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Old 18 December 2017, 01:59   #1
Devlin
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Something's wrong with my A1200

I think i've damaged my Amiga with my previous VGA project (thread here)
because when I plug my RGB/SCART cable in I get a super washed out barely visible white screen. Anyone know what i'm likely to have damaged and what I can do to fix it?

The odd part is that the VGA lead I made still shows it's somewhat awful picture just fine
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Old 18 December 2017, 09:54   #2
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Hmmm, are you sure the SCART cable is ok? And it's a different display you're trying it on, one that otherwise works ok? It was definitely fine before, yeah? Check the composite output too and see how that looks. If you're still getting an image but it's too white, it's likely the video DAC or the video reference voltage that's been upset. It's hard to know exactly though without a scope and some measurements.
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Old 18 December 2017, 10:28   #3
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A white screen with silhouettes of the Amiga screen can also be a symptom of the display not switching into RGB mode. You know that the Scart plug can use Composite, S-Video and RGB, all sharing the same pins, so the display has to select the right mode. Check the voltage on the selector pin. The other thread says that you connected +12V to some pins on the VGA plug, so maybe you destroyed the 12V line on the Amiga. Or some fuse is blown. Does the serial port still work? It needs 12V, too.
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Old 18 December 2017, 10:32   #4
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Yes, that's a good call, you could easily have burnt out a 12V trace on the motherboard by connecting it to ground like that. audio should also be distorted if you're missing 12V, but it's also possible that only the 12V output at the RGB port was affected, and supply to the rest of the machine still works fine.
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Old 18 December 2017, 15:40   #5
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All my devices either give "no signal" or the washed out white screen with the RGB cable.
The composite video output is fine, as is audio.

From what i'm reading it's highly likely I've burned out the 12v on the monitor port.

I followed the instructions in the other thread for the VGA but it's entirely possible the cable shield(which is ground) might have come into contact with something inside.

Is there an easy way to visually confirm a burned trace?
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Old 18 December 2017, 16:43   #6
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Shorting to ground might not have needed the 12V line to come into contact with the shield - connecting it to the ground on the display could be enough depending on aspects of the display's design. A burned trace should be fairly obvious, but could be on the underside of the A1200 motherboard. From memory, it could even be along the edge of an internal layer, which wouldn't be as easy to spot. There's also the possibility that the damage is to a component - there are filters on those lines that probably don't take kindly to shorts. Best bet would be to check the continuity from pin 22 to the power socket's 12V terminal with a multimeter when the machine's powered off (there should be a low resistance). Alternatively, if you're careful, you could check for 12V present on pin 22 when the machine is powered on.

Last edited by Daedalus; 19 December 2017 at 16:54.
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Old 18 December 2017, 18:31   #7
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I get nothing on pin 22 in either of those cases.

No visible or easy to spot burned traces either.
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Old 19 December 2017, 16:54   #8
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That looks like your issue so. Parts of the 12V trace travel in an internal layer on the board, so the damage could easily be in there. Check out the A1200 board on http://www.amigapcb.org/, click on pin 22 and use that to trace the connection by measuring the resistance between the pin and each point backwards until you know where the fault is.
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Old 19 December 2017, 18:22   #9
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check your RGB/SCART cable is correctly wired with no extra or incorrect connections using a mutimeter in resistance/continuity mode. Without removing the connector covers, check all pins and shields on each connector to all pins/shields on both connectors.

Also, check the connection resistance and double check that the pin numbers are correct. Especially if you're using a DB25 25 pin connector that has been modified (cut down) to be a DB23 23 pin connector.

For the harder part of checking for A1200 damage, you'll need some Amiga 1200 schematics:
http://www.amigawiki.de/doku.php?id=...ice:schematics
https://archive.org/details/A1200_Sy...1992_Commodore
http://www.jammarcade.net/computer-schematics/
and Amiga 1200 hardware info from
http://aminet.net/search?query=A1200+hardware (in amiga iff format!!)

There are also many Youtube videos on fixing Amiga 1200
[ Show youtube player ]
[ Show youtube player ]
[ Show youtube player ]

The A1200 schematics should match the A1200 board version.

http://www.amigapcb.org/ only has the Amiga 1200 revision 2b Motherboard at present.

Suspect that the pin 22 to +12v line connectivity issue was because the connection at the +12v end was bad. Try from video port pin 22 to each side of nearby SMD resistor E264 instead; one should give near 0 ohms resistance and the other about 200 ohms resistance. Then try pin 22 to the positive pin of the large 47 micro-farad SMD electrolytic capacitor C821 near the Amiga 1200's power input socket; we expect about 200 ohms resistance.

Doubt that shorting any of the VCC (+5v), +12v, or -12v pins on the Video port to the 0v pin or shield on the same port would actually damage the A1200 Motherboard traces as each is currrent limited by an in-series 200 ohm resistor to a maximum short to 0v current of 25mA, 60mA, and 60mA respectively.
Given that you're VGA cable using the same port is giving a good signal then it is probably an incorrectly wired RGB/SCART cable causing the wrong voltage levels (higher than expected) to appear on the RGB (or composite video) pins of the SCART plug.

Last edited by jlin_au; 19 December 2017 at 19:02.
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Old 26 December 2017, 03:03   #10
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I've done all I can do that I'm comfortable with to try and figure out what's wrong, so I'm going to look into getting it diagnosed and repaired by someone who can do it. I'll be sad to be without my miggy for any amount of time but it's better than having a busted one I can't even play on
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