Amiga.org

Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: Richard42 on July 03, 2011, 06:17:09 PM

Title: a1200 display sync problems with VGA cable
Post by: Richard42 on July 03, 2011, 06:17:09 PM

I have an A1200 with a DB23<-->VGA cable and can't get it to work with my video converters and VGA CRTs.  I have a GBS-8220 and an Ambery (http://www.ambery.com/rgbcgatovgac.html) and neither of them will sync to the A1200 display.

I took the cable apart and found that it was wired to take the A1200's CSYNC composite sync signal and route it to the HSYNC pin of the VGA.  The converters are supposed to handle composite sync as well.  So I hooked up a scope to the csync signal and found this:

(http://i53.tinypic.com/2w4lnv6.gif)

The hsync component of the composite sync doubles in frequency (to 31khz from 15khz) around the time of the vsync pulse.  Is this normal, or a problem with my Amiga?

Title: Re: a1200 display sync problems with VGA cable
Post by: desiv on July 03, 2011, 07:51:49 PM

Good luck..
There's some good tech people here, but there are also some very serious bit-heads over at EAB (English Amiga Board).
You might want to post this over there as well...

desiv

Title: Re: a1200 display sync problems with VGA cable
Post by: ncafferkey on July 04, 2011, 02:54:47 AM

Don't you need a special Amiga VGA adapter with some resisters or something in it, rather than just a plain cable?

Title: Re: a1200 display sync problems with VGA cable
Post by: Stedy on July 04, 2011, 09:25:00 PM

@Richard42

That looks like a typical Amiga video signal to me. It is sort of interlaced, though the odd even frame delimiter is missing. Take a look here for my handy guide:
http://www.ianstedman.co.uk/Amiga/amiga_hacks/Amiga_SCART/amiga_scart.html

The Csync signal is possibly too big in amplitude, the info pages states 2v peak to peak for H/V sync signals, the Amiga H/V/C Sync signals are >4.5V. Adding a 150 ohm resistor in series with the Amigas CSYNC signal will reduce the levels nicely.

Ian

Title: Re: a1200 display sync problems with VGA cable
Post by: Richard42 on July 05, 2011, 06:23:20 AM

Thank you for your answers.  I got it working today, and there were 2 issues causing my problems.  First, the csync voltage was too high.  The input impedance on the GBS-8220 is about 500 ohms.  By putting a 570 ohm resistor in series it knocked the voltage down to around 2v.  The other issue is that the GBS-8220 appears not to be able to handle 50hz.  It will only sync to the signal when the Amiga is in NTSC (60hz) mode.

I had this working a few months ago with an Ambery AV-1 converter, and it could do the 50hz modes as well, but now it doesn't work.  I think that this converter is dead.  I'll have to get another one and try it again.