Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-ses!hpcea!hpnmdla!waynec
From: waynec@hpnmdla.HP.COM (Wayne Cannon)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 386 PCs and FCC Class A and Class B certification.
Message-ID: <340005@hpnmdla.HP.COM>
Date: 27 May 89 00:51:08 GMT
References: <6930@bunny.GTE.COM>
Organization: HP Network Measurements Div, Santa Rosa, CA
Lines: 29


As mentioned elsewhere, rf energy escapes through the slimmest of
joints and ventilation holes in any conductive covering.
Conductive paint (except expensive silver paint) doesn't really
do a very good job, and most manufacturers use an arc or flame
sputtered zinc coating on the inside of plastic cases (looks like
a silver or gray paint).  For a home-brew job, aluminum foil
works great, cemented to the inside of the case.  Adhesives can
come loose and drop foil on your precious circuit cards, but I
had good luck inside my terminal (next to my ham rig) with rubber
cement.  Make sure to wrap the foil well into any joints between
mating pieces so you will have a continuous conducting surface
with which to seal the joint.

To seal the joints, you can get springy brass fingers to connect
between cover pieces, but I found some self adhesive tape that
looks like a coarse steel wool ribbon that worked very well for
me.  It requires a small gap so will not work for very tight or
sliding joints.  Make sure that all joints are tight enough for a
good electrical connection continuously along their length, or
for a weaker choice, that there is a conductive serpantine path.

Cover ventilation slots with a wire screen.  If you have a fan,
this will increase air noise somewhat.  Make sure not to forget
the bottom and any uncovered paths through any CRT, if present.
Most CRTs are pretty well shielded except near the faceplate and
along the neck.  You can get metal sleeves that fit the necks of
most CRTs.  After the above precautions, I didn't have any
noticeable problems without covering the CRT neck.