a short.

One thing I forgot - measure the resistance of the thermistor. It's in there to limit surge current when the monitor is plugged in. If that shorted, the initial surge could blow the fuse.

PlainBill

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Re: ViewSonic Model VG920 - No Display/No Power Light

Resistance of the thermistor read 11.6

With the light bulb...

The light bulb flashes. Standard 60w 120v GE household light bulb. I never expected that.

DC voltage across the large capacitor reads 158v. The green power light comes on...

When the light bulb flashes, the green power light turn amber then goes out. After the flash the green power light comes back on.

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

Resistance of the thermistor read 11.6

With the light bulb...

The light bulb flashes. Standard 60w 120v GE household light bulb. I never expected that.

DC voltage across the large capacitor reads 158v. The green power light comes on...

When the light bulb flashes, the green power light turn amber then goes out. After the flash the green power light comes back on.

Ohhh, SOMETHING appears to be drawing too much power. Is it the LCD panel or the inverter?

Or you could try going to a 250 watt heat lamp or putting a couple of lamps in parallel.

Dang!!! I just realized that in a few years we aren't going to be able to do this. I'm not at all sure that a CFL would take kindly to this treatment.

PlainBill

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Re: ViewSonic Model VG920 - No Display/No Power Light

I put a couple of light bulbs in parallel. The green light comes on, turns amber, then goes off, to come on again... Same as with one light bulb, except the light bulbs do not light up at all.

Also there is a very faint electric click when the amber light goes out.

I don't think I mentioned that the voltage on the large cap drops when the amber light goes out, somewhere around 20-40v, the second the green light comes back on the voltage at the cap jumps back to 157/158.


What does the Canadian Football League (CFL) have to do with compact fluorescent light bulbs?

[browork]

Re: ViewSonic Model VG920 - No Display/No Power Light

Now to put the light bulbs in a parallel circuit instead of a series circuit... My bad, had some type of brain fart...

With the bulbs in parallel this time, still not light... The Green power light comes on, then switches to amber and stays amber... Hitting the power switch turns the LCD off (no power light)... Hitting the power switch again turns the LCD back on, green light, approx five seconds later the light turns amber... Amber light stays on without issue.

Looking at the display, it comes on... Says no signal... I plugged a computer in, LCD comes on and stays on... been running 15 minutes now fine... The light bulbs are rather warm however... Is that normal? Could it just be a bad fuse?

[browork]

Re: ViewSonic Model VG920 - No Display/No Power Light

Ok correction number two...

For some reason a few light bulbs in parallel told me three bulbs... With three bulbs in parallel the LCD works fine... Three light bulbs should be similar to a 360v fuse if I am following you correctly... The fuse on the board is a 250v... So I took one bulb out of the circuit....

Now with two bulbs in parallel...

The lights come on, mid range, gets brighter to full elimination, then goes out... The LCD comes on, shows a screen when plugged into a computer, then when the light bulbs go out so does the screen... And the green power light goes out...

I am a slow learner, but learning...

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

Now to put the light bulbs in a parallel circuit instead of a series circuit... My bad, had some type of brain fart...

With the bulbs in parallel this time, still not light... The Green power light comes on, then switches to amber and stays amber... Hitting the power switch turns the LCD off (no power light)... Hitting the power switch again turns the LCD back on, green light, approx five seconds later the light turns amber... Amber light stays on without issue.

Looking at the display, it comes on... Says no signal... I plugged a computer in, LCD comes on and stays on... been running 15 minutes now fine... The light bulbs are rather warm however... Is that normal? Could it just be a bad fuse?

It could have been a bad fuse, momentary surge, etc. I would STRONGLY suggest installing a new fuse. Using a length of 14 gauge wire would be a VERY bad idea.

PlainBill

[browork]

Re: ViewSonic Model VG920 - No Display/No Power Light

We seemed to cross posts, posting at the same time...

I now doubt a new fuse would fully fix my issue, as I stated below

Ok correction number two...

For some reason a few light bulbs in parallel told me three bulbs... With three bulbs in parallel the LCD works fine... Three light bulbs should be similar to a 360v fuse if I am following you correctly... The fuse on the board is a 250v... So I took one bulb out of the circuit....

Now with two bulbs in parallel...

The lights come on, mid range, gets brighter to full elimination, then goes out... The LCD comes on, shows a screen when plugged into a computer, then when the light bulbs go out so does the screen... And the green power light goes out...

I am a slow learner, but learning...

Edit: I ordered the fuses (5) today... Same type and style that is on it now.

[browork]

Re: ViewSonic Model VG920 - No Display/No Power Light

here is a video of what I see...

http://www.youtube.com/watch_private...-wsFQsmA4BKTOQ

it is private and will be deleted probably within 24 hours...

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

Ok correction number two...

For some reason a few light bulbs in parallel told me three bulbs... With three bulbs in parallel the LCD works fine... Three light bulbs should be similar to a 360v fuse if I am following you correctly... The fuse on the board is a 250v... So I took one bulb out of the circuit....

Now with two bulbs in parallel...

The lights come on, mid range, gets brighter to full elimination, then goes out... The LCD comes on, shows a screen when plugged into a computer, then when the light bulbs go out so does the screen... And the green power light goes out...

I am a slow learner, but learning...

Sometimes I explain the theory behind a test, then forget to describe the test. Sometimes I just tell people how to do a test, expecting them to pick up on the theory. Sometimes these don't work.

Some event caused the monitor to suddenly draw enough current to blow the fuse. Usually there is another failure and the fuse protects the circuit board. Rarely there is no apparent cause. This could have been one of those rare events. It could have been a power surge, a defective fuse, bad karma, whatever.

In the event of a 'hard' failure (shorted power FET), bypassing the fuse could do serious damage. I'm talking about shorted rectifier bridge, shorted thermistor, lands burnt off the circuit board. (Recall that a 15 Amp circuit breaker will handle well over 20 amps for a short time before tripping).

My preferred way to test things like this is to use a Variac - a variable transformer. That way I can increase the voltage slowly while monitoring the current, and avoid nasty (expensive) surprises. Most people don't have one sitting on their workbench. By using a light bulb (or several light bulbs in parallel), we limit the current. Think of it as a layman's variac.

A 100 watt 120, volt light bulb has a resistance of just over one ohm when hot. Three of them in parallel have a resistance of about .35 ohms. By starting with one bulb and adding them in parallel you are increasing the voltage to the monitor, making sure it doesn't suddenly draw a large amount of current. The fact that the monitor worked for an extended period of time with three bulbs in parallel leads me to believe it will work with a fuse in place of the lamps.

PlainBill

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Re: ViewSonic Model VG920 - No Display/No Power Light

Funny part is that I think I have a a variable transformer in storage. When I started my small town computer repair business I took over an old TV repair shop. It was complete with thousands of old TV parts and test equipment. I cataloged everything and put it in storage in case I might need to use it later.

I will update this thread when I get the fuses and replace it, doing a 24 hour burn-in test to make sure it will work fine or if it fails again.

Odd part is that it is a 250v fuse, so that is one huge power serge to take it out and nothing else. Of course more then 2A would take it out too, so who knows.

[PlainBill]

Re: ViewSonic Model VG920 - No Display/No Power Light

Funny part is that I think I have a a variable transformer in storage. When I started my small town computer repair business I took over an old TV repair shop. It was complete with thousands of old TV parts and test equipment. I cataloged everything and put it in storage in case I might need to use it later.

I will update this thread when I get the fuses and replace it, doing a 24 hour burn-in test to make sure it will work fine or if it fails again.

Odd part is that it is a 250v fuse, so that is one huge power serge to take it out and nothing else. Of course more then 2A would take it out too, so who knows.

A few words on fuse ratings. The voltage rating is the voltage that they will interrupt without arcing. Actually, I believe it is a fraction of the voltage they are tested at. IIRC, the standard test for a household 15 Amp fuse was hitting it with 2000 Volts at 500 amps. It had to 'blow' without exploding. That 2A 250 volt fuse wouldn't have any problems operating at 500 or even 5000 VAC as long as the current didn't exceed 2 amps.

PlainBill

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Re: ViewSonic Model VG920 - No Display/No Power Light

Thanks for the info and the help. New Fuse arrived last week. The monitor has been running great for 5 days now with no additional issues. It appears to have just been a bad fuse.