Amiga hardware info, help and support with a focus (but not limited to) North American NTSC experiences. Open to all.
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Olem
Norway

by Olem posted Wed May 01, 2019 6:57 am

Hi everybody, haven't posted anything around here before, love the forum. I'm mostly into the C64, but got myself a nice boxed A500 from a german Ebay seller a couple of years back. It works fine with an ACA500plus, but has some sort of issue with the floppy drive. It reads disks fine. However, when I try to copy a disk, I get mixed results. Most often it seems to work fine, no errors during writing, but the resulting disk is unreadable.
A few days back I eventually got around to opening the case and checking out the drive. It seems very clean and nice, and the small caps seem to be ok, at least there's no physical signs of "bulging" or leakage. It's a Chinon. More surprisingly, I found something strange on the motherboard: The Gary chip has a resistor soldered on to two of the legs! Has anyone got any idea why this was done? Can this be related to the disk drive issue?
The motherboard is a PAL revision 6A. The only other possibly unusual thing I found, was that the Fat Agnus was version 8375, and seems to be a bit never than the other chips.
20190426_222536.jpg
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Zippy Zapp
CA, USA

by Zippy Zapp posted Wed May 01, 2019 8:05 am

Interesting. It is hard to tell without seeing the rest of the board. Is there any other jumper wires or cut traces anywhere? When people mod Gary it is usually related to adding RAM or CHIP RAM mods. Since you have an 8375, which is capable of 2MB chip RAM I would say that is a possibility. The Gary chip apparently also has floppy functions so that could be an issue. If the rest of the mod was removed and the Gary mod was not fixed then I would think it could cause issues.

Of course it doesn't hurt to clean the drive heads and clean and lubricate the drive rails and moving bits. :D
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intric8
Seattle, WA, USA

by intric8 posted Wed May 01, 2019 8:21 am

How bizarre. I've never seen anything quite like that. Usually when I see a weird resistor added to a motherboard it's often put in there at the factory to repair a boo boo they made. But this is really unusual.

If would be interesting if you had a spare GARY to put in there and see what happens. How much CHIP RAM does your board report?
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Olem
Norway

by Olem posted Wed May 01, 2019 1:42 pm

Hm, a chipram mod that was removed might explain it, though I have no idea what this actually does. I checked the motherboard once again, and found a jumper that seems to have been tampered with.
20190501_220229.jpg

20190501_220126.jpg

Without the 512 k expansion in the usual slot, it reports 362736 free memory in Workbench, so there's no extra chipram there now. This is the whole thing:
rev6A.jpg

Yes, I guess I'll change out the Gary and see what happens then. I haven't got a spare, but I found a german gentleman on Ebay that had a Gary with a production date that's just a few weeks off the rest of the chipset.
Somebody definitively has been working in here, the machine seem to have been built in 1990. The Fat Angus seems to be from '93, and is probably added later. Are there any compatibility issues with that?
Yep, I'll try the isopropanol and silicone grease routine on the floppy drive later, maybe that'll do the trick :D
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A10001986

by A10001986 posted Thu May 02, 2019 12:53 am

That resistor on Gary pulls EXRAM to high, so it's some kind of RAM hack. Nothing to do with the floppy drive.

(Mentioned here)
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Zippy Zapp
CA, USA

by Zippy Zapp posted Thu May 02, 2019 11:08 am

Yep, JP2 is what would have had been cut and possibly a wire soldered between it and a Gary adapter. It should be safe to replace the Gary or if you can desolder the resistor, clean up the legs and reinsert.
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Olem
Norway

by Olem posted Thu May 02, 2019 12:26 pm

Ok, thank you guys! Think'll try to desolder it, if all goes wrong I have a spare Gary in the mail anyway. But do you think that JP2 is ok? It's got solder all over it, kind of hard to tell what's been cut and what's been connected. Best to leave it like is, maybe? Kind of strange that these things doesn't seem to affect the system as it is, at least R-type runs happily :D

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