thad@cup.portal.com (Thad P Floryan) (08/15/89)
Just received an email from a person who "caught" a reference I made recently to the RF shield surrounding the hard disk in the UNIXPC. The person was asking if his removal of the shield may be contributing to the problems he's having with his HD. My answer (names censored to protect the guilty!): ---------- Hi ****! That shield for the HD is there for a reason! MOST of the "crap" from the CRT is directed downwards ... right onto the HD. It also goes off to the left (so to speak). I discovered this with several other computers, too, when attempting to place either floppies or HDs to the "left" of a monitor ... the "left" or "right" really depends where the HV flyback circuitry is located in a specific monitor. In the UNIXPC, operating the internal HD without the RF shield is asking for possible data corruption; the RF energy beneath the CRT is *VERY* strong; I discovered this by placing an AM/FM portable radio near the "base" of the CRT when I had the cover off and the system operating when testing some other things. In fact, operating the UNIXPC fully open jams my car radio (AM/FM, "premium sound", top-of-the-line unit in my T-Bird) at 20 feet; just THINK what that can do to your hard disk; I had to back my car out of the garage before the radio would again operate. If you can retrieve that shield, do so and re-install it! Thad Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]
mdapoz@hybrid.UUCP (Mark Dapoz) (08/16/89)
In article <21302@cup.portal.com> thad@cup.portal.com (Thad P Floryan) writes: > The person was asking >if his removal of the shield may be contributing to the problems he's having >with his HD. Umm, would you believe that mine never had a shield. I didn't know I was missing it until I opened up a friends machine and saw this huge thing sitting over the drive. Never really bothered me 'cause my drive has never given be a bit of a problem, and still doesn't give me problems. Of course, the monitor is now on a stand by itself off to the side so that I can get at the hardware. I beleive that my machine was manufactured sometime in mid 86 because all the parts have dates around 8618 stamped on them. -- Mark Dapoz (mdapoz@hybrid.UUCP) ...uunet!{mnetor,dptcdc}!hybrid!mdapoz I remind you that humans are only a tiny minority in this galaxy. -- Spock, "The Apple," stardate 3715.6.
motteler@umbc3.UMBC.EDU (Howard E. Motteler) (08/16/89)
In article <21302@cup.portal.com> thad@cup.portal.com (Thad P Floryan) writes: >That shield for the HD is there for a reason! MOST of the "crap" from the >CRT is directed downwards ... right onto the HD. It also goes off to the >left (so to speak). Well, sort of... But the machine meets FCC standards for radio noise as it comes out of the box, as does the CRT. All digital signals are sealed inside steel or tin, and the HD shield is part of this. The flyback transformer, which is essentially the same as found in a B&W TV, is not completely shielded, as the sawtooth waveform for the horizontal sweep at ~ 15KHZ puts out a lot less radio energy (even given the higher power levels) than a bunch of TTL level square waves around 10 or 20 MHZ, as found on the motherboard and the HD digital circuitry. The TTL video signal to the monitor probably puts out more RF noise than the flyback, and some care has been taken to shield the video signals. Inductive coupling of the flyback transformer to, say the HD or floppy read heads might be more of a problem, but interestingly, the floppy is not inside the machine's electrostatic shield, so the designer's weren't worried about this. Come to think of it, the switching power supply probably puts out more RF noise and has stronger magnetic fields than the flyback. Switchers typically run anywhere from 10 to 50 KHZ or so, and all power for the machine is running through that little transformer. I wouldn't leave floppies sitting on either side of the machine. (If you set floppies on the right side of the machine, you block the power supply air holes, anyway!) >In the UNIXPC, operating the internal HD without the RF shield is asking for >possible data corruption; the RF energy beneath the CRT is *VERY* strong; I >discovered this by placing an AM/FM portable radio near the "base" of the CRT >when I had the cover off and the system operating when testing some other >things. Well, an AM radio is quite sensitive as a switching noise detector, and if the "hood" (the steel lid above the motherboard) is open at all when the machine is running, it will put out even more RF noise. The very first computer I ever owned, a Z80 system from "the digital group" came with a demo that would buzz out the "star spangeled banner" an a nearby AM radio, while drawing a flag on the screen...ok, ok, it was 1976, the bicentennial and all... Since the HD platter and heads are sealed in a metal can, its vulnerability to noise should be restricted to the short path the head signals travel from outside the can, to the drive electronics under the can. Essentially, if noise from its own TTL signals, stepper motor pulses, etc., don't bother it, then noise or magnetic fields from the switcher or flyback probably won't, either. I would guess that the intent of the HD shield is to bring the machine into FCC specs for radio emmissions, more than to protect it from external noise. Not that it necessarily proves anything, but I've been running without the HD RF shields on my machines for a year and an half, and although they may put out a bit more RF noise, I can run an AM shortwave radio 10 feet from the machine, when it's closed up, with no interference, and I've never had any other problems. Reasons for leaving the shield off include: (1) On the big miniscribes (at least on mine) it defeats the drives' shock mounts by pressing very tightly aginst the top of the drive; (2) it restricts air circulation around the drive electronics, (3) the "tin fingers" that press aginst it in the back prevent the "hood" from propping up in the full open position; (I took those off, too); (4) lazyness on the part of the person putting the machine back together :-). >If you can retrieve that shield, do so and re-install it! If you can't, I can make you a great deal on a couple of barely used RF shields :-) -- Howard E. Motteler | Dept. of Computer Science motteler@umbc3.umbc.edu | UMBC, Catonsville, MD 21228