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Old 25 June 2019, 18:16   #1
Tony Gunk
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Exclamation Amiga 500 Issues

Hello,


I am very new to the Amiga scene, and I'm currently somewhat ignorant to the inner-workings of an Amiga, so I was hoping I could get some help on this subject.


I own an NTSC Amiga 500 with a rev5 motherboard, and OCS. When I first got the computer, I powered it on, and found that the startup becomes stuck on either a dark grey, light grey, or white screen; never progressing to the kickstart 1.2 floppy screen. Also of note, the drive activity LED always stays on, even if the drive is unplugged. The drive makes no click, and the caps lock key gives no sign of error. I got the same result using the B&W composite output, and on an RGB monitor.


When I opened the case, I noticed there was an A501 512k ram expansion installed. I removed it, knowing that they contain a real time clock battery that tends to leak over time. I also searched the board for any signs of corrosion caused by a leaking batter, but found that the board was remarkably clean, with the exception of some dust. I turned on the computer without the A501, and got the same result. While I had the computer open, I noticed that after being turned on for a while, the 68000 CPU would become VERY hot; hot to the point where you could not touch your finger to it for more than a second without burning yourself. I also noticed that while all the other chips would become somewhat warm after a while of being on, the Gary chip would stay cold. I'm not sure if this is how these particular chips are intended to act, but it seemed like a possible cause of my issues; especially that overheating CPU.


I really hope to get this computer working, because I know the capability of the Amiga for gaming and graphics production. Along with the computer, I was also given a massive stash of games and software on floppies that I hope to be able to use one day.


Any help on this topic would be greatly appreciated.
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Old 25 June 2019, 19:34   #2
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With the grey screens, it could be an issue with the CIA chips. What you can do is to try swapping them around to see is that helps. The two CIA chips, even though they are labelled even and odd, are interchangeable.

Also try pressing gently down on all the socketed chips to make sure they are all seated well.
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Old 25 June 2019, 23:42   #3
Tony Gunk
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I swapped the CIA chips, without any change in behavior. I've also noticed that instead of just dark grey, light grey, or white, the screen also becomes stuck on blue, or green. From what I've read, a blue screen indicated an issue with the OCS (Denise, Paula, Fat Agnus), and green indicates a ram error. However, I find it strange that these colors are appearing in conjunction with the grey and white screens. I've gone and reseated all the chips; Fat Agnus in particular, as well as the 68000 CPU without any change.

What else is there that I could possibly do?

*Edit*
Looking at the boot-up screen color list again, I noticed that light green (one of the colors I noticed coming from the computer) indicates an issue with one of the CIA chips.

Last edited by Tony Gunk; 25 June 2019 at 23:52.
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Old 25 June 2019, 23:52   #4
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It could also be a power supply issue. Is the PSU the original PSU? Very strange behaviours can occur if a PSU doesn't supply the correct voltages and currents. I would test the PSU to make sure it's working okay.
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Old 25 June 2019, 23:56   #5
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The PSU is the original. Now that you mention it, I find it worth noting that sometimes, the computer just won't turn on at all. Perhaps it is a power supply issue? If so, I'd still like to run a few tests. I've ordered a few replacement parts, and I'll see if this makes a change.


Thanks for the help with this, as I'm not sure what I could have done otherwise.

*Edit*
I do own a multimeter, but I'm not very acquainted with it. How do I check the voltage coming from the PSU with it?
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Old 26 June 2019, 00:06   #6
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Reading a PSU without a load attached can give misleading results, so perhaps the simplest way of testing it might be to disconnect the floppy drive power cable and measure the voltage present there. The two middle pins are ground, the outer pins are 5V and 12V. Set your meter to the V setting (or 20V setting if it has manual ranging), hold the black probe against the shielding (or one of the square screw hole pads if the top shielding is missing), and touch the red probe off one of the outside pins of the floppy power connector. Be *very* careful of letting the red probe touch off *anything* else while it's touching one of the outer power pins...

The 12V rail is less critical, but if the 5V rail isn't within around 0.3V of 5V, you'll probably have problems running the machine reliably.
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Old 26 June 2019, 00:07   #7
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You're very welcome.

I'd probably not connect the PSU back up to the A500 until you've got it tested. Just in case there is a short in the PSU.

With the PSU, you've probably got leaky caps in there. There are a few options, getting the PSU recapped, replacing the internals (like with a Meanwell RT-50B), converting an ATX PSU, or getting a new PSU for your A500.
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Old 26 June 2019, 06:22   #8
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Quote:
Originally Posted by Daedalus View Post
Reading a PSU without a load attached can give misleading results, so perhaps the simplest way of testing it might be to disconnect the floppy drive power cable and measure the voltage present there. The two middle pins are ground, the outer pins are 5V and 12V. Set your meter to the V setting (or 20V setting if it has manual ranging), hold the black probe against the shielding (or one of the square screw hole pads if the top shielding is missing), and touch the red probe off one of the outside pins of the floppy power connector. Be *very* careful of letting the red probe touch off *anything* else while it's touching one of the outer power pins...

The 12V rail is less critical, but if the 5V rail isn't within around 0.3V of 5V, you'll probably have problems running the machine reliably.

I tested the PSU with my multimeter as you instructed, with the 12V and 5V rails on the floppy power connector. The 12V rail showed around 11.28V, and the 5V rail showed around 4.36V. Based on what you said, the 5V is not within 0.3V, which would be a cause to assume there is, in fact, and issue with the power supply?
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Old 26 June 2019, 06:26   #9
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4.36 is way to low so Check the Power supply first.
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Old 26 June 2019, 18:04   #10
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Yep, time for a new power supply there.
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Old 27 June 2019, 02:06   #11
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Thanks for the advice, it probably wouldn't have crossed my mind if not for the suggestion.


However, I still have questions regarding the CPU. Is it in fact supposed to get so incredibly hot? I understand the chip are supposed to get at least somewhat warm after a bit of use, but this case seems unnatural, and it seems to get this hot in no short time. I was just wondering if this is part of the intended behavior.
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Old 27 June 2019, 18:09   #12
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Generally no, the CPU shouldn't get that hot. Unless the CPU has internal issues, or there's a short sending the wrong voltage to the wrong pin, or it's under very heavy load. My hopeful thought is that due to the low voltages on the board, the CPU and other chips aren't initializing correctly and the CPU is in a continuous fast loop creating all the heat. But I don't know enough about the internals of the 68000 to say for sure. With a new power supply, things may then start up correctly and the temp should then be in the normal operating range.
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Old 27 June 2019, 19:26   #13
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Sounds like another issue that could be fixed by a new power supply. If it is something wrong internally with the 68000, I've got a spare on hand. The one that's in the machine currently isn't an original Motorola anyhow; some kind of rebrand with a large "S" for a logo.
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Old 27 June 2019, 19:53   #14
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Hmm... Not sure on what brand of 68k would be. But I think it may be a good thing you have a spare on hand. Knock off chips like that generally don't do too well. The knock off CPU could actually be the issue.
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Old 27 June 2019, 20:51   #15
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Quote:
Originally Posted by DofD View Post
Hmm... Not sure on what brand of 68k would be. But I think it may be a good thing you have a spare on hand. Knock off chips like that generally don't do too well. The knock off CPU could actually be the issue.
Not true! My own A500 Rev 5 from back in the day has exactly this Signetics CPU and it is still going strong See here for an example of a list of 68000 CPUs used: https://www.bigbookofamigahardware.c...t.aspx?id=1497
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Old 28 June 2019, 07:12   #16
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I replaced the CPU with an original Motorola, and had no change in behavior regarding the computer's output. However, the new CPU does not heat up the way the old one would, which means the old CPU might have had an issue of it's own. Also, as I pulled out the old CPU from it's socket, one of the pins on the end which had become loose, broke of in it's socket, rendering it completely useless.
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