[Coco] 720K 5.25" drives....

[Gene Heskett]

On Wednesday 23 December 2009, Frank Swygert wrote:
>Won't the 720K 5.25" drives work fine as a 360K drive? I know they are
>80 track instead of 40, but I seem to recall they would work just fine
>as 360K drives as well as 720K. I know they were used in the Tandy 2000,
>but don't know about any others. There have been a few Teac models in
>the surplus channels lately. Just thought that might be a good one for
>CoCo users since 360K drives have pretty much dried up.

Iffy at best Frank.  #1, the track width is about 40-45% of the 360k track 
width.  This will work on one condition.  And that is that before being 
formatted by double stepping the heads to maintain some semblance of the same 
track to track spacing, they absolutely must be erased on a very strong bulk 
tape eraser such as broadcast facilities had years ago to erase their audio 
carts and video tapes with.  If not done, then the leftover data on what 
would be an unused adjacent track in between the tracks being formatted will 
scramble the reads when that disk is then put in a true 360k drive and a read 
is tried with that wider head.

Such a disk can then be read in a true 360k drive about 99% of the time, but 
really should be write protected as the wider write track of the 360k drive 
might scramble the adjacent track, I have had exactly that happen to me on 
several occasions.

No such problem exists in the 3.5" realm because they are all 135 tpi and the 
only thing that changes between a DD and an HD is the write currents are 
about doubled in HD mode and the clock speed of the data rate goes from 
250kbaud to 500kbaud, or even 1 megabaud in the 2.88 meg versions.

There are also a very very few amiga hi density drives about that do their 
version (on DD disks only) via slowing the disk down to 150 rpms.  Obviously 
the dependability suffers, a lot, and the drive spindle actually visibly 
steps, creating a situation where the disk must slip on the drive hub until 
the hubs pin locks into the hole in the disk hub, thereby synchronizing the 
data speed wobbles that causes back out of the data stream for the most part.

-- 
Cheers, Gene
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