Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cmcl2!husc6!encore!xenna!paradis
From: paradis@xenna.Encore.COM (Jim Paradis)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 386 PCs and FCC Class A and Class B certification.
Keywords: business use ONLY, RF interference, low cost 386PCs
Message-ID: <7091@xenna.Encore.COM>
Date: 22 May 89 01:50:47 GMT
References: <6930@bunny.GTE.COM>
Sender: news@Encore.COM
Reply-To: paradis@xenna.UUCP (Jim Paradis)
Distribution: usa
Organization: The Whizzo Chocolate Company
Lines: 51

In article <6930@bunny.GTE.COM> js07@GTE.COM (Jack Shaio) writes:
>One way of turning a Class A into a Class B is to put conductive paint all
>over the inside of the box; any experience with this ?

Welll... here's a bit of experience AND a few extra observations for your
money.  First the experience:  the last company I worked for used
VT100-clone terminals made by MicroTerm as part of their product.  These
boxes were veritable fountains of RF energy... no WAY were we going to get
FCC approval for our computer systems if we hung THOSE babies off of it!
So, in an effort to reduce the RF interference, MicroTerm brought out an
ECO on the terminal which was to paint the inside with conductive paint.
OK, but there was just one problem:  the main circuit board was mounted
vertically about 1/8" from one wall of the terminal, CIRCUIT SIDE OUT!!
This meant that if you bumped the side of the terminal, you ran a chance
of shorting out the main board... the possible results were anything from
an unintended reset to terminal flambe'.  Last I heard, MicroTerm didn't
make that model of terminal anymore.  The moral of this story, anyway,
is that if you're GOING to go the conductive-paint route, make sure
that nothing that comes near the sides of the box depends on the fact
that the box is insulative!!

Now on to the observations:  my brother is an RFI testing/compliance
engineer, and he tells me that the only way to GUARANTEE FCC class B
compliance is to enclose the whole device in a lead-lined box with
perfectly flat surfaces and perfectly square corners (no fair poking
holes in the box to let wires out or air in, either!!).  Short of this,
making a device FCC-compliant is pretty much a black art.  That's why
you'll sometimes open up a device and you'll see a square of copper foil
or mesh seemingly randomly glued to the side of the case -- after diddling
with the device for WEEKS, the engineer found that placing EXACTLY that
square inch of copper in EXACTLY that spot would make the device compliant.

In the end, therefore, unless your brother is an RFI guru or you have
a spare lead coffin handy, you'd best just pay your money and take your
chances in the clone market... or else pay extra for a system that is
certified compliant.

Note, however, that even buying a certified system doesn't solve all
of your problems.  As soon as you buy an expansion card and plug it
in, you've just made a new machine and have to worry about compliance
all over again.  A single option card can turn a compliant PC into a
quasar (I found that out with my own PC -- when I first bought it, it
was a floppy-only system, and it was quiet as a kitten RF-wise.  As
soon as I installed a hard disk && controller, though, all hell broke
loose.  TV reception in our house became non-existant whenever the PC
was on... since I often used the PC as an escape when my wife wanted
to watch a movie I didn't care about, this was a bit of a problem.  My
solution was to get cable TV 8-) 8-) 8-)  )

Jim Paradis	paradis@encore.com		508-460-0500
(My other .signature is a witty saying)