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Old 22 November 2016, 02:06   #1
PARALAX
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Arrow A1200 - No screen over Composite & HF

There is a problem with my A1200 when trying to use over composite. I only get a black screen there. The HF output bring up white flickering stripes only moving down horizontal quickly. Only RGB works fine. All capacitors have been replaced, the SMD ones with ceramic capacitors. Any idea with other components could be blown? Maybe a resistor near the composite port?
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Old 22 November 2016, 10:25   #2
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Yep, a couple of resistors are used by the composite circuits, as are a couple of the through-hole capacitors, so it's worth checking them out. The video encoder chip itself is also worth looking at, as is the whole area of the PCB around there since if there was any capacitor leakage in the past it could have damaged traces in the area.

It could be a coincidence that both outputs have damaged parts nearby, but if they're both caused by the same failure, check the video encoder (U12), the sync signal into it and all the components around it. In particular, make sure the ceramic capacitors used for the video reference voltage are working properly - it's relatively easy for them to be damaged, especially when they're stacked to increase their capacitance, and a bad reference voltage will affect both outputs as well.
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Old 23 November 2016, 09:47   #3
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So, which components should I check and how do I have set the multimeter for that? I need the voltage values and the exact board position (Uxxx, Cxxx etc.). Is it enough to simple measure the voltage issues from special solder points? Someone told me to check the Delay Line (Z222+Z221) by connecting Pin 16 + 18 from U12 but I still get no signal so they should still work. What can I do next? And how can I check the U12 function with a multimeter or is this only possible with an oscillator?

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Old 23 November 2016, 11:21   #4
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If your multimeter has capacitance, you should check the capacitors involved for roughly the correct values. They're C459, C460, C461, and no harm ruling out C235-237 by checking them too (they're through hole capacitors). But really, for this sort of diagnosis you need an oscilloscope. You can check the continuity of the sync line from pin 4 of U23 to pin 10 of U12 - that should be pretty much 0 ohms - but that won't tell you if the sync signal is actually there or not. No harm to check the resistors for the correct value too, though they're unlikely to be the cause: R221 to R225, R234 to R237, and R459.
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Old 23 November 2016, 12:03   #5
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Why are pins 19 and 21( RGB blue out. and VCC2) bridged on the Video encoder chip ? this wont do it much good I will imagine
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Old 23 November 2016, 12:23   #6
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Hmmm, That's a very good point. I didn't pay any attention to that, just assumed it was one of the factory patches, but that could be the attempt at bridging pins 16 and 18 with a miscount of pins. If that's the case, remove it and try again - hopefully it hasn't damaged anything inside the chip.
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Old 23 November 2016, 21:18   #7
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Quote:
Originally Posted by supaduper View Post
Why are pins 19 and 21( RGB blue out. and VCC2) bridged on the Video encoder chip ? this wont do it much good I will imagine
Yes, sorry it was my fault. Did it again with the correct pins now but no change. The problem existed a long time ago before recapping and replacing the elkos with (new) capacitors, so I think they are working and the CXA 1145 could be faulty. But how can I find that out?
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Old 24 November 2016, 09:27   #8
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To be honest the composite on our amiga`s is not that reliable and quality is low thats why we all use RGB, as your RGB is working fine I will not worry too much about the Composite , anyway the delay line circuit in all Amiga`s is prone too fail through one component or another.
Like Daedalus says check those components mentioned or swap the Video Chip for a new one if you really want to try and get it working
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Old 24 November 2016, 13:42   #9
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Sometimes composite is the easiest way since it's practically guaranteed to work with any TV old or new. Many regions don't have RGB inputs on their TVs, and even in Europe, SCART is disappearing while composite is still present. I use RGB when I can but occasionally the composite comes in quite handy. All depends on your usage though... Quality is low of course, but I wouldn't say it's any more unreliable than any other port.

If you don't have the tools needed to check sync and video signals and replace SMT chips (an oscilloscope and hot air station), the simplest way to sort it would be to send it to someone who does for repair.
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Old 23 September 2018, 23:41   #10
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I've got a black screen on the composite output of my Amiga 1200 (rev 1D4) too. I've just got a steady 2V output on the composite RCA.

I've been probing around all evening and found that resister R225 is missing from the underside of the board. Since this resister is required to connect pin 18 of Z222 to pin 18 (LUM_IN) of U12, I suspect this is why it's not working. I have a lovely video signal on one R225 pad BTW. Have you checked if you have R225 or not?

I've owned this Amiga from new, so nobody has removed it... It just wasn't fitted in the factory.

I've got a 1K ohm resister on order now! ;-)

Last edited by Lord_Carnage; 24 September 2018 at 06:08.
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