Problem with AD724

RiP · 25 March 2019, 10:59

I've just found an AD724 but sadly, I got this wrong colors (magenta, and blue instead of black) plus some odd noise lines and sync issue

I followed this article (PAL) to make it: https://www.everythingamiga.com/2017...o-adapter.html
The tantalum capacitors i got are all polarized though

Any chance to fix it?

nogginthenog · 25 March 2019, 20:05

As far as I know tantalum capacitors are polarised.
I see what you mean. The circuit shows 5 non polarised caps.

Try Googling around. Seems this chip is popular with retro systems.

Mathesar · 25 March 2019, 20:24

I would replace all the 0.1uf capacitors with plain ceramic types. Or even better: Use some foil caps like mkt for the rgb input caps. Tantalum is not what I would use for a circuit like this. As for the color issues, I think the tuning is quite critical. Does the trim capacitor give any effect?

pandy71 · 26 March 2019, 17:22

Amiga HSync and VSync are VGA compatible, use CSync instead, AD724 datasheet provide information how to do it.

Code:

 If available, the AD724 will also
accept a standard CSYNC signal by connecting VSYNC to
Logic HI and applying CSYNC to the HSYNC pin

CSync carry TV like timing. Try to rewire your AD724 and use CSync.

Check XTAL frequency (oscilloscope could be a nice thing)

Agnus DS - check waveforms at page 39
https://computerarchive.org/files/co...ual-ENG%20.pdf

I agree with Mathesar - tantalum caps can be even dangerous when improperly aplicated.

RiP · 12 April 2019, 09:30

Well, I replaced the caps with non-polarized type but still no luck

The trim capacitor had no/less effect too. I tried to use CSync but got even worse result.
The only way I could get correct colors was when I removed C7 and C8 caps and had to touch the top of C6

Mathesar · 13 April 2019, 21:06

Quote:

Originally Posted by RiP

Well, I replaced the caps with non-polarized type but still no luck

The trim capacitor had no/less effect too. I tried to use CSync but got even worse result.
The only way I could get correct colors was when I removed C7 and C8 caps and had to touch the top of C6

Strange. Looks like you have a layout problem. This kind of circuitry needs a nice and tight layout, especially on the crystal circuit and the power decoupling.
Probably your wires are just too long. For example, your trimming cap and crystql are waaaaay on the other end of the board while ideally they should be as close as possible to the ad724.

RiP · 15 April 2019, 19:19

Quote:

Originally Posted by Mathesar

Strange. Looks like you have a layout problem. This kind of circuitry needs a nice and tight layout, especially on the crystal circuit and the power decoupling.
Probably your wires are just too long. For example, your trimming cap and crystql are waaaaay on the other end of the board while ideally they should be as close as possible to the ad724.

Possible to use an external 5V adapter instead?

Mathesar · 15 April 2019, 19:41

Quote:

Originally Posted by RiP

Possible to use an external 5V adapter instead?

Maybe, but to me it looks like the layout is the most likely culprit.
Make sure that your power supply decoupling caps are close to the chip wiith short wires to the A POS,DPOS, AGND and DGND pins. Also, the crystal/trimcap return wire should go directly to the AD724, not via a "shared" ground wire.

I hope that will help.

RiP · 30 April 2019, 12:45

Ok, but how to connect an external 5V adapter to it properly?
I know that I should disconnect the 5V from Amiga and use the one from adapter but what about the grounds from Amiga and adapter?

Mathesar · 30 April 2019, 20:32

Quote:

Originally Posted by RiP

Ok, but how to connect an external 5V adapter to it properly?
I know that I should disconnect the 5V from Amiga and use the one from adapter but what about the grounds from Amiga and adapter?

The grounds from the adapter and the amiga should be tied together. You are right about the 5v; disconnect the amiga 5v and use the adapter 5v.

RiP · 09 May 2019, 12:40

Thanks, I powered it externally but made no difference

cbmeeks · 12 June 2019, 04:30

I built a similar circuit (although for NTSC) and have had great luck with it. I had posted some details on my progress here but it appears my topic was removed (not sure why).

If you're interested, I could probably dig it out and post some closeup pics if that would help. Or maybe show how I built mine.

FYI, the original post was here:

http://eab.abime.net/showthread.php?...33#post1109933

RiP · 13 June 2019, 07:43

Thank you but your link doesn't work. What schematic did you use?
I have many AD724 chips but sadly couldn't get them to work, strange...

25 March 2019, 10:59	#1
RiP Q Join Date: Feb 2017 Location: CD0: Posts: 139	Problem with AD724 I've just found an AD724 but sadly, I got this wrong colors (magenta, and blue instead of black) plus some odd noise lines and sync issue I followed this article (PAL) to make it: https://www.everythingamiga.com/2017...o-adapter.html The tantalum capacitors i got are all polarized though Any chance to fix it?

26 March 2019, 17:22	#4
pandy71 Registered User Join Date: Jun 2010 Location: PL? Posts: 2,745	Amiga HSync and VSync are VGA compatible, use CSync instead, AD724 datasheet provide information how to do it. Code: If available, the AD724 will also accept a standard CSYNC signal by connecting VSYNC to Logic HI and applying CSYNC to the HSYNC pin CSync carry TV like timing. Try to rewire your AD724 and use CSync. Check XTAL frequency (oscilloscope could be a nice thing) Agnus DS - check waveforms at page 39 https://computerarchive.org/files/co...ual-ENG%20.pdf I agree with Mathesar - tantalum caps can be even dangerous when improperly aplicated. Last edited by pandy71; 26 March 2019 at 17:38.

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25 March 2019, 20:05	#2
nogginthenog Amigan Join Date: Feb 2012 Location: London Posts: 1,309	As far as I know tantalum capacitors are polarised. I see what you mean. The circuit shows 5 non polarised caps. Try Googling around. Seems this chip is popular with retro systems.

25 March 2019, 20:24	#3
Mathesar Registered User Join Date: Aug 2014 Location: Netherlands Posts: 695	I would replace all the 0.1uf capacitors with plain ceramic types. Or even better: Use some foil caps like mkt for the rgb input caps. Tantalum is not what I would use for a circuit like this. As for the color issues, I think the tuning is quite critical. Does the trim capacitor give any effect?

12 April 2019, 09:30	#5
RiP Q Join Date: Feb 2017 Location: CD0: Posts: 139	Well, I replaced the caps with non-polarized type but still no luck The trim capacitor had no/less effect too. I tried to use CSync but got even worse result. The only way I could get correct colors was when I removed C7 and C8 caps and had to touch the top of C6

30 April 2019, 12:45	#9
RiP Q Join Date: Feb 2017 Location: CD0: Posts: 139	Ok, but how to connect an external 5V adapter to it properly? I know that I should disconnect the 5V from Amiga and use the one from adapter but what about the grounds from Amiga and adapter?

09 May 2019, 12:40	#11
RiP Q Join Date: Feb 2017 Location: CD0: Posts: 139	Thanks, I powered it externally but made no difference

12 June 2019, 04:30	#12
cbmeeks Registered User Join Date: Dec 2015 Location: Soddy Daisy, TN USA Posts: 95	I built a similar circuit (although for NTSC) and have had great luck with it. I had posted some details on my progress here but it appears my topic was removed (not sure why). If you're interested, I could probably dig it out and post some closeup pics if that would help. Or maybe show how I built mine. FYI, the original post was here: http://eab.abime.net/showthread.php?...33#post1109933

13 June 2019, 07:43	#13
RiP Q Join Date: Feb 2017 Location: CD0: Posts: 139	Thank you but your link doesn't work. What schematic did you use? I have many AD724 chips but sadly couldn't get them to work, strange...

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