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Old 22 July 2013, 02:39   #1
kipper2k
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A1200 video repair problem & solution

Hi All,

I recently bought myself an a1200 (Escom Rev 2b) on Ebay that was stated as being broken (the video display colours were wrong).

I received it yesterday and of course first thing to do was to see what was wrong with it. I added the 3.1 KS roms and then plugged it in using my svideo adapter to my monitor. the white colour on the screen was more of a beige colour so something was definitely wrong. I plugged in the composite video output to my monitor and the colour was the same.

Now that i knew that the problem was related to both the RGB and composite signal, i was able to deduce that the problem was not the RGB inputs to the CXA1145 (as that only outputs the composite) and the same RGB inputs were fed directly to my svideo adapter. The video signal to both RGB and composite is fed by the video DAC so there was a possibility that the video DAC output was not putting out (so to speak).

So, i needed a quick way to test whether the video DAC output was good. The Video DAC receives its colour signals from Lisa. Lisa sends 8 bits of data to the video DAC for each colour. The Video DAC calculates the output and then forwards it on to both the CXA1145 or the 23 pin RGB output.

Fortunately i had a very easy way to test whether Lisa or the DAC was bad. The Indivision AGA video adapter plugs directly onto Lisa and generates its own video output so therefore it was safe to "assume" that if i plugged the MKII over Lisa and i obtained a good video signal through its own output then i had narrowed down the problem significantly...

Success, using the Indivision MKII output to my monitor the colours were perfect... This told me that Lisa was good and there was a problem with the tracing, resistors, Video DAC or a series of diodes (Doubtful). There could have been a short somewhere after Lisa but i believed the results would be far worse if that was the case.

So, next thing to do was to try to find out which portion of the colour signals was bad.. The way i done this was to feed Indivision output to one monitor and fed SVideo output to another monitor. I could see the difference in the colours immediately. Booting up from CF card I opened up palette in Workbench and i adjusted the RGB levels all the way down. I slid the slider for blue all the way up... perfect... Green... perfect. When i selected red i noticed that the red colour scale was not a smooth gradient all the way up to full intensity. As i adjusted the red intensity upwards i seen a dark spot. This told me that there was a problem with either the Video DAC, a trace(s) or 68ohm resistor. All 8 bits of each colour pass through a 68 ohm resistor from Lisa to the Video DAC. The easiest way to check if there was a trace/resistor problem was to measure resistance from each of the Red data bits from Lisa to the DAC. I should see 68 ohm on each data bit when measuring from Lisa to DAC.

I measured all resistors and luckily enough R7 was showing infinite resistance so i was hoping that this was the only issue as otherwise the DAC was next on the list to be removed/replaced. I applied a little flux to the affected pins and resistor and put a little fresh solder on all solder joints, rechecked continuity and all looked good.

Rebooted, checked colours through RGB, all were good, checked composite, good also. Rebooted from floppy and floppy drive refused to boot, i thought oh crap, after all this i gotta play more. I remembered that this A1200 has a jumper installed and realised that i had to do the Escom floppy fix so i removed it, put a jumper on in the correct place and was back in business.

time to fix was about 2 hours so very happy
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Old 22 July 2013, 05:17   #2
NovaCoder
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Nice bit of detective work my friend.

Way beyond my skills to fix something like this.
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Old 22 July 2013, 06:02   #3
demolition
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Nice work. Keep bringing back those Amigas.
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Old 22 July 2013, 06:57   #4
mfilos
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Awesome! Next time get some pr0n pics of before and after :P
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Old 22 July 2013, 20:37   #5
prowler
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Nice one, Kipper!
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Old 28 September 2015, 06:50   #6
TjLaZer
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Kipper, I have an Amiga 1200 that is doing the same thing, but for me Red and Blue are the culprits, so it might be a bad connection or a bad DAC. Can you help me fix it?
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