VG175 flickers, trying to locate cap.

VG175 flickers, trying to locate cap.

Joined: January 11th, 2005, 4:38 pm

April 29th, 2005, 7:27 pm #1

Hello everyone,

I have a viewsonic 17" VG175 LCD that When I turn it on, it flickers massively horrizontally with hundreds of white lines. After 10min it synchs and almost looks great.

I am thinking its a cap problem, but I dunno where to start.
Try resholdering some components to remove cold joints. But didn't see anything obvious. I dunno if it on the main board or on the HIGH voltage board.

Any suggestions?


Dave
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Richard B
Richard B

April 29th, 2005, 9:17 pm #2

Hi this will be on main board look for clock IC near crystal and a cap on its supply.

But check any SMD electrolytics.

Rich
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Joined: January 11th, 2005, 4:38 pm

May 2nd, 2005, 10:52 pm #3

Hi Richard,


I located the clock and I checked the SMD electrolytics next to it. It was ok.
Here is a bunch of photos of my monitor.
http://photos.yahoo.com/dmajaess

There is a photo called "close_up_clock"
Like I said, I removed the SMD capacitor to the right of it and measured it with my LC meter. It is ok.
I checked a bunch of other SMD caps (in circuit testing) with a multimeter looking for shorts. Couldn't find anything obvious.

In the photos, I showed my monitor symptom. When the monitor powers up from cold, it is really liney. After 10minutes it works better, after 30minute it is fine. I think it is a faulty cap. But I can't seem to locate it. Nothing is bulging, no odd colors on the solder from leakage.

I don't have an incircuit meter, just a multimeter and a LC meter. Is removing all the SMD caps the only way to find the problem?
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Richard B
Richard B

May 3rd, 2005, 12:10 am #4

Hi an LC meter will not find it unless it can measure the D of the capacitor and needs a seperate button/function for this measurement it is the same for capacitors as Q is for inductors.

It measures the goodness of the capacitor so a cap with high internal resistance will read a high D.

I always use a scope and look for electrolytics connected from GND to a voltage rail with high voltage ripple on them, also check the PSU if this is external.

Rich
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Dave
Dave

May 3rd, 2005, 4:00 am #5

I replaced most of the electrolity capacitor on the +5V side. There seem to be a +/-0.5Vripple on the 5V after one particualr diode. Spent 3hours trying to locate the ripple's source. Changing caps, etc... Couldn't find it, but it is in sync with a clock pulse coming from the main IC.


I then bypassed the diode with a wire, and got no ripple. But the problem wasn't fixed. (Took out the wire, got a the ripple )

I check all the caps with my Scope. They seem to be working alright.

Is there a service manual for this monitor?
Maybe I can test some points to isolate the problem
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Oliver
Oliver

May 10th, 2005, 8:03 pm #6

Hello Dave,

i have the simular problem with a 17" TFT Viewsonic Vg175.
Could you please tell me (or mark it in a picture of the TFT-mainboard) where i can find this nasty 1µF SMD cap?
This would be a great help for me.

Thanks a lot, Oliver
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Richard B
Richard B

May 10th, 2005, 10:54 pm #7

I replaced most of the electrolity capacitor on the +5V side. There seem to be a +/-0.5Vripple on the 5V after one particualr diode. Spent 3hours trying to locate the ripple's source. Changing caps, etc... Couldn't find it, but it is in sync with a clock pulse coming from the main IC.


I then bypassed the diode with a wire, and got no ripple. But the problem wasn't fixed. (Took out the wire, got a the ripple )

I check all the caps with my Scope. They seem to be working alright.

Is there a service manual for this monitor?
Maybe I can test some points to isolate the problem
Hi do you have any freezer spray you could try to bring it back by cooling individual parts until you find the component that is the cause.

Rich
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Dave
Dave

May 11th, 2005, 1:07 am #8

Check your email Oliver.
Dave
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David
David

May 11th, 2005, 6:03 am #9

Richard,

Thanks for the freezing idea! I tried it on a viewsonic va520 with flickering video. I found a power chip (AIC1526-1CS) on the board that was bad. when I froze that chip, the video was perfect for a few seconds. I replaced the chip with an equivalent and the monitor works great now.

I am curios as to why freezing the chip worked??
Maybe a thermal sensor in the chip??

Thanks
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Fred
Fred

May 12th, 2005, 8:24 pm #10

Hi do you have any freezer spray you could try to bring it back by cooling individual parts until you find the component that is the cause.

Rich
Hi Dave
Can you tell me which part to replace to fix it?
Thank You
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