326298-01 C64 Motherboard Assembly - Click to Enlarge - Thanks to Bob Woolley's donation of a C64 computer and the accessories that go with it, I've begun a journey to see what I can do within this arena.
So for starters I discovered that the video coming out of my unit was pretty poor when viewed on my VIZO LCD monitor, because it had very visible jail bars and the colors were kinda muted. So first order of business was to find the schematics covering my board, which I soon got in a 1992 C64 Service Manual in which I discovered that I had the very first board revision (326298-01) utilizing an early production ceramic VIC-II chip.
Looking at the schematic, I could see that the basic circuit for taking the Chroma and Luma from the VIC-II and making them suitable for driving a monitor looked pretty sound, but I didn't like the signal mixing being used to produce the composite output. It just presented too much opportunity for the Chroma signal to bleed into the Luma if I ever wanted to use this over S-Video. Speaking of S-Video, this wasn't actually broken out on my board, since having the early version also meant that I had the 5-pin DIN A/V jack which had no separate Chroma output.
So for starters I discovered that the video coming out of my unit was pretty poor when viewed on my VIZO LCD monitor, because it had very visible jail bars and the colors were kinda muted. So first order of business was to find the schematics covering my board, which I soon got in a 1992 C64 Service Manual in which I discovered that I had the very first board revision (326298-01) utilizing an early production ceramic VIC-II chip.
Looking at the schematic, I could see that the basic circuit for taking the Chroma and Luma from the VIC-II and making them suitable for driving a monitor looked pretty sound, but I didn't like the signal mixing being used to produce the composite output. It just presented too much opportunity for the Chroma signal to bleed into the Luma if I ever wanted to use this over S-Video. Speaking of S-Video, this wasn't actually broken out on my board, since having the early version also meant that I had the 5-pin DIN A/V jack which had no separate Chroma output.
Early 5-pin vs. Later 8-pin C64 A/V Jack Pin-Out
So I began work on creating a PCB to piggyback the VIC-II chip and break-out what I hoped would be a high quality video signal, minus the jail bars.
This is what that ended up morphing into...
This is what that ended up morphing into...
| C64-JBgone_schema.pdf |
I decided to use the FMS6400 Video Buffer/Combiner chip as the final stage, but retained a lot of the original C64's video conditioning circuitry, minus the composite mixing stage. And then adjusted a few values to render a better more vibrant image.
When taking this to a PCB, I opted for a 4-layer board having both 5V and ground plane(s) in the middle layers, and also incorporated a top and bottom layer ground plane fill. Because as I was prototyping and researching information I came across an interesting article on noise bleeding from the AEC pin of the VIC-II over to the Luma pin that sat next to it, which seemed to be the real culprit for the jail bar issue. I figured using a triple ground plane scheme would be my best bet for isolating the Luma pin from the AEC pin.
The VIC-II's AEC pin is very similar to the Antic HALT pin on an Atari 8-bit computer, allowing the video chip to halt the CPU and take over full control during RAM access (DMA).
After applying some careful PCB layout considerations, this should end up being a very quiet video output stage, which I'll know for sure when the sample boards come back from JLCPCB later this week. The board's title is very optimistic in this regard.
When taking this to a PCB, I opted for a 4-layer board having both 5V and ground plane(s) in the middle layers, and also incorporated a top and bottom layer ground plane fill. Because as I was prototyping and researching information I came across an interesting article on noise bleeding from the AEC pin of the VIC-II over to the Luma pin that sat next to it, which seemed to be the real culprit for the jail bar issue. I figured using a triple ground plane scheme would be my best bet for isolating the Luma pin from the AEC pin.
The VIC-II's AEC pin is very similar to the Antic HALT pin on an Atari 8-bit computer, allowing the video chip to halt the CPU and take over full control during RAM access (DMA).
After applying some careful PCB layout considerations, this should end up being a very quiet video output stage, which I'll know for sure when the sample boards come back from JLCPCB later this week. The board's title is very optimistic in this regard.
 VIC-II PiggyBack Video Output Board |  Back Side of VIC-II PiggyBack Board |
My future goal is to re-design, hopefully improve, and condense the C64 down into something that I can squeeze into a NUC style case like my 576NUC+ and CV-NUC+ projects. I think it'd be rather cool to have a miniature collection of these 80's retro consoles that could all sit on my desk at the same time without taking up a lot of space.
So in that regard, there is more that I have been busily working on. So stay tuned to find out what's to come!
- Michael
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So in that regard, there is more that I have been busily working on. So stay tuned to find out what's to come!
- Michael
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