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Old 12 March 2018, 23:04   #1
Overmann
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Amiga 600 black screen, no post

Hi there,

I was hoping I'd get some help troubleshooting my A600. It won't post and I'm afraid I don't know a great deal about diagnosing.

My PSU is fine, I've used it with my 1200.
The green led comes on when I turn the 600 on, but the screen remains black.
The caps lock button blinks once.
No response from the floppy drive, and no activity on the floppy led.
Resetting via the keyboard makes the caps light blink again.
All the caps LOOK fine. :P
The cpu gets warm when the machine is on for a while, and some of the other chips get warmish, but the ROM remains cool to the touch.

I have been looking online, and I find similar symptoms, but none that are spot on. I'd like to diagnose some before I start changing caps and buying new roms etc.

I have a multimeter and an oscilloscope if there is any checking I can do, but I have little experience in using them to diagnose anything other then audio circuits. :P
Any help will be GREATLY appreciated!

Last edited by Overmann; 12 March 2018 at 23:30.
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Old 13 March 2018, 00:18   #2
Akira
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You will always hear this as thw first advice: have you recapped it?
SMD caps can look fine but be fucked.
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Old 13 March 2018, 00:34   #3
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No, I have not recapped it. Is there a way to test the caps before removing them? Is there some point on the bord where I can make a reading to verify if a cap is bad?
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Old 13 March 2018, 01:14   #4
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Common A600 problem. http://eab.abime.net/showthread.php?t=73265
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Old 13 March 2018, 01:26   #5
Overmann
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Quote:
Originally Posted by idrougge View Post
I came across this when looking for solutions. So you're thinking that my problem is the same as this even though my caps lock led is not always on? I didn't want to start pulling caps off without being certain.

Last edited by Overmann; 13 March 2018 at 01:33.
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Old 13 March 2018, 09:06   #6
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Most electrolytic capacitors in Amiga systems are there only for supply voltage filtering purposes (one may slowly start to experience crashes or interference in the video etc. as the capacitors dry), which is why their leakage often goes unnoticed until the electrolyte has wreaked havoc on the board.

However, that particular electrolytic in the A600 controls the reset circuitry. I can't say if the symptoms are always exactly the same but that cap is definitely the number one cause of boot failures in an A600.
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Old 13 March 2018, 18:55   #7
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Ok, I'll try taking it off tonight when I get home from work. The board deserves a recap either way (as does my A1200). I understand that 500 and 2000 are not prone to these errors. Should I recap my A500 and my A2000 as well? :P
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Old 13 March 2018, 19:33   #8
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The through-hole caps in the earlier models are physically larger, better sealed and not very prone to leaking so replacement is rarely necessary. Of course it does no harm either (unless you damage the board while desoldering ).

However, you'll want to make sure there isn't a leaky battery in the A2000 or the A500 (if it's a plus model or has a real time clock expansion). The batteries contain a large amount of electrolyte and can really spoil a board if left unattended.
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Old 13 March 2018, 23:08   #9
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Removing the cap made no difference, other then making the CPU A LOT hotter. Actually painful to touch.
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Old 14 March 2018, 01:07   #10
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That sounds very strange. So if you install a new capacitor (even normal through-hole ones will do, or ceramics), it goes cold again?
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Old 14 March 2018, 01:11   #11
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I will have to order some caps to try. It was warm before, if I left the machine on for some minutes, but now it's to hot to touch if left on for only a minute.
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Old 15 March 2018, 13:28   #12
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Does anyone have any other suggestions based on the symptoms? I should mention that I am using the videoport with a scart cable, and that Ive mostly been testing without keyboard and floppy drive attached. I tried once with floppy drive and keyboard but it acted the same way.

Is there any more info that I can give that will help, or is the next logical step to change the caps regardless of wether removong the above mentioned cap had any effect or not?
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Old 15 March 2018, 14:38   #13
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Is the power LED starting dim, then becoming bright after a second or so? Are you sure you don't have any corrosion on the board from capacitor leakage? If the LED deosn't get brighter after a second or two, a good first step would be to look at the reset lines with your scope. They should start low on power up, then quickly go high and stay high. Check the schematics for pin numbers, in particular the reset line going into the CPU. Careful you don't short things out with your probe though!
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Old 15 March 2018, 20:28   #14
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Quote:
Originally Posted by Daedalus View Post
Is the power LED starting dim, then becoming bright after a second or so? Are you sure you don't have any corrosion on the board from capacitor leakage? If the LED deosn't get brighter after a second or two, a good first step would be to look at the reset lines with your scope. They should start low on power up, then quickly go high and stay high. Check the schematics for pin numbers, in particular the reset line going into the CPU. Careful you don't short things out with your probe though!
The LED starts bright and strong. I have inspected for corrosion, and on closer inspection there might be some around the caps. I've ordered a new set and will be replacing these next week hopefully.

Would checking the reset lines still be interesting? The reset seem to be behaving normally to me. It resets when pressing the reset-combo on the keyboard and the caps-led behaves normally.
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Old 15 March 2018, 23:05   #15
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So the LED doesn't change brightness at all? Checking the reset line to the CPU would tell you if the reset part of the motherboard is fully working.
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Old 26 June 2018, 17:23   #16
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I've recapped the macine now but as far as I can tell there is no difference.
@Daedalus No it does not change brightness. I will look into checking the reset line.

The strangest thing, apart from it not working, is how hot the cpu is getting. Really uncomfortable to touch even for a second.
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Old 26 June 2018, 17:35   #17
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Hmmm, something serious is wrong there so. The CPU will normally get warm during use, but never so hot that it hurts. Yeah, check the reset line, and though it might sound strange, check the voltage at the floppy power connector. If the 5V is too high you'll get hot chips, and I've recently repaired an A600 that had a partial short between 5V and 12V, so it's not beyond the reals of possibility that the 5V line is being dragged high.
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Old 26 June 2018, 17:42   #18
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There's a tiny LM555 timer chip next to some commonly leaking caps to the right of the motherboard. AFAIK this chip is used to hold the CPU reset line for some time (for things to stabilize). If it doesn't function correctly, the CPU will be in reset mode forever. Check for continuity around this area with a multimeter.

It's very common for newbies to think that the capacitors look fine while they have actually spread their electrolyte already, and the corrosion have developed. The corroded metals would not be shiny anymore, but rather have a matt green look.
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Old 26 June 2018, 20:41   #19
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Quote:
Originally Posted by Overmann View Post
I've recapped the macine now but as far as I can tell there is no difference.
@Daedalus No it does not change brightness. I will look into checking the reset line.

The strangest thing, apart from it not working, is how hot the cpu is getting. Really uncomfortable to touch even for a second.
I would also recap the PSU. This one holds bigger capacitors.
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