js07@GTE.COM (Jack Shaio) (05/16/89)
A recent issue of PC Magazine tested 100 386 PCs from many manufacturers. A good proportion, around 30%, have only FCC Class A certification, meaning they can cause enough RF (radio frequency) interference to damage reception of TV and AM radio signals. Most of the high performance/low cost 386s fall into this category, including ALR, Tandy 5000, Northgate. I've heard that you can turn on a Class A device in your home, but if you interfere with someone's TV or receiver, the FCC can force you to stop using your device, or modify it to stop interfering. Amateur ham radio operators run into this often. Does anyone know more about this topic ? Some very competitive brands, like ALR, are class A, and would make a terrific buy, if only one could be sure of being able to turn them on at home. There are enough Class A PCs on the market that it would be helpful to know what can be done about the Class A bummer. 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 ? ---------------------------------------------------------------------------- P.S.: if you MUST email a reply, please don't bother including the FCC's number. I have it, but feel embarassed to ask about fines/jail terms.
jborza%burgundy@Sun.COM (Jim_Borza) (05/16/89)
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 ? It's not nearly that simple. In effect, a device is considered Class A if it meets the following: a) It is intended for use in non-residential,i.e. industrial/office environments (this can be determined by ,among other things, the sale price, power requirements, bulk/weight, etc.) b) The manufacturer tests and certifies the device as meeting the Class A requirements. Class B is for equipment either intended for, or easily adapted for use in residential environments, or offered for sale through channels sel- ling into residential environments. Class B requires the test data to be submitted to the FCC. The FCC determines whether to certify the unit as Class B and may require a sample of the equipment to be submitted to them prior to issuing (or denying) the certification. Spraying conductive paint may bring the emissions from a unit within the Class B limits, but it will never make it a Class B product - that re- quires the FCC's certification. It's also interesting to note that if your computer interferes with someone's TV/Radio reception, it doesn't matter if it's Class A, B or whatever, you're on the hook to stop the interference. The above isn't legal advice - just general guidelines learned the pro- verbial "hard way". Jim Borza - Sun Microsystems Disclaimer? Sure, why not?
toma@tekgvs.LABS.TEK.COM (Tom Almy) (05/16/89)
In article <6930@bunny.GTE.COM> js07@GTE.COM (Jack Shaio) writes: >A recent issue of PC Magazine tested 100 386 PCs from many manufacturers. >A good proportion, around 30%, have only FCC Class A certification, meaning >they can cause enough RF (radio frequency) interference to damage >reception of TV and AM radio signals. >Some very competitive brands, >like ALR, are class A, and would make a terrific buy, if only one could >be sure of being able to turn them on at home. There are enough >Class A PCs on the market that it would be helpful to know what can be >done about the Class A bummer. > Don't worry about it! Unless you put your TV right next to your computer you'll never notice it. I get more interference from adjacient televisions than from my (class A) computer. Extra added disclaimer: The above may not apply if you live in a "fringe" reception area, or you are in an apartment and your computer is located next to your neighbor's TV (through the wall). Tom Almy toma@tekgvs.labs.tek.com Standard Disclaimers Apply, as well as the one above
davidsen@sungod.steinmetz (William Davidsen) (05/17/89)
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 ? This does wonders on a machine with a plastic case. Of course, I've never seen a 386 with a plastic case. This was done for a few PC clones which came in plastic or other non-conducting cases. All it seems to do for a metal case is give you a chance for a paint peeling to drop somewhere someday. I remember doing it, copper worked better than aluminum, and adding aluminum foil stuck on with rubber cement was best. As much as this brings back memories of "the old days," if it was that easy the vendors would do it themselves. I once had a system that took out every TV in the house, and a hard disk that opened the neighbor's garage door on power up. EMI is much less these days, and the cheap solution is gone. Careful placement of the system will *usually* allow you to use them without problems, although if you are in an apartment building you will want to think before you buy. Most class A machines will *almost* make class B, except the rack mounted things with no back and sides. bill davidsen (davidsen@crdos1.crd.GE.COM) {uunet | philabs}!crdgw1!crdos1!davidsen "Stupidity, like virtue, is its own reward" -me
BVAUGHAN@pucc.Princeton.EDU (Barbara Vaughan) (05/17/89)
In article <5160@tekgvs.LABS.TEK.COM>, toma@tekgvs.LABS.TEK.COM (Tom Almy) writes: >In article <6930@bunny.GTE.COM> js07@GTE.COM (Jack Shaio) writes: >>A recent issue of PC Magazine tested 100 386 PCs from many manufacturers. >>A good proportion, around 30%, have only FCC Class A certification, meaning >>they can cause enough RF (radio frequency) interference to damage >>reception of TV and AM radio signals. > >Don't worry about it! Unless you put your TV right next to your computer >you'll never notice it. I get more interference from adjacient televisions >than from my (class A) computer. My Compaq Portable III is upstairs; my TV is downstairs. Whenever I turn on the computer, the TV picture becomes noticeably "grainier". I don't remember whether it's Class A or B, and I suppose Princeton, NJ might be called a fringe area (about 50 miles from both NY and Philadelphia transmitters), but we normally have no reception problems. Barbara Vaughan
ray@ole.UUCP (Ray Berry) (05/18/89)
Conductive paint is helpful in shielding non-metallic enclosures, but
would be of no value in reducing the EMI from a pc in a steel cabinet.
Electromagnetic energy will escape from an enclosure in two ways -
on the cables entering/leaving the box, and through any openings in the
enclosure, such as drive bays and even the seal between the slide-on
cabinet top and the bottom of the chassis. There is an aperture effect
such that the minimum frequency energy which will escape is a function
of the size of an opening in the enclosure (e.g., the size of the holes
in a perforated panel).
To shield a machine well, you must provide an "rf-tight" box. It is
not enough to screw the top on at 2-3 tops on the chassis- good electrical
contact must be maintained at every joint with gaps not exceeding an inch
or so. Any conductors entering and leaving the box must be filtered, or
well shielded. (Obviously you can't put a 1 Mhz low pass filter on your
video output :-)..
The incremental mfg cost of producing such a computer is probably not
that great (<$100) but few companies have the resources to do the requisite
engineering and testing. Plus, there really isn't any public demand for
it. There are a few companies who make "Tempest" rated pc's (VERY well
shielded, for the NSA types)- but those machines are priced so that only
the "TAXPAYERS" can afford them...
The only "quiet" pc I have seen offered to the public is a unit manu-
factured by Hal Communucations that is intended for use in ham radio in-
stallations.
trivial, but
--
Ray Berry KB7HT uucp: ...uw-beaver!sumax!quick!ole!ray CS 73407,3152
Seattle Silicon Corp. 3075 112th Ave NE. Bellevue WA 98004 (206) 828 4422joel@peora.ccur.com (Joel Upchurch) (05/18/89)
In article <8371@pucc.Princeton.EDU>, BVAUGHAN@pucc.Princeton.EDU (Barbara Vaughan) writes: > My Compaq Portable III is upstairs; my TV is downstairs. Whenever I turn > on the computer, the TV picture becomes noticeably "grainier". I don't > remember whether it's Class A or B, and I suppose Princeton, NJ might > be called a fringe area (about 50 miles from both NY and Philadelphia > transmitters), but we normally have no reception problems. I don't know if this is relevent, but if you are using a roof antenna, how far is the antenna lead from the computer? And is it shielded or unshielded cable? -- Joel Upchurch/Concurrent Computer Corp/2486 Sand Lake Rd/Orlando, FL 32809 joel@peora.ccur.com {uiucuxc,hoptoad,petsd,ucf-cs}!peora!joel Telephone: (407) 850-1040 Fax: (407) 857-0713
paradis@xenna.Encore.COM (Jim Paradis) (05/22/89)
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)
waynec@hpnmdla.HP.COM (Wayne Cannon) (05/27/89)
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.