Wavy Video with S-Video Mod

[OldAtAtari]

Hi, everyone.

 

I completed the Saundby S-Video mod on my 7800. I now have it plugged into my LCD screen. I know my choice of TV could be the problem, but before I go find a CRT, I figured it was worth posting my problem here in case anyone has a quick answer.

 

The video is wavy, particularly noticeable at the edges of lightly colored objects. Here's a YouTube video of the problem. https://youtu.be/dcwT4k0Y16U

 

Any ideas?

 

Thank you!

Edited May 14, 2018 by OldAtAtari

[OldAtAtari]

Just an update. After wiggling some wires around on my PC board I found that I can get the waving image to stop, while at the same time causing the picture to go black and white. I'm pretty sure this points to user error. I've probably got some faulty solder joints in there somewhere. I'm thinking I'm going to disolder everything and start over, this time with a better focus on quality.

 

Or maybe I'll just install a UAV. That thing looks pretty great.

 

Thanks.

[Osgeld]

yea check your solder joints, and keep in mind analog video from that era is going to crawl a little bit, maybe not that bad, but over svideo its not going to be like looking at a bluray menu or your pc

 

(and you would never notice it on a CRT cause the persistence of the phosphorous glow, but I am one of those if you want CRT, that's great have at it, myself, my only CRT is my model 1 NEC multisync and I have not even turned it on in 20 or more years, actually at this point I am a little afraid to as its been in my parents garage since the late 90's lol)

Edited May 15, 2018 by Osgeld

[OldAtAtari]

Thank you, Osgeld!

 

Yep, it was definitely a problem with my soldering. I'm planning now on getting a UAV from Bryan, so then I won't have to build something myself, and I'm even considering having CrossBow install it for me. If I can take myself completely out of the equation, I've got a better chance of this working. :-)

 

Thanks again.

[OldAtAtari]

Ok, we have a solution here.

 

I spoke to an electronics engineer at work, and he pointed me to the diode. Saundby, on his instructions page, says that he used an FD333 diode, but he says that a 1N4001 should work fine. So I used a 1N4001. As it turns out, these diodes are not interchangeable. The FD333 has a capacitance (yes, diodes have capacitance - I guess everything does) of 6pF. The 1N4001 has a capacitance of 15pF. My electronics friend explained it to me, something about the added capacitance causing the sync signal to overshoot. I hooked it up to an oscilloscope, and sure enough, the sync, which should ideally be a square wave, was spiking at its peak before settling back down. That was with the 1N4001. With the FD333, there is a smaller spike, and it settles more quickly. And the wiggle is gone from the picture.

 

Point being that you should use the FD333, and even though Saundby's page says that the FD333 isn't common (probably wasn't common back in the day), you can easily get it online now.