Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!ncsuvx!ccvr1!mark
From: mark@ccvr1.uucp (Mark Boyd)
Newsgroups: comp.sys.atari.st
Subject: Re: INTERNAL HARD DRIVE SEQUENCER CIRCUIT
Summary: info about 1040 modification
Message-ID: <3421@ncsuvx.ncsu.edu>
Date: 17 Jul 89 15:37:45 GMT
References: <890716.01230852.087234@SFA.CP6>
Sender: news@ncsuvx.ncsu.edu
Reply-To: mark@ccvr1.ncsu.edu (Mark Boyd)
Organization: Univ. of North Carolina - Asheville
Lines: 22

I took apart a 1040 ST to check the reset timer. It is a 556 ( a dual
555 )  instead of the Mega's 555. I traced the wiring from the reset
switch and found that ( on an older 1040 ) the timing resistor is 22K
and the capacitor is 2.2 uF. This would give a delay of less than .1
second. If you want a reasonable delay for HD startup, a 10 Meg 
potentiometer should work with the existing capacitor. Put it in series
with the resistor ( marked R8 on the system I checked ). You'll get
more reliable timing by going to a larger capacitor - just solder
it in parallel with the 2.2 uF. ( marked C4 on the old 1040ST ). I'd
start with a 22 or 33 uF capacitor ( these are polarized, so make sure
the + is connected  to the + of the 2.2 ) and a 1 M potentiometer.

All of the above is purely hypothetical for me.  My hard disk is too
noisy to use all of the time, so I'm not going to make this mod myself. 
I have done lots of hardware stuff and I've used lots of 555's, so I'm
offering an little 'informed opinion'.  This does sound like a simple,
reliable, way to get a proper power on boot from the hardisk. 

	Mark Boyd
	Computer Science
	mark@ccvr1.ncsu.edu
	mbunca@ecsvax.uncecs.edu