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Amiga A520 Modulator – S-Video Out

Whilst trying to capture the Amiga via a USB Capture device, I ran into problems. I couldn't get anything other than a black screen and considered the fact that the A520 modulator was producing a bad signal. The USB Capture device worked fine on Windows 10 with the playstation, so I knew that it worked.

I'd been using an A520 on the Amiga for a while as my secondary PC monitor has Composite Video in and it was easy to switch back and forth. I also wanted colour, so I chose the A520 over the A500's standard mono video output.

The USB capture device also accepted S-Video, so I thought I'd try and 'upgrade' the A520 to S-Video, cleaning the signal in the process, in an attempt to capture the video on my desktop. Another advantage would be that I'd stop having to switch video inputs all the time, which would also help standard windows usage as half the time a window would open on the screen that was currently displaying the Amiga.

So, what to do? Google! Zenithia's blog was a first hit with this post where he uses another post's instructions to modify an A520. The other post is S-Video from the A520 at Dave's Amiga Hardware Page. Zenithia seemingly does the circuit-board conversion correctly but, at the end, he describes how Dave leaves out the information on adapting the two Y and C signals from the modified A520 to an S-Video plug. Instead, Zenithia uses a capacitor to fake the sync... let's sort all this out and build the cable correctly!

Required Components

One thing that Dave's page doesn't have is an obvious list of required components... so... here's your shopping list:

capacitors:
	22uf electrolytic - 16v (x3)
	100uf electrolytic - 16v 
	220uf electrolytic - 16v 
	56pf ceramic
	0.1uf (100n) ceramic

resistors:
	- 180 ohm
	- 220 ohm
	- 270 ohm
	- 330 ohm
	- 1 Kohm
	- 10 Kohm
	- 470 Kohm
	- 1 Mohm
	- 2.2 Mohm

I've included the four stock electrolytics on the board (not mentioned by Dave) as purchases also. The 100uf on my A520 pretty much fell off... there's no real chance that capacitors from 1989 are any good anymore!

Modifying an A520

First up, grab your A520 and prize it open. It's held together with plastic pins that insert into sleeves on the opposite half-shell. Grab a flat-head screwdriver and gently separate the case at each corner. The case will come apart with minimal force, so never apply too much pressure! Finally remove the two screws that hold down the RGB plug.

Once apart, turn on your soldering iron. There's 19 steps to follow from the instructions at Dave's page. Make sure that any wiring that you use to bridge pins or points are insulated! Also try and place any bridging capacitors or resistors away from the soldered pins of other components so they aren't pierced when the case it put back together!

Dave's page has three images at the end, showing the original circuit, the circuit after modifications and finally the deltas between both. You'll note that the modified circuit shows how the C and Y are now available from the two RCA sockets on the modulator.

The modifications are pretty straightforward. One tricky part was the installation of a resistor in R18. My board had the holes drilled, but no solder pads. So instead I just piggy-backed the resistor under the board, soldering to points along the traces. If you look really closely below, you'll see that the holes have no pads, directly below the hacked on resistor.

Another point is to be super-careful when cutting traces! Too often my knife would go flying to towards another trace trying to do other damage.

Finally, the hack inside the modulator is pretty tricky. Make sure you have a sharp pair of snips when you're cutting the white plastic housing away. Once done, solder a pin through the RCA plug rear lead and push it down towards the header. From there, it'll be held perfectly in place to solder the final connection.

Wiring up the cables

So, you now have both Chroma (Colour) and Luma (Intensity) available on your modulator. The previous Audio-In RCA plug is Intensity, with Video Out being Chroma. From here, you need to wire up an S-Video Cable appropriately. I grabbed an S-Video cable I had spare along with a component cable that I have absolutely no use for.

After chopping them both in half, I bared the ends of both and wired everything together. The pinout of my S-Video cable was as follows:

	1	gnd		black
	2	gnd		brown
	3	intensity	red
	4	chroma		orange

With that, I just made sure the two channels were paired and that the grounds were attached to the shields of each RCA lead. The other two wires were the signals and, officially, I could wire them either-which-way as I can swap the plugs at the Amiga end!

Does it even work?

It sure does, but the USB Capture device still doesn't pick up the signal! The Amiga must output some very weird frequencies or incorrect signal voltages. Regardless, my secondary monitor also takes S-Video, so I plugged it in...

Ahh... crisper... I mean, it's not perfect, but it's much clearer! Disregard the last shot... one of my A500s is throwing those vertical lines and they're on the todo list to fix. As for getting the picture USB-capture'd, I suppose I could start toying with the RGB port to get a better signal, but I have no idea if this USB Capture device will ever support it... instead I've gone and sourced an internal AverMedia PCI-e Card. Will update later when that arrives.