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Old 27 November 2016, 15:47   #1
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Amiga 1200 serious Video port issues

I am having serious scrambled gfx, coloured vertical lines appearing and freezing issues when using my A1200 Rev2B with anything attached to the Video (RGB) port.

To put it in a context, I have been using my A1200 with a ScanJuggler (very early version made back in 2000) and its VGA port on an old GABA 15" SVGA CRT monitor and everything worked fine for about 15 years. During this time period, I have not even removed the ScanJuggler from the chips it is seated on. My poor old CRT monitor passed away about a year ago and I started to use it on an 8-year old LCD monitor (with ScanJuggler's VGA port) and my LCD TV with an RGB to SCART cable (the very same SCART cable I use with my A500s without any issues).

About this time, unusual freezings started to happen, usually with the screen first getting scrambled, then everything frozen to halt. It is getting more and more serious and now I can not use my A1200 for more than 5 minutes without these problems appearing. Sometimes coloured vertical lines appeared even following a reset and sometimes even after turning it off for a while and then back on.

I recently had it recapped, but it did not affect these issues.
I have a Blizzard MKIV 1230, but problems appear even when it is not plugged in.
I tried to use it as a vanilla A1200 plugged to different screens through the video port, but all the same happens after a while.

I have been testing this issue for a year now, but I just could not figure out what the problem is.

Yesterday brought a major breakthrough, when I connected the A1200 to the LCD TV through the RF port. I used it for more than 5 hours different games and demos running with no problem at all (with 1230/50). Then I tried video port and issues were there again. Then again with RF and no problems.

So basically, the machine works fine when connected to anything via the RF port, but is failing when connected to anything with any kinds of cable (VGA or SCART) via the Video-RGB port.

Can anyone help how I could fix this issue?
Thank you very much in advance, I would really appreciate any kind of advice.
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Old 04 December 2016, 02:25   #2
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Any ideas, please...
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Old 04 December 2016, 10:55   #3
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It sounds like you have a dry solder joint somewhere, I would inspect all the way from the actual video port to the video chip and its components to see if there was a bad solder joint. As you are getting a good result from the RF port I would have to say that the likelihood of anything other than a bad joint, broken resister or SMD cap is unlikely but someone much wiser may have experience in this matter and respond (I hope)
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Old 04 December 2016, 21:39   #4
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Thank you for the tip, I am going to check.
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Old 04 December 2016, 22:46   #5
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If you haven't got them, great redrawn schematics are here -


You'll want to look first at the the pages on Video out and Lisa.

As Allen1 says, since the RF works fine the composite will likely also be fine. In turn this means that the analogue RGB signals from the video DAC U30 are also likely ok. So I agree the first thing to check is physical damage/dodgy soldering at the video connector CN9 and all the Rs and Cs that connect signals there.

In particular you should look at the power connections to the connector and at the signals which are not used by the composite/RF out. HSYNC, VSYNC, DR, DG, DB, DI, XCLK, _XCLKEN, _CCK, PIXELSW. If you can't find anything wrong at the connector, follow these signals back through the schematic to see where they come from and again look for physical damage on the board to any components in the signalpath.

Can you display RF and video port at the same time? You can check if the garbled stuff appears on the RF when it starts to go wrong on the video port which may give further hints to what is wrong.
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