Wavy video on 1040 STF

Troubles with your machine? Just want to speak about the latest improvements? This is the place!

Moderators: Mug UK, Zorro 2, spiny, Greenious, Moderator Team

User avatar
telengard
Atari nerd
Atari nerd
Posts: 44
Joined: Mon Dec 26, 2005 10:33 pm

Wavy video on 1040 STF

Postby telengard » Sat Apr 28, 2018 8:33 pm

I'm getting slightly wavy video on my 1040. I'm using an SC1435 monitor. My 520ST looks fine using the same monitor and cable.

I was prepared to start doing a power supply recap, but I swapped out with another (already recapped) PS and it looks the same.

Seems like I should be looking at caps near the video circuit, any suggestions on what could be the culprit?

siriushardware
Captain Atari
Captain Atari
Posts: 401
Joined: Thu Aug 21, 2014 7:55 pm
Location: UK

Re: Wavy video on 1040 STF

Postby siriushardware » Sat Apr 28, 2018 9:10 pm

Before you go pulling anything out, read this thread and see if your problem seems like the same problem:

viewtopic.php?f=15&t=33234

User avatar
telengard
Atari nerd
Atari nerd
Posts: 44
Joined: Mon Dec 26, 2005 10:33 pm

Re: Wavy video on 1040 STF

Postby telengard » Sun Apr 29, 2018 4:00 pm

Thanks for that info.

I went over the videos and his is truly wavy. Not sure exactly how to describe what mind is doing, maybe it is dot crawl. There is slight shifting in where the pixels end up on the screen, like maybe over one or two randomly. It's not wavy in unison like those videos.

I also took my STF apart and I'll be danged if I can find an adjustable cap anywhere. Looked at the schematic for the STE and C117 seems to be PAL only. This one is NTSC.

Anything else I could check out?

User avatar
telengard
Atari nerd
Atari nerd
Posts: 44
Joined: Mon Dec 26, 2005 10:33 pm

Re: Wavy video on 1040 STF

Postby telengard » Wed Jun 06, 2018 2:29 pm

I've been researching this a bit more, and based on what I've read, the issue is with the hsync (possibly jitter?). I can't find a schematic for my exact board rev, so I've been trying to trace where the hsync goes after coming out of the glue chip. It seems to go to an inverter chip but from what I've read that is for the signal going to the modulator, which I don't have. In the schematics I do have, it seems the signal goes straight out from the glue with one cap somewhere in the signal chain.

Also, I found an animated image which shows the type of waviness I'm seeing, although mine is a a little more subdued than this.

Click on the pic to see it a little better, it doesn't show up as well when the image is shrunk. The distortion goes kinda from side to side. Almost like random noise is in the picture. Very fuzzy.

image1.gif


Any suggestions would be very helpful. And I do have an oscilloscope.
You do not have the required permissions to view the files attached to this post.
Last edited by telengard on Mon Jun 18, 2018 2:49 pm, edited 4 times in total.

User avatar
telengard
Atari nerd
Atari nerd
Posts: 44
Joined: Mon Dec 26, 2005 10:33 pm

Re: Wavy video on 1040 STF

Postby telengard » Mon Jun 18, 2018 1:55 pm

I spent the weekend pinning out the connection of the HSYNC from the glue to the rgb connector since I can't find a schematic that matches my board rev.

What I see is the signal going through 2 hex inverters via a chip and there is 22pf (filter) cap across the output. Why is the signal put through 2 inverters? Here's a pic

st_schematic.png


I also tried powering the STF off my Rigol power supply and the problem was worse. In both cases the signal looks pretty clean coming out of the glue, but it doesn't look as good coming out of the inverters. I have no idea what is acceptable for an hsync signal and what would cause the type of distortion I'm seeing. It is the right freq though @ 15.77Khz.

I know there are folks much more knowledgeable about this stuff than I so I'm hoping for some good advice. :cheers:
You do not have the required permissions to view the files attached to this post.
Last edited by telengard on Mon Jun 18, 2018 2:26 pm, edited 1 time in total.

User avatar
Gaiyan
Hardware Guru
Hardware Guru
Posts: 336
Joined: Tue Jun 29, 2004 3:39 pm
Contact:

Re: Wavy video on 1040 STF

Postby Gaiyan » Mon Jun 18, 2018 2:09 pm

telengard wrote:I spent the weekend pinning out the connection of the HSYNC from the glue to the rgb connector since I can't find a schematic that matches my board rev.

What I see is the signal going through 2 hex inverters via a chip and there is 22pf (filter) cap across the output. Why is the signal put through 2 inverters? Here's a pic

st_schematic.png


I'd imagine two inverters are used for boosting the signal and perhaps timing as well and probably as a buffer. At least those are the typical reasons for having two NOT gates in series iirc.
Image

User avatar
telengard
Atari nerd
Atari nerd
Posts: 44
Joined: Mon Dec 26, 2005 10:33 pm

Re: Wavy video on 1040 STF

Postby telengard » Mon Jun 18, 2018 2:49 pm

That makes sense. Any thoughts on the distortion I'm seeing, and maybe what could be involved? (the distortion shows up better in the pic if you click on it).

ijor
Hardware Guru
Hardware Guru
Posts: 3418
Joined: Sat May 29, 2004 7:52 pm
Contact:

Re: Wavy video on 1040 STF

Postby ijor » Mon Jun 18, 2018 6:47 pm

telengard wrote:What I see is the signal going through 2 hex inverters via a chip and there is 22pf (filter) cap across the output. Why is the signal put through 2 inverters?


It is a common method to buffer the signal. There is no point in stuffing a buffer chip when there are inverter gates available already. If you check the published schematics, the inverted signal is used for syncing with the video clock. Even when your board has no modulator, some of the logic is always present anyway. So it makes all the sense to invert it once again to obtain a buffered signal instead of adding a non inverter buffer chip.

mlynn1974
Captain Atari
Captain Atari
Posts: 194
Joined: Mon Mar 03, 2008 10:33 pm
Contact:

Re: Wavy video on 1040 STF

Postby mlynn1974 » Mon Jun 18, 2018 11:55 pm

This may sound silly but does your STF have all the shielding in place?
In particular there should be a shielding box around the video shifter. I'm not an expert on this but the American FCC at the time had some strict regulations about Radio Frequency Interference (RFI). The shielding should be there to prevent the component from accepting and causing interference.

My STF (UK, PAL) is missing some shielding due to a large memory upgrade board but it doesn't cause any problems at least with the monitors and TVs I've used.
Still got, still working: Atari 4Mb STe, 520STFM, 2.5Mb STF.
Hardware: Cumana CSA 354, Ultimate Ripper, Blitz Turbo, Synchro Express II (US and UK Versions).

User avatar
telengard
Atari nerd
Atari nerd
Posts: 44
Joined: Mon Dec 26, 2005 10:33 pm

Re: Wavy video on 1040 STF

Postby telengard » Tue Jun 19, 2018 3:25 pm

Not silly at all. I've seen behavior like that in the past, esp with my C64. There's definitely a metal case over the shifter and related components in the center, and also a metal cage that goes over everything. I was seeing this behavior with all of that in place though.

I guess it is possible there is another part of the original shielding that is missing, but based on the STs I have, I'm not sure what it would be.

Thanks everyone for chiming in. I'm hoping someone might have some ideas for me to try out. I'm wondering if that inverter chip (or the voltage being supplied to it) might be the issue?

Everything I've read seems to indicate the issue as you can see in the above picture is the HSYNC being off. It looks rock solid coming out of the Glue chip, but somewhere along the line it seems to be affected. I'm not an expert so it's possible what I see on my scope coming out of the Glue isn't great either, but after adjusting the trigger holdoff, I see no jitter, etc.


Social Media

     

Return to “Hardware”

Who is online

Users browsing this forum: No registered users and 5 guests