Some time I bought Amiga 500. It was mostly working. It turned out it needed CIA replacement (it got IRQ line stuck low permanently) and floppy cleanup and disk present switch de-oxidized. But to do all that I needed to connect it to some monitor to see what I am doing and most importantly what Amiga is doing. For troubleshooting the black and white, stock video output was enough but once fixed I wanted some color!

This is my first Amiga but I know there are well known solutions to that. But they suffer from being more expensive than Amiga itself (my Amiga was 65€) or requiring custom plugs like 23 pin video connector. (or both). My 1st idea was to just build A520HD video adapter. But since I was building it from scratch I thought I’ll make an internal version. Then I though I wanted SVideo more - my 4/3 monitor does not have component input. Common solution seems to use Sony CXA1145 chip from A600 but it is unavailable (I think) and it requires an external delay line. I went with more modern AD724 instead which is much simpler. Designed and made a board: Board_FBoard_B

I don’t have the schematics from that time - never thought to publish it… Anyway - it didn’t obviously work. I naively assumed that DENISE pin 18 called “burst” on the schematics will output PAL 4.433MHz color burst. So it doesn’t. :) Turnes out the same people who made Atari 800 worked on Amiga and did it the same way. So burst is “base clock” / 8: 28.37512MHz / 8 = 3.54689MHz which is 4/5 of the required frequency. Hmmm Atari… Let’s borrow colorburst from Atari then: I should probably tidy up my desk. Nah...

And it kind-of worked: Should I be happy or sad?

Hardly ideal…I need to move PAL clock to my board and colors are off… The colors issue was somebody mixed op-amp legs… and most of distortion is gone once we have own clock.: Better now

Not ideal but promising. The board looks bad now though: Frankenstain of a board

The issues:

  • New board needed - this one served well as a prototype but it is not reliable at all.
  • PAL clock is not synchronized to pixel clock - there was visible moving artefacts on the pixels edges.
  • Still a lot of noise

I decided to re-do the schematics, add A600 /4*5 clock circuit (almost identical as in Atari) and order board from PCB manufacturer.

Sooooo 2years have passed. I did other things :) But finally: Beatiful

With some more fixes - I made a mistake and swapped R and B AD724 inputs and also replacing input cables with coaxial grounded at input - the image quality is great. So here it is at GitHub. Fixes are not there yet but maybe sometime ;) installed_1st_attempt an eagle eye can spot external PSU replaced with 2 internal boards and 12V input next to new video output. installed_back

The image finally looks great over s-video: installed_image svideo

And even beter over component although TV makes it darker and little over-saturated. This does not happen on a monitor with component input. component

References: