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willowmoon93
December 17th, 2009, 02:09 AM
Hi everyone,

Well I hope someone can help me regarding this. I recently picked up an Amiga 500 computer with a whole bunch of 3.5" Amiga disks. The problem I'm having is that when I put a disk in the internal drive, it says something like "Not valid DOS disk (retry/cancel)". Now when I hit retry same message comes up. When I hit cancel, if I hit the eject button on the drive and the disk comes out, I put it right back in immediately, it goes to Workbench and starts the disk up fine. What I'm wondering is this: is there a way to eliminate the "Not valid DOS" fiasco so that I can just simply have the A500 autoboot the disk. Please let me know -- thanks!

John

carlsson
December 17th, 2009, 02:47 AM
Unfortunately the Amiga 500 floppy drives are infamous for breaking down. That is why you see relatively many 500's with broken or removed floppy drives. It may just as well be that your internal drive is about to give up, thus failing to read the floppy disk on the first attempt.

If you own an external floppy drive, you can try to boot from that one. Usually it should work, and the external one may be less used or of better quality.

willowmoon93
December 17th, 2009, 07:43 AM
Thanks Anders -- I'll give the external drive a shot when I hook it back up. I was reading that I need to hold both mouse buttons at the same time upon booting and then it should boot from the external drive instead of the internal one.

Speaking of the internal drive, what are the main reasons it would break down? Dirt accumulation, overall wear on the drive mechanisms? If it's something fixable (within reason), I'd be interested in trying to make a go of it.

carlsson
December 17th, 2009, 09:41 AM
I am not sure. The Amiga reads floppy disks in a different way than e.g. a PC would. From what I understand it reads a full track and then in software divide it into sectors. The data is as well GCR encoded. Some people claim the way the Amiga access the drive, it wears it out faster than if the same mechanism had been used in another computer. Besides I don't know if it is fixable, but by all means research the topic.

jthiemann
December 17th, 2009, 09:53 AM
I am not sure. The Amiga reads floppy disks in a different way than e.g. a PC would. From what I understand it reads a full track and then in software divide it into sectors. The data is as well GCR encoded. Some people claim the way the Amiga access the drive, it wears it out faster than if the same mechanism had been used in another computer. Besides I don't know if it is fixable, but by all means research the topic.

I'm working off memory here, but could look up the details when I get home. I remember the whole-track operation of the Amiga drives, and I think this was part of the reason the Amiga had the stupid habit of corrupting disks if you ejected during a read operation(!).

Amiga disks are MFM, but have a different sync structure, possibly just one sync per track. The Denise chip is capable of doing GCR, but I think that was only ever used to allow reding/writing of C64 5.25" floppies (the 1581 is also MFM - ISTR that Apple was the only one with GCR on 3.5" floppies)

Now regarding the floppy reading problem: I had similar problems occurring once with my A500. It turned out to be inadequate power. I have both a C128 and a A500, and while the power supply plug is the same, the A500 PSU is rated for higher currents to drive the floppy drives. I had accidentally used the C128 supply with the A500.

However I also have piles upon piles of Amiga floppies that will not read properly even with good drives. It's just age. Perhaps your floppies were all stored inadequately, and the information is permanently gone. Floppies never were a reliable medium.

willowmoon93
December 17th, 2009, 11:24 AM
Now regarding the floppy reading problem: I had similar problems occurring once with my A500. It turned out to be inadequate power. I have both a C128 and a A500, and while the power supply plug is the same, the A500 PSU is rated for higher currents to drive the floppy drives. I had accidentally used the C128 supply with the A500.

However I also have piles upon piles of Amiga floppies that will not read properly even with good drives. It's just age. Perhaps your floppies were all stored inadequately, and the information is permanently gone. Floppies never were a reliable medium.

Wow, I will have to check out the power supply for sure -- I have a boxed Commodore 128 elsewhere so I'll have to compare the power supplies and see if they are the same --I think possibly I have a spare Amiga power supply so that's good info to know -- thanks!

carlsson
December 17th, 2009, 01:42 PM
The C128 has 9VAC on the same pins the Amiga 500 has 12VDC. I suppose the Amiga has rectifiers and voltage dividers inside so a too low voltage may do funny things without permanently damaging it. At least I know swapping them the other way around is almost certainly going to kill your C128.

Of course it could be a bad PSU even if it is the right model and voltages.

willowmoon93
December 18th, 2009, 02:42 AM
Well I found out I was using the correct power supply all along -- and here's what is weird -- the internal drive is now working perfectly, no error messages or anything -- don't know why, but hey that's fine by me!

AB Positive
December 22nd, 2009, 08:29 AM
For future reference, there is a dead simple mod that can be done on many PC floppy drives that will allow them to be Amiga drives as well.

It's two short wires, and one trace cut. If you can solder, you can do the mod. :)