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Old 12 December 2018, 23:46   #1
Uplink7
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A1200 Boot/video problems

I need some help with a strange issue that is confusing me. I have an old A1200 motherboard that I kept for a spare and decided to do a recap on it to get some practice before I tackled my other in use boards. (I think it may have had an issue before this that led me to do a recap.) Well I got myself a bit too excited and started before I had all the parts/tools I needed it and yes butchered the board a bit. I know I should have practiced on something else. Well I got the tools I needed and began the recap again, fixed a couple of bad pads, and the later cap replacements came out fine. I replaced all but the 22uf audio caps. Well the issue now is if the board has been off for say overnight, it will boot to a black screen and stay that way, no disk animation etc. I can see good NTSC composite video on a scope, just black with no OS boot. However if I turn it off for a bit and then back on good video shows just for about 2 seconds and then goes to baseline nothing, dead like it got reset. I have tried different ROMs, different power supplies, keyboard attached, no keyboard attached, disk drive attached, no disk drive attached all with the same results. First boot of the day gives me NTSC video black that will stay on for minutes on end but if I shut it down a couple of times it just flashes NTSC video for 2 seconds and then baseline every time till I keep it off for overnight or at least many hours.

This seems to me like a cap issue as if they need to discharge overnight for it to not reset itself after 2 seconds. But I would think if I screwed up the recap it wouldn't work at all. I've tried checking traces, I have clocks, power seems good etc. Seeing I didn't replace the audio caps yet could they be the issue? I don't think they would stop it from booting and they don't look leaky.

I am enjoying the challenge but I'm not sure what to try next.
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Old 13 December 2018, 23:45   #2
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Hi .
Did you analized the Reset Line ?
I'm only really used to to ocs and ecs machines but in the end Garry or Gayle in your case should be the the one in charge of resetting it all.
I don't know the pin number but you will find it.
Immediate after power up the reset line has to go low for half a second or so and then high again.
And by low i mean not over 1.5 Volts... otherwise it is may shorted out giving you all sorts of fail.
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Old 14 December 2018, 23:29   #3
Uplink7
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Thanks. It goes high when I turn the A1200 on but stays high never goes low briefly. Interesting that when I probe the reset line on the CPU, video (black) will come back and sometimes stay on or flash off again. My scope must be taking it low randomly. One thing I'm now noticing is if I leave it on (no video out) Alice (U2) gets warm, not hot but a little warm and no other chip gets warm at all. It doesn't get hot, more normal warmth but its the only one.

I did end up changing the 22uf audio caps but as I suspected it didn't change anything.
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Old 15 December 2018, 09:02   #4
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I would suspect that there is a hairline crack in a solder joint or trace somewhere on the board. Probably once the board has been running for a little while, thermal expansion causes something to shift just slightly and causes the issue.

You could try poking at various chips or places on the board (with something non-conductive, of course) or using a cold spray to cool them down and see if there is any change.

I once had a board that required cooling of Alice to run. There was seemingly nothing wrong with the solder joints, but even a replacement chip didn't help. But installing a little heatsink and fan kept the system running. It's still a mystery to me what exactly the issue was; most likely a microscopic failure of the PCB somewhere I just could not find.
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Old 15 December 2018, 20:30   #5
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I agree. I bought myself one of those cheap microscope cameras to help trace things out and see if I can find it. I did find some cracked traces under the ROM's from a previous owner removing the chips with a screwdriver and not a chip puller. I expect there may be more especially in some of the chip solder joints.
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Old 17 December 2018, 21:19   #6
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Well using the camera and the Amiga PCB Explorer site I found a couple of bad connections between Alice and Paula. Looks like something hit the pins and broke a couple of them on the chip making it un-salvageable and a couple of solder joints as well. I used my heat gun to remove it and found a couple of pads to repair while I wait for a replacement chip. I just hope I have the skills to replace it.
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Old 17 December 2018, 21:36   #7
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Nice work. Soldering the new chip back in should not be too hard, just inspect carefully for any solder bridges once you are done.
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