Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!watmath!ahhills
From: ahhills@watmath.waterloo.edu (Arthur Hills)
Newsgroups: comp.sys.mac.hardware
Subject: 1.44 Meg Drive Problem Cause etc
Keywords: Drive Problems
Message-ID: <1990Aug1.190114.600@watmath.waterloo.edu>
Date: 1 Aug 90 19:01:14 GMT
Distribution: comp
Organization: University of Waterloo
Lines: 45




There has been  discussions on the net about the hardware problems with the
1.44 Meg floppy disk drives. I am the hardware supervisor of one lab which 
contains 50 SE'30 all with this drive.

	We have been noticing the same problems as every one else on the net.
Frequently the floppies that are in these drives would no longer boot the 
systems.  They could not be formatted on a apple system. However they could be 
formatted on an 386 clone system.Usually between 1 and 8 different bad blocks 
would be found.  

	At the start of term in January I decided to start with fresh disks in 
all of the machines. I took the floppies and started to look at them without 
reformatting them. After checking for bad blocks with the "SUM Tuneup" software,
I found that there was a very interesting problem. The same blocks were showing 
up as bad on a number of different disks and quite often if more than one block was found to be bad the separation of the bad blocks was a multiple of 36. This was  very unique.
	
	I contacted the Apple Technical Response Group in Toronto Canada as well
as the floppy disk manufacture. After much discussion and sending disks to
both of these facilities I finally received a verbal response from Apple and a 
written response from Kao Didak the manufacturer of the floppies. Apple informed
me that there 2 different types of floppy drives were shipped in their products.
An early version from Sony with a GREEN board as the controller and a later 
version with a BLUE board also from Sony. The early version did not have a 
phase-lock-loop circuit to regulate the speed of the floppy drive. The BLUE 
board unit has this circuit.

Quoting a letter from Kao Didak
 " A Phase Lock Loop is an electronic control system which is used to 
maintain a specific drive speed. The system automatically compensates for load 
variations thereby guaranteeing a specific rotational speed within (1%). A 
drive with no such circuitry could have a rotational speed which varied 
considerably.

	Will drive speed affect writing/reading? Yes if this drive speed was 
varying more that 1% when writing, we would experience problems with bit 
placement. Later when we go back to read the same portion of the disk track, 
the bit would appear to have shifted. This in turn could cause CRC (Cyclic 
Redundancy Check) errors."

 	The solution has been to replace all the floppy drives in my 
machines with the BLUE board controller.