From: ellens@bnr.ca (Chris Ellens)
Subject: Re: Upgrades not limited to si's?
Followup-To: comp.sys.mac.hardware
Organization: Bell-Northern Research
Date: Thu, 19 Nov 1992 14:08:00 GMT

In article <chips-091192131141@maverick.wr.tek.com>, chips@tek.com (Chip
Schnarel) wrote:
> 
> Look people,  do you really know what you're doing when you modify your
> machine?  Are you considering the consequences?  I'm not flaming those
> who've performed the upgrade, indeed I may perform it myself when my
> warranty expires.  But this whole-sale discussion of sticking a new crystal
> in anything should be couched in extreme caution.  (I'm NOT picking on
> Chewy, I'm talking about this whole thread.)
> 
> I've been waiting for someone with manufacturing experience to speak up,
> but since no-one has I'll stick my feet into my own mouth.
> 
> If you choose to upgrade your crystal speed, by all means go ahead, but
> before you make that decision, consider the following.
> 
> Electronic circuits are designed with a particular clock speed in mind. 
> Some margin is built into these designs.  The need for margin comes from
> two primary sources.  1) The speed of each component varies from machine to
> machine.  Margin assures that the worst possible combination of components
> still works.  2) The speed of all the components within a system varies
> with changes in temperature and powersupply voltage.  Margin assures that
> the machine still works under the worst possible combination of influences.
> 
> In manufacturing, the product is tested, typically over temperature.  On
> the old compact Macs this used to be a twenty-four hour burn in.  I don't
> know what they do now.  When you modify your Mac do you test it for
> twenty-four hours?  Do you test it over the whole temperature range?
> 
> If you don't, you might create a Mac that works fine right now, but gets
> flakey next summer when the room temperature is eighty degrees.  Or a Mac
> that gets flakey as the components age and the individual timings shift
> slightly.
> 
> This thread began with a discussion of Apple designing to 25MHz but
> shipping at 20MHz.  Maybe so, maybe not.  Who knows?  I do know that the
> CPU in my Mac is labeled -20 not -25.  Same with the FPU.  So at least two
> critical components are not rated for this speed.  Also, the manufacturing
> process tests at the operating speed, not the design speed.  No
> (production) Mac IIsi is tested at 25MHz until you do it yourself.  Let's
> say you modify your machine and it works.  It may be that you have removed
> most of the margin and that it works right now, but if anything else
> changes, you have no margin in reserve.  Your machine will become flakey or
> might just die.
> 
> It's your machine.  Do what you want with it, but make your decision from a
> position of knowledge.
> 
> Chip
> 
> BTW - Rosin core solder is hydroscopic.  That means that over time it soaks
> up water and becomes slightly conductive.  At 50MHz this can (and HAS in my
> experience) effect operation.  I hope that you have all cleaned the rosin
> off with alcohol before you replace the board.

I agree with Chip in that anyone embarking on a hack of their valuable Mac
hardware should know what they're doing. I hope that all of the warnings
posted here on the net (such as Chip's point about rosin core solder) don't
discourage people from such hacking.

A few decades ago, nobody who knew anything about cars drove a stock model
off
of the dealer lot. Customization to enhance performance was half the appeal
of
owning one. I hope I never see the day when computers are so perfect that
we
just let them sit there and run. So keep posting the warnings, caveats, etc
-
lets all learn from each other and see what we can come up with. (Hmmm.. do
you think I could get a little more muscle out of my LC if I machined the 
cylinder heads...)

Chris Ellens     
ellens@bnr.ca
Newsgroups: comp.sys.mac.hardware