Built my own: Amiga RGB to S-Video + composite adaptor small issue though...

[prowler]

Hey Munchausen,

Thanks for making the effort to join up and add that very relevant point to this thread.

It's very much appreciated.

No problem

There are a few similar posts around the interwebs where people have the same problem, but none found a solution, so hopefully I can save someone the pain I went through trying to figure this out!

[Johan1973]

I use this circuit, just add a svideo connector on the chroma and luma output, works a treat

[kipper2k]

Circuit is shown on the AD724 datasheet on page 10. Oscillator capacitor is 10 - 30 pf (i found 18pf ideal.

http://www.analog.com/static/importe...eets/AD724.pdf

Yeah, building the circuit itself isn't hard.

It's just that if you are using strip board you need to be very careful to keep the traces short or you'll get too much capacitance, the oscillator wont work at the right colourburst frequency, and you end up with a blurry black and white picture. There isn't anything about this in the layout considerations section of the AD724 datasheet.

You may need a much lower capacitance than the datasheet specifies due to parasitic capacitance in your circuit being higher.. I'm now using 4.8pF.

[TCH]

Hi!

I built that circuit in Narmi's topic, but i got a black and white screen. I have a PAL machine, so i put a 4.43 MHz oscillator between ground and FIN and connected STND to the ground to select PAL mode.
Any ideas why i got a bw screen?

[Johan1973]

Yes. I dont remember where it was, but I needed to jumper two legs on the IC together in order to get colour on pal. It has been long enough time that I dont remember much of that build. But I know I found the information in the datasheet, so I would recommend looking at that (for the AD724).

[TCH]

I already checked the datasheet but i did not found anything about it.
The pins wich are connected: STND/AGND/SELECT/DGND (these connected to ground) and APOS/ENCD/DPOS (these are connected to the +5V).
I think the pins you connected together were STND and AGND, but i already did too.

[Johan1973]

Jsr: modified mine to use Svideo out, connecting Chroma and Luma to a svideo connector.
Black and white colour is most likely due to you only getting luma signal out. Its late and I try to remember stuff that was long time ago. Sorry if I am not of much help.
The crystal must be a very specific 4.43 crystal with even the 4.43xx, xx numbers being important, to get proper video timing. If your black and white pictures are not distorted in any way, most likely that is not what cause it.

[TCH]

Thank you for helping. The crystal is on 4.433619 MHz.

[Johan1973]

That should do it.

If you followed the schematic precisely above, disconnect pin 1 on AD724 from Pin 15, and instead connect it to ground.

[TCH]

I already did that, STND is connected to ground.

[kipper2k]

It sounds like you have the PAL/NTSC selector backwards, thats why it is black, also you may have to change your capacitor value up a bit. If using it for PAL only you can remove the NTSC part of the circuit

[Johan1973]

(managed to make a time delayed double post, removed)

If you have an oscilloscope, it would be easy to check if you get chroma out at all.

[TCH]

klipper2k: The STND pin is grounded, PAL mode is selected.
I tried to put a bigger capacitor than the shematics said, instead of 15 pF i put in a 22 pF capacitor parallel to the oscillator. Still black and white and some weird noise (horizontal lines).
I tried it with a 27 pF one too, same result. The datasheet said 10-30 pF, maybe i need a larger than 30 pF capacitor for the oscillator?
I did not build the NTSC part of the circuit, there is only a PAL crystal and STND is permanently grounded.

Johan1973: I do not have an oscilloscope.
I use the composite output, did not mentioned this, sorry.

[TCH]

I uploaded my drawing to postimg, this is what i built:
http://postimg.org/image/jiw1nskcn/
The only modification to Narni's design, that this is a PAL circuit and there is no S-Video part, because my TV does not have S-Video input. And currently the capacitor beside the oscillator is not 15 pF but 27.

[TCH]

Okay, now it works, i checked the schematics of the A1200, and it's PAL oscillator had an 56 pF capacitor. I did not have an 56 pF one, but had two 27 pF, worked that too.

Johan1973, kipper2k: Thanks for the tips.

[waltermixxx]

howdy folks... glad to see this thread is still alive. I will have to go and re-visit my littleboard... I still have it, infact it's usually right in my face when I am at my work bench... I saw a memory expansion made available by Kipper2k, it made me want to hop back on this board and see what's new.... I think I would like to buy one of those memory upgrades...