Amiga 500 video to vga build

[Divarin]

Hi all I have an Amiga 500 and I'm trying to build a video cable to get it going on a VGA monitor that does support the 15khz signal.

First off I know that you can buy these but with my setup, to try to save space and not have too many cables, switch boxes, etc... lying around, I am trying to do basically the same thing I did with this spectrum where I built a cable to go out of the spectrum and into the custom Atari ST to vga cable I had previously built.

So I sourced another 13 pin din extension cable and a 23 pin d-sub connector and set out to build this cable. The issue I have is that the signal seems to lack any blue, it's very yellow indicating that only the red and green are coming over.

I know you're not supposed to combine the grounds but in my case I really do only have one ground to map to so on my first attempt I combined all of the grounds, (amiga pins 16-20, going off this pinout) That's when I saw the yellow video. It's stable and the sync is good but yellow.

So then I thought maybe I'll just connect my one and only ground to pin 20 (Video ground) but it's still yellow.

I did double check that the pin mapping is accurate so I'm assuming either there's an issue with the video port on the computer itself or how I'm connecting the ground(s).

 

[Divarin]

I just realized my 12 volts weren't coming in through the power supply due to a broken solder joint on the power connector. That may or may not have affected this. I'll have to test more later when I get a chance.

 

[Divarin]

Nope, fixing the power socket doesn't change the video issues.

I took a look at each of the R, G, and B pins coming out through my scope and they all look good. I confirmed that my ST to VGA cable is still good by connecting the spectrum and ST and white is white (not yellow) so I know the problem isn't there.

I also noticed that all of the grounds from pin 16 through 20 are connected on the motherboard anyway.

 

[Divarin]

well I think it might have just been some strangeness about my monitor. It looks find with the ST or spectrum but it seems with the amiga I needed to adjust the colors/brightness a bit. it still looks green in the test program when I try "full white" but when I click on "RGB Bit Levels" or "RGB 15 Levels" all 3 colors are there.
the white bar is pretty white on full intensity but fades to a green in the darker intensities.

 

[powerlot]

What are your voltages for the RGB signals when connected to the monitor? It should be 0.7V for maximum color and 0V for black (at 75ohm impedance). Also, how are your grounds combined? I'd try to keep video ground and digital ground separate.
Is your cable proper for analog video signals?

 

[Divarin]

What are your voltages for the RGB signals when connected to the monitor? It should be 0.7V for maximum color and 0V for black (at 75ohm impedance). Also, how are your grounds combined? I'd try to keep video ground and digital ground separate.
Is your cable proper for analog video signals?

as for the voltages/impedance that's a good question I didn't measure that. Unfortunately I won't be able to today and not sure if I'll be home (or have time) tomorrow either.
Right now I have the ground only going to the "video ground" pin (pin 20), which is electrically connected on the motherboard with 19, 18, 17, and 16. I don't have anything connected to any of the digital signal pins at all only analog R (3), G (4), B (5), analog video ground (20), hsync (11) and vsync (12).

So the way I have this hooked up is, this new cable is only about 4 inches long has the 23 pin dsub on one side going into the Amiga and a 13 pin din (female) on the other. That goes into a 13 pin (male) to VGA which I have run to the monitor and I use that originally just for an Atari ST and later I build a similar adapter cable to connect a ZX Spectrum to the monitor as well. The spectrum looks perfect through this connection, the ST's colors look fine but the picture is a bit dark and I end up having to increase the brightness when I hook up the ST.

 

[Divarin]

Okay well I haven't been back at this for a while because I was waiting for a replacement keyboard membrane to get shipped before I could really do any more testing.
I am noticing strange behavior which I thought would be too difficult to describe with words so I've uploaded a video to demonstrate:

A few things to note:

• Looking at the color of the window borders on startup, which I expect to be white, sometimes they're white (as they are at the start of this video), sometimes they're yellowish, and sometimes purple. That's not to say that only the window borders are discolored, the whole screen is. It's just that the window borders are the first place you'd notice this as the computer starts. Compare the window borders between the first boot-up at 0:16 and the 2nd one at 5:30.
• Sometimes during startup before anything comes on screen the screen flickers.
• This same flickering and tearing can be seen in the video which is most visible at around 4:00 (especially bad at 4:30)
• Also around this time, when I'm cranking up the brightness of the background notice that the window borders are changing color.
• The last thing to note is that the apparent "gradient" on the screen (where it's darker at the top and lighter at the bottom) seems to be an artifact of my cellphone's camera and isn't actually visible in person.

It does not appear to be my cable or the video port's soldering I tried putting pressure on it and wiggling the cable around but that had no effect. I also tried hooking up the spectrum to this same cable (as explained earlier in this thread) and it shows up normal with good bright (and correct) colors.

I still can't rule out that there's an issue with my adapter that takes the video from the Amiga's video port and puts it into the 13-pin din to VGA cable but I've double and triple checked that things are wired correctly and there's no loose connections.

I'm hoping these video artifacts will look familiar to someone and can point me in the right direction.

 

[Divarin]

BTW, I should mention that I'm an Amiga newb so if anything you see is expected behavior (like window border changing color in response to background changing color) then just let me know.