Atari STe - Where to grab un-amplified RGB

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thgill
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Atari STe - Where to grab un-amplified RGB

Postby thgill » Wed May 07, 2014 11:05 pm

So, I have mentioned on multiple times (and even started a thread) about my displeasure with the video out from the STe.

I have had 4 or 5 different 520 and 1040 STe's. NTSC and PAL. RGB or composite output.

They all exhibit the same issue; ringing in the video. Most noticeable to the right of objects. It shows up as a shadowing of sorts.

I recently got to thinking about the issue some more. Perhaps it's noise or the method in which its amplified on the motherboard.

What I want to know is: Where can I grab un-amplified RGB video at? I presume off the Shifter?

I have looked at a couple STe schematics, but it isn't terribly clear.


My plan is to grab unamped video and run it through the THS7314 video amp circuit to see if that improves the video quality.
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Re: Atari STe - Where to grab un-amplified RGB

Postby Hippy Dave » Thu May 08, 2014 2:38 am

Ringing can be caused by a bad cable, or splitter box, or the monitor. The Atari ST/E has 3 emitter follower transistors, each with a 75 ohm resistor connected to the emitter. Check a schematic.

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Re: Atari STe - Where to grab un-amplified RGB

Postby troed » Thu May 08, 2014 8:26 am

No I agree with thgill - this is definitive an inherit problem with the STE specifically. I've had it show up on all machines (STE) I've had since 1989 - but not on STM/STFM. Throughout the years no cable, monitor, switchbox etc has been the same of course ...

As to where to find the signals, see the GST Shifter section here: http://info-coach.fr/atari/hardware/STE-HW.php#hardware

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Re: Atari STe - Where to grab un-amplified RGB

Postby exxos » Thu May 08, 2014 8:46 am

gst.png


The STFM has 22pF capacitors on the RGB output lines, you could try that on the STE to see if it improves matters.

The STFM is bias slightly different, but its probably down to voltage differences on the GST vs Shifter. Though the circuit is pretty much identical otherwise.

Also C411, I would replace with a good ESR value capacitor, 10uF, and place a 1uF ceramic across it to make sure you are getting a reasonable smooth supply to the RGB amps.
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