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netbeui

Please help me identify this 800xl mod

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Hi fellow Atari lovers,

 

I picked up an 800xl today and after bringing it home I noticed a switch on the back. When I opened up the case and took a closer look I see two thin wires soldered to two resistors. 

 

The machine works fine when the switch is in one position but the video goes weird when I switch it to the other position. 

 

Does anyone know what this mod is for? I’ve attached a few pics below. 

 

Thanks,

Sal

9561467A-AB73-4557-84CB-89EDCCD9F43D.jpeg

0FD97B4D-0A21-4792-8933-892C5B988458.jpeg

D010F30D-CBAE-46D5-98CF-D0D0B37E55FA.jpeg

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Just guessing here, I could be way off base......

Chroma mod for S-video output via the 5-din plug with a switch so the resistors did not have to be cut?

 

 

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It looks like a mute switch because it looks like one side of the switch is connected to pin 37 of U22 (POKEY) and the other side to earth. The switch when closed would ground out the audio signal from POKEY.

 

If that is the case it's weird that it disrupts the video signal. Are you using the composite out or the TV out for video?

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No. The switch is connected to pin 10 of U23 (PIA - PB0) and turn-off the OS-ROM.

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I don’t understand the need for a chroma mod. I have a cable that I plug into the monitor port of this 800xl which has 3 rca connectors that go directly into the chroma, luma and audio ports on the back of my commodore monitor. The picture looks really good, is the mod you’re referring to supposed to make it even better?

 

I’ve tried booting the computer with this mystery switch flipped on but the computer won’t boot. If I flip the mystery switch after running a game or basic it just garbles and freezes the screen. I also sent a message to the previous owner of this xl on Craigslist but he said he has no idea what that switch is for. 

 

Anyway thanks for your help and ideas. If you have any other suggestions please let me know!

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Hard reset maybe. Try flipping it, then flipping it back after booting. Then press the reset key.

Edited by JR>

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The stock 800xl does not support s-video out of the box, only rf and composite, if you're getting chroma on your commodore monitor, it was more then likely modded and that switch with the video seemed suspect to me, but it seems unlikely after reading the other posts. 

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Looks like one wire is connected to U22 (POKEY), pin 21 = K2 which is one of the Keyboard Scan inputs and the other to Audio Out Pin 37.

 

So doesn't seem to make any sense 

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7 minutes ago, TGB1718 said:

Looks like one wire is connected to U22 (POKEY), pin 21 = K2 which is one of the Keyboard Scan inputs and the other to Audio Out Pin 37.

 

So doesn't seem to make any sense 

A homebrew external keyboard mod that connected through the DIN? 

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I believe that Simius is correct.  The wire going to the resistor follows a trace that appears to pass between the pins on the Pokey and ends up reaching one of the Port B pins on the 6520.  The other wire goes to a capacitor on the side that ties to the ground plane.  I think closing the switch grounds a pin on Port B...why?  I have no idea.

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7 hours ago, JR> said:

Hard reset maybe. Try flipping it, then flipping it back after booting. Then press the reset key.

I tried that but it doesn’t seem to make any difference. I flipped the switch and booted to nothing. Then flipped it back and still nothing. Then pressed reset and got the pause before ready prompt. And now when I press reset it just resets back to the ready prompt. 

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Thanks everyone for the tips and trying to help. Still no idea what this mystery switch was installed for but I’m gonna ignore it, stop obsessing over it and just enjoy this awesome little computer :)

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Well, it has nothing to do with SVideo, but I wonder if it disables the internal OS and and keeps the RAM under the OS enabled.  Speculation of course.

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1 hour ago, netbeui said:

I tried that but it doesn’t seem to make any difference. I flipped the switch and booted to nothing. Then flipped it back and still nothing. Then pressed reset and got the pause before ready prompt. And now when I press reset it just resets back to the ready prompt. 

Switch to the working position and boot to basic.  At this point pressing reset should just immediately returns to ready(warmstart). At this point flip the switch, then flip it back.  What happens when you press reset now?  Immediate jump to ready(warmstart)....pause before ready (coldstart), or something else?

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1 hour ago, JR> said:

Switch to the working position and boot to basic.  At this point pressing reset should just immediately returns to ready(warmstart). At this point flip the switch, then flip it back.  What happens when you press reset now?  Immediate jump to ready(warmstart)....pause before ready (coldstart), or something else?

I followed your steps and switched to the working position and booted to basic then pressed reset which initiated a warmstart. I flipped the switch and flipped it back, now pressing reset just goes to a warm start. Here’s a pic of what an initial start looks like and what it looks like when I first flip the switch in case it helps. 

5BD72A55-FD44-44CC-8AA4-C8733C802484.jpeg

7CC967B8-3BF1-4B0D-93E1-F4054CDF91B0.jpeg

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I think it was an attempt at just that JR> - it just didn't work as hoped

for as the slap easy one without the support logic of a real freezer. What's

the harm in trying?  I was clueless until your suggestion too. You got my

vote.

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20 hours ago, netbeui said:

I followed your steps and switched to the working position and booted to basic then pressed reset which initiated a warmstart. I flipped the switch and flipped it back, now pressing reset just goes to a warm start. Here’s a pic of what an initial start looks like and what it looks like when I first flip the switch in case it helps. 

5BD72A55-FD44-44CC-8AA4-C8733C802484.jpeg

7CC967B8-3BF1-4B0D-93E1-F4054CDF91B0.jpeg

This last image is definitely the ROM disabled. It locks up because there is no OS and it looks like this because there is no font.

 

23 hours ago, kheller2 said:

Well, it has nothing to do with SVideo, but I wonder if it disables the internal OS and and keeps the RAM under the OS enabled.  Speculation of course.

You were right. Funny you say this, because I used to have such a switch myself between 92 and 96 or something. I used it for OS experiments and wanted the OS to run from the RESET vector, hence the disable OS ROM completely switch, which I only switched after I had run software to install a RAM based OS.

 

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This mod most likely is for the software based "Freezer" that was made around 1987/88 or so by a Mr. Frank Malisch of Germany. Name of this "Freezer" was something like "Replay" (IIRC, could be wrong) and it was just some wire, a switch, and a floppy disk. So it was much cheaper than my hardware based "Turbo Freezer" and the school kids could afford it better. Actually, I helped Frank with the design of hardware concept and the 1050 TURBOs which produced the unbreakable copy protection he did use. This software freezer works by keeping the freezer software in the RAM behind the OS ROM and when the switch is flicked and an interrupt occurs then the freezer software is able to take over the machine. The truly ingenious part follows - reconstruction of the unreadable hardware registers of ANTIC, GTIA and POKEY by the freezer software only. Some of that was automatic, some required user inputs based on observations and listening to the sounds. This took a while and was not straightforward. Had a learning curve. But it generated a bootable floppy disk and if all you wanted was to make one of those to defeat some copy protection of a game, then Frank's software freezer was the cheapest route to have it. As for the

copy protection of his freezer disk, it was never broken and even professional duplication machines couldn't copy it. Years later, some interested third party wanted to buy the rights to the software freezer - but even Frank, who created it, could not remove the copy protection. It was so much intrinsic to the freezer code that even with the source code it could not be cracked. So Frank could not sell the rights to that third party. Needless to say, the software freezer disks now are very rare and very hard to find. - Bernhard Engl    

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