Another VIC20 SVideo mod

Modding and Technical Issues

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oracle_jedi
Vic 20 Amateur
Posts: 51
Joined: Sat Nov 08, 2008 6:54 pm

Another VIC20 SVideo mod

Post by oracle_jedi »

I know there are several great posts on modding the VIC20 for SVideo.

If this method has been done before I couldn't find it. Based on the option by A4000bear, this version is designed for an original NTSC 2-prong gold label VIC with a socketed VIC chip.

It differs from Bear's version in that this version requires no trace cuts. Besides soldering equipment you only need a 75ohm resistor and a 470pF capacitor. It also differs in that I reconfigure the 5 pin DIN monitor port to match the pin out of the Atari 800. That is:

Pin 1 - Luma
Pin 2 - GND
Pin 3 - Audio
Pin 4 - Composite Video
Pin 5 - Chroma

This allows me to use the same video cables I already have to connect the VIC to a composite color monitor and my big SVideo TV.

Note - this mod renders the RF modulator useless since I remove the +5V from Pin 1.
Note - I accept no responsibility for any damage you do to a VIC20 trying to follow this. Use at your own risk!

Okay - first let me show you my VIC's video circuit.
01.jpg
On the foil side of the board, we can see the 5 pin DIN socket:
03a.jpg
Step 1: To remove the power from pin 1, we are going to remove the capacitor C31 and the small jumper between pads E2 and E3. In the following I show you the solder holes from the foil side:
04a.jpg
On the component side, you can see C31 and the jumper removed here:
05.jpg
Step 2: Next we will disable composite video on pin 5.

Remove the unnamed ferric bead to the lower right of C31.
06a.jpg
Step 3: Next we will add luma to pin 1.

Take the ferric bead you just removed, and use it to jump left pad of its original position to the left pad of C31.
07.jpg
Test: If you connect your VIC20 with an SVideo cable now, you will have a noisy monochome display.

Step 4: Next we will remove chroma from the luma signal.

Locate and remove FB9 which is directly above pin 2 of the VIC chip (shown in previous picture).

Test: If you connect your VIC20 with an SVideo cable now, you will have a very sharp monochome display.

Step 5: Next we will add chroma to pin 5.

Take a 75ohm resistor, and use it to connect the lower pad of FB9 to the right pad of what was the unnamed ferric bead from step 2. In my example I solder the resistor on the foil side, but you can also do it on the component side if you prefer, just make sure the legs of the resistor are not touching any other component.
08.jpg
Test: If you connect your VIC20 with an SVideo cable now, you will have a very sharp color display, but only monochrome if you use a composite cable.

Step 6: Next we will add color back to pin 4.

Take a 470pF capacitor, and solder it between the empty pad above where the unnamed ferric bead was removed in step 2 to the left pad of C18. The component at C18 is not removed, so you will have to solder the leg of the capacitor to the pad of C18 as best you can.
09.jpg
Test: If you connect your VIC20 with a composite video cable you should have a good color display. Using an SVideo cable you should have a better color display.

And that's it. In Bear's schematic he used a 0.1uF capacity in line with the 75 ohm resistor. I didn't have one in my box so I was unable to test what difference it makes to the output, but if you have one and can test it I am interested in feedback and improvements.
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