From: chips@tek.com (Chip Schnarel)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Upgrades not limited to si's?
Date: 10 Nov 92 00:01:11 GMT
Article-I.D.: maverick.chips-091192131141
Followup-To: comp.sys.mac.hardware
Organization: Tektronix, Inc.

In article <z7m12hg@rpi.edu>, simsh@marcus.its.rpi.edu (Chewbacca) wrote:
> 
> I just heard last night that it's also possible to upgrade the oscillator in
> your PB 140 or Quadra 700/900 the same way as in the IIsi. Anyone tried this
> or know anything about it? I'm curious, but I don't know how reliable the 
> source is. It'd be loads of fun to pick up a cheap 140 and run it like a 145.
> 
> -- 

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.