Bad Electrical Noise from Mobo in VGA Signal *video of problem* \ VOGONS

Bad Electrical Noise from Mobo in VGA Signal video of problem

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First post, by kingcake

Posted on 2020-08-10, 17:33

kingcake Offline

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I'm working on a Packard Bell 386 with Oak VGA integrated onto the motherboard. It has horrible electrical noise in the VGA output and I can't figure out the source.

Here's a video of the noise:
https://www.youtube.com/watch?v=bBMaL1LD3zU

Things I've tried:

Recapping ENTIRE mobo -- somehow made it a little worse

Swapping power supplies

unplugging drives and stuff

shoving a couple of low esr caps across the 5v molex connector -- this improved it about 25%

This is on my normal test bench setup. So I know my cable and monitor are good. I use them every day.

Any ideas on what to do next?

Reply 1 of 7, by Doornkaat

Posted on 2020-08-10, 17:46

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The video looks like you're using a TFT monitor, is that correct?
Some video adaptors just put out a lot of noise that many TFT controller boards can't handle. I have a couple of cards that will output an image that looks ok on CRTs but very noisy on TFTs. Other than trying to add filters I don't think this can be helped.

Reply 2 of 7, by kingcake

Posted on 2020-08-10, 18:21

kingcake Offline

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Doornkaat wrote on 2020-08-10, 17:46:

The video looks like you're using a TFT monitor, is that correct?
Some video adaptors just put out a lot of noise that many TFT controller boards can't handle. I have a couple of cards that will output an image that looks ok on CRTs but very noisy on TFTs. Other than trying to add filters I don't think this can be helped.

Hmmm I will try hooking up a CRT to see what it looks like. But I mean, the noise is really, really bad.

Reply 3 of 7, by Oetker

Posted on 2020-08-10, 18:26

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It's a long shot but try if you can adjust sync polarity of the video card. I've a similar noisy image before I did so, however it was a P3 with an Ati Rage Pro at 1600x1200, so quite a different system. And have you tried a discrete video card?

Reply 4 of 7, by kingcake

Posted on 2020-08-10, 19:11

kingcake Offline

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Oetker wrote on 2020-08-10, 18:26:

It's a long shot but try if you can adjust sync polarity of the video card. I've a similar noisy image before I did so, however it was a P3 with an Ati Rage Pro at 1600x1200, so quite a different system. And have you tried a discrete video card?

Yeah I tried two discrete ISA cards. An Oak card and a Cirrus Logic card.

The Oak card looked much better. The Cirrus Logic card looked just as bad.

Reply 5 of 7, by kingcake

Posted on 2020-08-11, 13:21

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I tried a CRT. The noise is much better, but still noticeable.

Reply 6 of 7, by kingcake

Posted on 2020-08-11, 19:35

kingcake Offline

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Welp. I just bought a digital oscilloscope. I will find the source of this extreme voltage ripple. lmao.

Reply 7 of 7, by hwh

Posted on 2020-08-12, 01:03

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I have an Axcel 386. Same Oak integrated setup. When I ran it, the graphics were "wavy" and unsteady. I assumed it had to do with storage or capicitors. That was on a CRT.

I tried an external card and the problem was 100% gone.

No, I have no clue how to fix it.

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