viewsonic vg720 no power, blown transistor on mainboard

[Th3_uN1Qu3]

Re: viewsonic vg720 no power, blown transistor on mainboard

Or it's blind luck that the cpu starts without it.

Or the guys designing the CPU know what they are doing, as opposed to those who did the board.

[cadillacman]

Re: viewsonic vg720 no power, blown transistor on mainboard

yeah im my opinion this is like a viewsonic appendix. It came with one, but eventually mr viewsonic gets sick and you remove it. It works fine without it lol.

there must be some other way the thing resets itself without this push-pull totempole ic. Ive tried and tried to make it glitch, lockup, and otherwise fail and the thing just works like it should. Its good, im calling this a valid repair at least in my particular case.

[PlainBill]

Re: viewsonic vg720 no power, blown transistor on mainboard

yeah im my opinion this is like a viewsonic appendix. It came with one, but eventually mr viewsonic gets sick and you remove it. It works fine without it lol.

there must be some other way the thing resets itself without this push-pull totempole ic. Ive tried and tried to make it glitch, lockup, and otherwise fail and the thing just works like it should. Its good, im calling this a valid repair at least in my particular case.

It's under the heat sink, but the answer is the Gemini processor. There are several threads started about a year and a half ago where two people had the same problem on two different versions of this board. If you want to locate them do an advanced search for my posts with 'reset controller'.

The short form is a full datasheet is not available for this version of the Gemini chip, but the short form available mentions that it has an internal reset controller.

Note that letting the monitor run for hours does little to test this. The reset controller is functional for only a few milliseconds when power is applied.

PlainBill

[edo]

Re: viewsonic vg720 no power, blown transistor on mainboard

Hello everyone,
My ViewSonic VG720 just went the same way yesterday: no power , no led. Glad you found the problem and solution, I intend to remove the U406 part and wish for advice. I might sound dumb but after removing the stand I can't find a way to open the monitor. Being new to the insides of computers and electronics, it would be great if you could tell me about how to remove this reset controller.

[edo]

Re: viewsonic vg720 no power, blown transistor on mainboard

Managed to open it.
The U406 doesn't seem burned from the outside (not that I can check it's fine or not).
Also there's something that seems odd to me on the other side of the board. There's a connexion between two rows like it has melted. But maybe it's the way it's supposed to be.
Here are the pictures (what would we do without pictures).

[PlainBill]

Re: viewsonic vg720 no power, blown transistor on mainboard

Managed to open it.
The U406 doesn't seem burned from the outside (not that I can check it's fine or not).
Also there's something that seems odd to me on the other side of the board. There's a connexion between two rows like it has melted. But maybe it's the way it's supposed to be.

Use a DMM to measure the voltage on each of the pins. Use one of the screws holding the board in place as the ground point. That will tell us if it's working or not.

The solder blob on the second picture is normal.

Here are the pictures (what would we do without pictures).

Surprisingly (frustratingly) a significant number of people think they can simply describe (or not describe) a problem and we can tell them which part to replace. We ARE good, but not that good.

PlainBill

[edo]

Re: viewsonic vg720 no power, blown transistor on mainboard

You may not be that good but you certainly are that helpful. In no way could I have found by myself the solder blob was normal, as I'm absolutely not familiar with electronic terms and therefore lost with any related web search. Thanks for that information.
I have no measurement tool neither the means to get one (back in a hurry from Japan, I just assembled a computer with parts from people's donations ) so I will try to remove the U406 and see if that solve the problem. I suppose melting the three solders with a hot screw driver is better than cutting them with a wire cutter. Sounds a bit Macgyverish but I'm very short on tools and need this screen to get power again.

[edo]

Re: viewsonic vg720 no power, blown transistor on mainboard

It worked ! Thank you very much for the advice. My first guess was the solder blob would have been the problem as the U406 seemed OK from the outside. You prevented me from removing the blob and that saved my screen !
Thanks again !

[Longbow]

Re: viewsonic vg720 no power, blown transistor on mainboard

A bad reset i.c. is a very interesting problem. I have seen a few failures over the course of many years, but they are on the whole very reliable. Their function is to keep the main uPC reset pin low for a specific time, maybe a few milliseconds after power is applied. uPC's must have a specified reset pulse in order to boot up correctly. With few exceptions, processors do NOT contain their own reset circuitry. They may boot up without reset, but that would be unpredictable and accidental. That part is there to make sure the circuit starts up under worst case conditions. And that's what we want.

No reset pulse, or one with the incorrect timing, risks having the uPC start up in an unknown condition. Most often, the reset IC brings up the main uPC in a multi processor unit. The main uPC then has a reset output pin that is connected to all the secondary processors. Unless that reset IC is burned (very unusual), the only way I've ever located a bad one is to watch the output with a scope while applying power to the unit. The general symptom, as you would expect, is a dead set.

[Th3_uN1Qu3]

Re: viewsonic vg720 no power, blown transistor on mainboard

Well, i haven't been here for too long - less than an year if you look at my profile, and there have been at least four reset controller failures documented here already.

[PlainBill]

Re: viewsonic vg720 no power, blown transistor on mainboard

And as discussed in my previous post, the Gemini processor on this board DOES have an internal reset controller.

To correct another error, a reset input can be active high or active low. It so happens that the Gemini line favors an active high reset.

PlainBill

[Longbow]

Re: viewsonic vg720 no power, blown transistor on mainboard

Active high reset is certainly good information to know. I don't think I've come across it, and I would guess that 99% of processors use reset=LOW. The active high reset IC would then be a different part and easily identified because the other pin would be connected to B+ instead of ground. In any case, the part is important and necessary, but also an unlikely candidate for failure. There's really no way to check it other than looking at the reset pin on the uPC with a scope, when the power is applied.

What is really easy to spot, on the other hand, is a uPC with a reset line at intermediate voltage. The reset pin on any uPC will either be @ ground or @ B+. Anything in between is incorrect and will prevent the processor from working.

[PlainBill]

Re: viewsonic vg720 no power, blown transistor on mainboard

Active high reset is certainly good information to know. I don't think I've come across it, and I would guess that 99% of processors use reset=LOW. The active high reset IC would then be a different part and easily identified because the other pin would be connected to B+ instead of ground. In any case, the part is important and necessary, but also an unlikely candidate for failure. There's really no way to check it other than looking at the reset pin on the uPC with a scope, when the power is applied.

What is really easy to spot, on the other hand, is a uPC with a reset line at intermediate voltage. The reset pin on any uPC will either be @ ground or @ B+. Anything in between is incorrect and will prevent the processor from working.

Here's a suggestion: Look at this thread and then read this thread.

PlainBill

[matstermind]

Re: viewsonic vg720 no power, blown transistor on mainboard

thanks jetadm123, removing the reset chip fixed my monitors!

[cadillacman]

Re: viewsonic vg720 no power, blown transistor on mainboard

and as a followup, mines been running a-ok this whole time, day after day...

[spiscean]

Re: viewsonic vg720 no power, blown transistor on mainboard

I have a Viewsonic q20wb. Replaced the caps and all I get is the amber light when plugged in. No green light and nothing when I press the power button. Could this be having the same issue as mentioned here? Thank you.

[PlainBill]

Re: viewsonic vg720 no power, blown transistor on mainboard

I have a Viewsonic q20wb. Replaced the caps and all I get is the amber light when plugged in. No green light and nothing when I press the power button. Could this be having the same issue as mentioned here? Thank you.

Let's see - different model, different symptoms. Not likely.

This type of question comes up repeatedly, and each time it does I wish I could reach through my monitor and slap the offender upside the head. At this time there are thousands of models of monitors sold under dozens (hundreds?) of brand names. Since service manuals are only slightly less rare than honest politicians, it is impossible to determine what components were used to build a particular monitor. Frequently two monitors of different brands will be identical internally. What is worse, frequently a manufacturer will sell the same model of monitor with different internal cards (I have seen 4 different service manuals for one popular model of Acer 19" wide screen monitor. The highest version number on those manuals is 6. Yes, at least 6 versions of one model number).

To get a useful answer to your question, take your monitor apart and take pictures of the power supply and the signal card. Take them from directly above the card, no flash, well lit, sharp focus, and 1000 - 2000 pixels on a side. Attach them using 'Manage Attachments' - below the text entry area - to a NEW thread.

PlainBill

[W4LGC]

Re: viewsonic vg720 no power, blown transistor on mainboard

I've removed the two screws at the bottom rear of the VG720, but can't determine what continues to hold the enclosure together. I need to remove U406 because last night the LED refused to come on with power confirmed at the case input. Help, please.

[selldoor]

Re: viewsonic vg720 no power, blown transistor on mainboard

SM is here
https://cdn.badcaps-static.com/pdfs/...b2174bad8f.pdf
Page 60 something

[W4LGC]

Re: viewsonic vg720 no power, blown transistor on mainboard

Thank you, the service manual was very helpful for getting the case off.

U406 was fried, so I carefully removed it. Leaving the circuit open, or shorting pin runs 1 and 2 did not allow the power LED to come on, so I'm studying the schematics now. I'm finding 5V and 12V all over the place, so now focusing primarily on the board where U406 was originally mounted.